Transmission: References – CR05

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New References (5th Edition)

The following pages add short comments to the papers published since the previous edition (June-October). The comments are from


The Virus

Zhou J, Otter JA, Price JR, et al. Investigating SARS-CoV-2 surface and air contamination in an acute healthcare setting during the peak of the COVID-19 pandemic in London. Clin Infect Dis. 2020 Jul 8:ciaa905. PubMed: Full-text:

In a cross-sectional observational study in a London hospital, SARS-CoV-2 was detected on 114/218 (52.3%) of surfaces and 14/31 (38.7%) air samples but no virus was cultured. As expected, viral RNA was more likely to be found in areas immediately occupied by COVID-19 patients than in other areas (Zhou J 2020).

Schlottau K, Rissmann M, Graaf A, et al. SARS-CoV-2 in fruit bats, ferrets, pigs, and chickens: an experimental transmission study. Lancet Microbe July 07, 2020. Full-text:

When intranasally inoculated with TCID50 of a SARS-CoV-2 isolate, twelve fruit bats (Rousettus aegyptiacus) showed characteristics of a reservoir host and 12 ferrets (Mustela putorius) mimicked subclinical human infection with efficient spread. Pigs (Sus scrofa domesticus) and 20 chickens (Gallus gallus domesticus could not be infected by SARS-CoV-2 (Schlottau 2020).

Routes of Transmission

Meyerowitz EA, Richterman A, Gandhi RT, Sax PE. Transmission of SARS-CoV-2: A Review of Viral, Host, and Environmental Factors. Ann Intern Med 2020, published 17 September. Full-text:

Eric Meyerowitz et al. present a comprehensive review of the evidence of human SARS-CoV-2 transmission (Meyerowitz 2020). Their key points:

  1. Respiratory transmission is the dominant mode of transmission.
  2. Vertical transmission occurs rarely; transplacental transmission has been documented.
  3. Cats and ferrets can be infected and transmit to each other, but there are no reported cases to date of transmission to humans; minks transmit to each other and to humans.
  4. Direct contact and fomite transmission are presumed but are likely only an unusual mode of transmission.
  5. Although live virus has been isolated from saliva and stool and viral RNA has been isolated from semen and blood donations, there are no reported cases of SARS-CoV-2 transmission via fecal–oral, sexual, or bloodborne routes. To date, there is 1 cluster of possible fecal–respiratory transmission.
Aerosol, Droplets

Prather KA, Marr LC, Schooley RT, et al. Airborne transmission of SARS-CoV-2. Science 05 Oct 2020: eabf0521. Full-text:

According to Kimberly Prather and colleagues, we should clarify the terminology to distinguish between aerosols and droplets using a size threshold of 100 μm, not the historical 5 μm (Prather 2020). This size more effectively separates their aerodynamic behavior, ability to be inhaled, and efficacy of interventions. Viruses in droplets (larger than 100 μm) typically fall to the ground in seconds within 2 m of the source and can be sprayed like tiny cannonballs onto nearby individuals.


Bax A, Bax CE, Stadnytskyi V, Anfinrud P. SARS-CoV-2 transmission via speech-generated respiratory droplets. Lancet Inf Dis September 11, 2020. Full-text:

Spit happens. This group published the impressive NEJM video, visualizing speech-generated oral fluid droplets and suggesting that normal speaking might be an important mode of transmission (Bax 2020). Here, the four authors vigorously resist the criticism of other authors who argued that the video experiments were unrealistic. They also provide nice new videos showing speech droplets emitted by four people, when speaking the phrase “spit happens” with the face positioned about 10–15 cm behind a thin sheet of intense green laser light.

Anfinrud P, Stadnytskyi V, Bax CE, Bax A. Visualizing Speech-Generated Oral Fluid Droplets with Laser Light Scattering. N Engl J Med. 2020 May 21;382(21):2061-2063. PubMed: Full-text:

New video:


Fennelly KP. Particle sizes of infectious aerosols: implications for infection control. Lancet Respir Med, July 24, 2020. Full-text:

Is there really evidence that some pathogens are carried only in large droplets? (Fennelly 2020) Or would cough aerosols and exhaled breath from patients with various respiratory infections show striking similarities in aerosol size distributions? In case of doubt, how would you protect your family and yourself?

Santarpia JL, Rivera DN, Herrera VL et al. Aerosol and surface contamination of SARS-CoV-2 observed in quarantine and isolation care. Sci Rep 10, 12732 (2020). Full-text:

After evacuation from the Diamond Princess cruise ship in March 2020, 11 were admitted to a hospital in Nebraska, two in a biocontainment unit and 9 in a quarantine unit. Key features of both units included: (1) individual rooms with private bathrooms; (2) negative-pressure rooms (> 12 ACH) and negative-pressure hallways; (3) key-card access control; (4) unit-specific infection prevention and control (IPC) protocols including hand hygiene and changing of gloves between rooms; and (5) personal protective equipment (PPE) for staff that included contact and aerosol protection. Joshua Santarpia and colleagues collected air and surface samples to examine viral shedding from isolated individuals and detected viral contamination among all samples. Their data suggest that SARS-CoV-2 environmental contamination around COVID-19 patients is extensive, and hospital IPC procedures should account for the risk of fomite, and potentially airborne, transmission of the virus (Santarpia 2020).


Klompas M, Baker MA, Rhee C. Airborne Transmission of SARS-CoV-2: Theoretical Considerations and Available Evidence. JAMA. 2020 Aug 4;324(5):441-442. PubMed: . Full-text:

Brief review. It is impossible to conclude that aerosol-based transmission never occurs, write Michael Klompas and colleagues, but the balance of currently available evidence suggests that long-range aerosol-based transmission is not the dominant mode of SARS-CoV-2 transmission (Klompas 2020).


Chagla Z, Hota S, Khan S, Mertz D; International Hospital and Community Epidemiology Group. Airborne Transmission of COVID-19. Clin Infect Dis. 2020 Aug 11:ciaa1118. PubMed: Full-text:

Zain Chagla and colleagues discuss the paper by Morawska L, Milton DK, It is Time to Address Airborne Transmission of COVID-19 (Clin Infect Dis 2020, 6 July). They agree that there is potential for the transmission by aerosols, especially in poorly ventilated indoor crowded environments. However, they argue that the main mode of transmission of SARS-CoV-2 is short range through droplets and close contact. Explore this one-page comment to see how the debate continues (Chagla 2020).


Asadi S, Gaaloul ben Hnia N, Barre RS, et al. Influenza A virus is transmissible via aerosolized fomites. Nat Commun 11, 4062 (2020). Full-text:

SARS-CoV-2 can be transmitted via droplets, fomites and possibly aerosol. Will we need to get accustomed to a fourth transmission route, aerosolized fomites? That’s what Nicole Bouvier and colleagues suggest, although for now only for influenza A virus. They show that dried influenza virus remains viable in the environment, on materials like paper tissues and on the bodies of living animals, long enough to be aerosolized on non-respiratory dust particles that can transmit infection through the air to new mammalian hosts (Asadi 2020). Will we soon see a paper about SARS-CoV-2 transmission via aerosolized fomites?


Kang M, Wi J, Yuan J, et al. Probable Evidence of Fecal Aerosol Transmission of SARS-CoV-2 in a High-Rise Building. Ann Intern Med 2020, published 1 September. Full-text:

Nanshan Zhong, Min Kang and colleagues report 9 infected patients in 3 families. While the first family had a history of travel to the coronavirus disease 2019 (COVID-19) epicenter Wuhan, the other 2 families had no travel history and a later onset of symptoms. The families lived in 3 vertically aligned flats connected by drainage pipes in the master bathrooms. The authors suggest that virus-containing fecal aerosols may have been produced in the associated vertical stack during toilet flushing after use by the index patients (Kang M 2020). This report reminds us of a SARS-1 outbreak in March 2003 among residents of Amoy Gardens, Hong Kong, with a total of 320 SARS cases in less than three weeks (see, page 65).

See also the comment by Michael Gormley [Gormley M. SARS-CoV-2: The Growing Case for Potential Transmission in a Building via Wastewater Plumbing Systems. Ann Intern Med 2020, published 1 September. Full-text:] concludes that that wastewater plumbing systems, particularly those in high-rise buildings, deserve closer investigation, both immediately in the context of SARS-CoV-2 and in the long term, because they may be a reservoir for other harmful pathogens.


Mondelli MU, Colaneri M, Seminari E, et al. Low risk of SARS-CoV-2 transmission by fomites in real-life conditions. Lancet Infect Dis September 29, 2020. Full-text:

Some arguments that environmental contamination leading to SARS-CoV-2 transmission is unlikely to occur in real-life conditions, provided that standard cleaning procedures and precautions are enforced. The chance of transmission through inanimate surfaces is likely less frequent than hitherto recognized (Mondelli 2020).


Yamagishi T, Ohnishi M, Matsunaga N, et al. Environmental sampling for severe acute respiratory syndrome coronavirus 2 during COVID-19 outbreak in the Diamond Princess cruise ship. J Infect Dis. 2020 Jul 21:jiaa437. PubMed: Full-text:

In the early epidemic in Japan, many infections occurred among the passengers and crew members on board the Diamond Princess cruise ship in February, 2020. By March 1, 2020, there were approximately 700 individuals with laboratory-detected SARS-CoV-2 infection (see the previous articles by Russell et al., Yamagishi et al. and Tabata et al.). The authors performed environmental sampling on the Diamond Princess cruise ship on 22-23 February 2020 (prior to disinfection of the vessel and while some passengers and crew members remained aboard) and obtained specimens from cabins in which confirmed COVID-19 cases stayed (case cabins), cabins with no confirmed case at any point (non-case cabins), and common areas. SARS-CoV-2 RNA was detected from 58 out of 601 samples (10%) from case cabins 1-17 days after the cabins were vacated, but not from non-case cabins (Yamagishi 2020). There was no difference in the detection proportion between cabins for symptomatic (15%, 28/189) and asymptomatic cases (21%, 28/131). No SARS-CoV-2 virus was isolated from any of the samples. The authors conclude that transmission risk of SARS-CoV-2 from symptomatic and asymptomatic patients may be similar and environmental surfaces could be involved in viral transmission.


Chen Y, Qin G, Chen J, et al. Comparison of Face-Touching Behaviors Before and During the Coronavirus Disease 2019 Pandemic. JAMA Netw Open 2020;3(7):e2016924.

Is wearing face masks really associated with reduced face-touching behaviors? To answer the question, Xing Li and colleagues from Sun Yat-sen University, Guangzhou, China, used videos recorded in public transportation stations, streets, and parks among the general population in China, Japan, South Korea, Western Europe (ie, England, France, Germany, Spain, and Italy), and the US to analyze mask-wearing and face-touching behavior in public areas. The authors found that mask wearing was associated with reduced face-touching behavior, especially touching of the eyes, nose, and mouth (Chen Y 2020). They conclude that the reduction of face-touching behaviors by mask wearing could contribute to curbing the COVID-19 pandemic. Excellent news for the coming months.


Joonaki E, Hassanpouryouzband A, Heldt Cl, et al. Surface Chemistry Can Unlock Drivers of Surface Stability of SARS-CoV-2 in Variety of Environmental Conditions. Chem, August 06, 2020. Full-text:

Nice overview of existing knowledge concerning viral spread, molecular structure of SARS-CoV-2, and the stability of the virus surface. Edris Joonaki and colleagues discuss potential drivers of the SARS-CoV-2 surface adsorption and stability in various environmental conditions (Joonaki 2020).


Deng W, Bao L, Gao H, et al. Ocular conjunctival inoculation of SARS-CoV-2 can cause mild COVID-19 in rhesus macaques. Nat Commun 11, 4400 (2020). Full-text:

If you are exploring extra-respiratory routes of SARS-CoV-2 transmission, read the article by Chuan Qin, Wei Deng and colleagues. The authors inoculated five rhesus macaques with SARS-CoV-2 conjunctivally, intratracheally, and intragastrically. The conjunctivally infected animal had a higher viral load in the nasolacrimal system than the intratracheally infected animal but also showed mild interstitial pneumonia, suggesting distinct viral distributions (Deng W 2020).


Vivanti AJ, Vauloup-Fellous C, Prevot S, et al. Transplacental transmission of SARS-CoV-2 infection. Nat Commun2020. Full-text:

Maybe the first documented case of transplacental transmission. French doctors report on a 23-year-old COVID-19 patient who gave birth by cesarean section to a baby found to have the infection (Vivanti 2020). The viral load was much higher in the placental tissue than in the amniotic fluid or maternal blood: this suggests the presence of the virus in placental cells, which is consistent with findings of inflammation seen at histological examination. Good news: baby is fine.


Chambers C, Krogstad P, Betrand K, et al. Evaluation for SARS-CoV-2 in Breast Milk From 18 Infected Women. JAMA August 19, 2020. Full-text:

There are some case reports on the detection of SARS-CoV-2 in breast milk. Christina Chambers and colleagues examined 64 breast milk samples from 18 infected women. Although SARS-CoV-2 RNA was detected in one milk sample, the viral culture for that sample was negative. These data suggest that SARS-CoV-2 RNA does not represent replication-competent virus and that breast milk may not be a source of infection for the infant (Chambers 2020).

Cats and dogs

Patterson EI, Elia G, Grassi A, et al. Evidence of exposure to SARS-CoV-2 in cats and dogs from households in Italy. bioRxiv 23 July 2020. Full-text:

Nicola Decaro and colleagues assess SARS-CoV-2 infection in 817 companion animals in northern Italy at the height of the spring 2020 epidemic. Although no animals tested PCR positive, 3.4% of dogs and 3.9% of cats had measurable SARS-CoV-2 neutralizing antibody titers, with dogs from COVID-19 positive households being significantly more likely to test positive than those from COVID-19 negative households (Patterson 2020). From their experience, the authors conclude that it is unlikely that infected pets play an active role in SARS-CoV-2 transmission to humans. Only under special circumstances, such as the high animal population densities encountered on infected mink farms, animal-to-human transmission might be likely.


Garigliany M, Van Laere AS, Clercx C, et al. SARS-CoV-2 Natural Transmission from Human to Cat, Belgium, March 2020. Emerg Infect Dis. 2020 Aug 12;26(12). PubMed: Full-text:

Mutien Garigliany from Liège, Belgium, and colleagues report a human-to-cat transmission. A household cat was productively infected with the SARS-CoV-2 virus excreted by its owner, and the infection caused a non-fatal but nevertheless severe disease (Garigliany 2020).


Transmission Event


Rockett RJ, Arnott A, Lam C, et al. Revealing COVID-19 transmission in Australia by SARS-CoV-2 genome sequencing and agent-based modeling. Nat Med 2020 Jul 9. PubMed: Full-text:

These researchers examined the added value of near real-time genome sequencing of SARS-CoV-2 in a subpopulation of infected patients during the first 10 weeks of COVID-19 containment in Australia. Genomic evidence was used to cluster 38.7% (81 out of 209) of cases for which the available epidemiological data could not identify direct links (Rockett 2020). This included clustering 12.4% (26 out of 209) of cases with a history of recent arrival from overseas with other cases without a travel history and 5.3% (11/209) of locally acquired cases with unknown epidemiological links. Twenty-two (10.5%) of the 209 cases were epidemiologically classified as ‘locally acquired—contact not identified’.


Park YJ, Choe YJ, Park O, et al. Contact Tracing during Coronavirus Disease Outbreak, South Korea, 2020. Emerg Infect Dis October 2020. Full-text:

The authors analyzed 59,073 contacts of 5,706 COVID-19 index patients. Of 10,592 household contacts, 11.8% had COVID-19; rates were higher for contacts of children than adults. Of 48,481 non-household contacts, 1.9% had COVID-19. Interestingly, the highest COVID-19 rate (18.6%) was found for household contacts of school-aged children (Park YJ 2020) and the lowest (5.3%) for household contacts of children 0–9 years in the middle of school closure.


Milani GP, Bottino I, Rocchi A, et al. Frequency of Children vs Adults Carrying Severe Acute Respiratory Syndrome Coronavirus 2 Asymptomatically. JAMA Pediatr. Published online September 14, 2020. Full-text:

Early reports suggested that children, often asymptomatic, might be facilitators of SARS-CoV-2 transmission and amplify local outbreaks. Here, Carlo Agostini, Gregorio Milani and colleagues conducted a study among individuals hospitalized in Milan. About 1% of children and 9% of adults without any symptoms or signs of SARS-CoV-2 infection tested positive for SARS-CoV-2. The authors conclude that their data do not support the hypothesis that children are at higher risk of carrying SARS-CoV-2 asymptomatically than adults (Milani 2020). Attention: a retrospective analysis.


Luo L, Liu D, Liao X, et al. Contact Settings and Risk for Transmission in 3410 Close Contacts of Patients With COVID-19 in Guangzhou, China: A Prospective Cohort Study. Ann Intern Med. 2020 Aug 13. PubMed: Full-text:

Chen Mao and colleagues traced 3410 close contacts of 391 SARS-CoV-2 infected index cases between 13 January and 6 March 2020. 127 contacts (3.7%) were secondarily infected. Compared with the household setting (10.3%), the secondary attack rate was lower for exposures in healthcare settings (1.0%) and on public transportation (0.1%). Interestingly, although not unexpectedly, the secondary attack rate increased with the severity of index cases, from 0.3% for asymptomatic to 3.3% for mild, 5.6% for moderate, and 6.2% for severe or critical cases (Luo L 2020). Index cases with expectoration were associated with higher risk for secondary infection (13.6% vs. 3.0% for index cases without expectoration).


Xie W, Campbell S, Zhang W. Working memory capacity predicts individual differences in social-distancing compliance during the COVID-19 pandemic in the United States. Proc Natl Acad Sci U S A. 2020 Jul 10:202008868. PubMed: Full-text:

Among 850 US residents participating in a survey, the authors found that social distancing compliance could be predicted by individual differences in working memory (WM) capacity. WM retains a limited amount of information over a short period of time at the service of other ongoing mental activities. Its limited capacity constrains our mental functions, such that higher WM capacity is often associated with better cognitive and affective outcomes. Of note, the unique contribution of WM capacity to the individual differences in social distancing compliance could not be explained by other psychological and socioeconomic factors (e.g., moods, personality, education, and income levels). The message that the authors hide using scientific language can be said more clearly: if you see a guy sitting in the bus not wearing a mask: poor idiot, don’t get closer. His WM capacity is poor (Xie W 2020).


Rhee C, Kanjilal S, Baker M, et al. Duration of SARS-CoV-2 Infectivity: When is it Safe to Discontinue Isolation? Clinical Infectious Diseases, 25 August 2020, ciaa1249. Full-text:

Persistently positive RT-PCRs generally do not reflect replication-competent virus. SARS-CoV-2 infectivity rapidly decreases thereafter to near-zero after about 10 days in mild-to-moderately-ill patients and 15 days in severely-to-critically-ill and immunocompromised patients (Rhee 2020). This review summarizes evidence-to-date on the duration of infectivity of SARS-CoV-2.


Singanayagam A, Patel M, Charlett A. Duration of infectiousness and correlation with RT-PCR cycle threshold values in cases of COVID-19, England, January to May 2020. Euro Surveill. 2020;25(32). Full-text:

More on “viral load” and infectivity. Virus culture was attempted from 324 samples (from 253 cases) that tested positive for SARS-CoV-2 by RT-PCR. RT-PCR cycle threshold (Ct) values correlated strongly with cultivable virus. Probability of culturing virus declined to 8% in samples with Ct > 35 and to 6% (95% CI: 0.9–31.2%) 10 days after onset; it was similar in asymptomatic and symptomatic persons (Singanayagam 2020).


Lesho E, Reno L, Newhart D, et al. Temporal, Spatial, and Epidemiologic Relationships of SARS-CoV-2 Gene Cycle Thresholds: A Pragmatic Ambi-directional Observation. Clinical Infectious Diseases, 25 August 2020, ciaa1248. Full-text:

Same direction. This prospective serial sampling of 70 patients revealed clinically relevant cycle thresholds (Ct, “viral load”), namely a Ct of 24 (“high viral load”), 34, and > 40 (“negative”) that occurred 9, 26, and 36 days after symptom onset. Of note, race, gender, or corticosteroids did not appear to influence RNA-positivity. A retrospective analysis of 180 patients revealed that initial Ct did not correlate with requirement for admission or intensive care (Lesho 2020).


Lopez AS, Hill M, Antezano J, et al. Transmission Dynamics of COVID-19 Outbreaks Associated with Child Care Facilities — Salt Lake City, Utah, April–July 2020. MMWR Morb Mortal Wkly Rep. ePub: 11 September 2020. Full-text:

Cuc Tran, Adriana Lopez and colleagues describe 12 children who acquired SARS-CoV-2 infection in child-care facilities. All had mild or no symptoms. They transmitted the virus to at least 12 (26%) of 46 non-facility contacts (Lopez 2020). The authors conclude that testing children who might not have symptoms could improve control of transmission from child-care attendees to family members.


Schwartz NG, Moorman AC, Makaretz A, et al. Adolescent with COVID-19 as the Source of an Outbreak at a 3-Week Family Gathering — Four States, June–July 2020. MMWR Morb Mortal Wkly Rep. ePub: 5 October 2020. Full-text:

Children can serve as the source for COVID-19 outbreaks, even when their symptoms are mild (Schwartz 2020). In this outbreak that occurred during a 3-week family gathering of five households, an adolescent aged 13 years was the suspected primary patient. Among the 14 persons who stayed in the same house, 12 experienced symptoms. Of note, none of the additional six family members who maintained outdoor physical distance without face masks during two longer visits (10 and 3 hours) to the family gathering developed symptoms.


Lewis NM, Chu VT, Ye D, et  al. Household Transmission of SARS-CoV-2 in the United States. Clinical Infectious Diseases, 16 August 2020. Full-text:

Nathaniel M Lewis and colleagues sought to estimate the household secondary infection rate (SIR) of SARS-CoV-2 and evaluate potential risk factors for secondary infection among 58 households in Utah and Wisconsin. Fifty-two of 188 household contacts acquired secondary infections (SIR: 28%, 95% CI: 22–34%). Of note, household contacts to COVID-19 patients with immunocompromised conditions had increased odds of infection (OR: 15.9, 95% CI: 2.4–106.9) as well as household contacts who themselves had diabetes mellitus (OR: 7.1, 95% CI: 1.2–42.5) (Lewis 2020).


Wilson RF, Sharma AJ, Schluechtermann S, et al. Factors Influencing Risk for COVID-19 Exposure Among Young Adults Aged 18–23 Years — Winnebago County, Wisconsin, March–July 2020. MMWR Morb Mortal Wkly Rep. ePub: 9 October 2020. DOI:

Still in the US: Which are the drivers of behaviors that might influence risk for COVID-19 exposure among young adults? In a remote US County, these were low severity of disease outcome; peer pressure; and exposure to misinformation, conflicting messages, or opposing views regarding masks (Wilson 2020). A scientifically inspired national prevention policy would have been helpful.


Asadi S, Cappa CD, Barreda S, et al. Efficacy of masks and face coverings in controlling outward aerosol particle emission from expiratory activities. Sci Rep 10, 15665 (2020). Full-text:

Masks work with super-emittors! William D. Ristenpart, Sima Asadi and colleagues measured outward emissions of micron-scale aerosol particles by healthy humans performing various expiratory activities while wearing different types of medical-grade or homemade masks. Both surgical masks and unvented KN95 respirators reduced the outward particle emission rates by 90% and 74% on average during speaking and coughing. These masks similarly decreased the outward particle emission of a coughing super-emitter, who for unclear reasons emitted up to two orders of magnitude more expiratory particles via coughing than average (Asadi 2020). An interesting collateral finding: people speak more loudly, but do not cough more loudly, when wearing a mask.


Hendrix MJ, Walde C, Findley K, Trotman R. Absence of Apparent Transmission of SARS-CoV-2 from Two Stylists After Exposure at a Hair Salon with a Universal Face Covering Policy — Springfield, Missouri, May 2020. MMWR Morb Mortal Wkly Rep. 14 July 2020. Full-text:

Have we ever mentioned masks? Among 139 clients exposed to two symptomatic hair stylists with confirmed COVID-19 while both the stylists and the clients wore face masks, not a single symptomatic secondary case was observed; among 67 clients tested for SARS-CoV-2, all tests were negative (Hendrix 2020). At least one hair stylist was infectious: all four close household contacts (presumably without masks) became ill.


Wang X, Ferro EG, Zhou G, Hashimoto D, Bhatt DL. Association Between Universal Masking in a Health Care System and SARS-CoV-2 Positivity Among Health Care Workers. JAMA. 2020 Jul 14. PubMed: Full-text:


Again, universal masking: in March 2020, the Mass General Brigham, the largest health care system in Massachusetts (12 hospitals, > 75,000 employees), implemented universal masking of all HCWs and patients with surgical masks. During the preintervention period, the SARS-CoV-2 positivity rate increased exponentially, with a case doubling time of 3.6 days. During the intervention period, the positivity rate decreased linearly from 14.65% to 11.46%, with a weighted mean decline of 0.49% per day and a net slope change of 1.65% additional decline per day compared with the preintervention period (Wang X 2020).

Contejean A, Leporrier J, Canouï E, et al. Comparing dynamics and determinants of SARS-CoV-2 transmissions among health care workers of adult and pediatric settings in central Paris. Clin Infect Dis. 2020 Jul 15:ciaa977. PubMed: Full-text:

This prospective study compared a 1,500-bed adult and a 600-bed pediatric setting of a university hospital located in central Paris. From February 24th until April 10th, 2020, all symptomatic HCW were screened. Attack rates were of 3.2% and 2.3% in the adult and pediatric setting, respectively (p = 0.0022). In the adult setting, HCW more frequently reported exposure to COVID-19 patients without PPE (25% versus 15%, p = 0.046) (Contejean 2020). The total number of HCW cases peaked on March 23rd, then decreased slowly, concomitantly with a continuous increase in preventive measures (including universal medical masking and PPE). Residual transmissions were related to exposures with undiagnosed patients or colleagues but not to contacts with children attending out-of-home care facilities.


Brooks JT, Butler JC, Redfield RR. Universal Masking to Prevent SARS-CoV-2 Transmission—The Time Is Now. JAMA July 14, 2020. Full-text:

Data is clear now. First, public health officials need to ensure that the public understands clearly when and how to wear cloth face coverings properly. Second, innovation is needed to extend physical comfort and ease of use. Third, the public needs consistent, clear, and appealing messaging that normalizes community masking (Brooks 2020). According to the authors, broad adoption of cloth face coverings is a civic duty, a small adaption in our daily lives reliant on a highly effective low-tech solution that can help turn the tide.


Stewart CL, Thornblade LW, Diamond DJ, Fong Y, Melstrom LG. Personal Protective Equipment and COVID-19: A Review for Surgeons. Ann Surg. 2020 Aug;272(2):e132-e138. PubMed: Full-text:

Are you a surgeon? Then your particular medical association has been using personal protective equipment (PPE) for more than a century (Stewart 2020). This review addresses both the mechanism of SARS-CoV-2 transmission and the capabilities of PPE in the perioperative COVID-19 setting.


Bhaskar ME, Arun S. SARS-CoV-2 Infection Among Community Health Workers in India Before and After Use of Face Shields. JAMA August 17, 2020. Full-text:

This observational study describes transmission before and after the use of face shields (made of polyethylene terephthalate) in health workers in Chennai, India. Before the introduction of face shields, 12/62 workers were infected, while visiting 5,880 homes with 31,164 persons (222 positive for SARS-CoV-2). After the introduction, among 50 workers (previously uninfected) who continued to provide counseling, visiting 18,228 homes with 118,428 persons (2682 positive), no infection occurred (Bhaskar 2020).


Link-Gelles R, DellaGrotta AL, Molina C, et al. Limited Secondary Transmission of SARS-CoV-2 in Child Care Programs — Rhode Island, June 1–July 31, 2020. MMWR Morb Mortal Wkly Rep. ePub: 21 August 2020. Full-text:

Ruth Link-Gelles et al. report a possible secondary transmission in four of the 666 child-care programs in Rhode Island that were allowed to reopen. The apparent absence of secondary transmission within the other 662 child-care programs was likely the result of efforts to contain SARS-CoV-2 transmission, in particular maximum class sizes and use of face masks for adults (Link-Gelles 2020). The authors conclude that adherence to current CDC recommendations remains critical to reducing transmission in child-care settings, including wearing of masks by adults, limiting mixing between established student-teacher groups, staying home when ill, and cleaning and disinfecting frequently touched surfaces.


Simha PP, Rao PSM. Universal trends in human cough airflows at large distances featured. Physics of Fluids 32, 081905 (2020). Published 25 August. Full-text:

Fine droplets can pass through layers of masks and are carried away by the exhaled airflow unlike larger droplets that settle down due to gravity. Now Padmanabha Prasanna Simha and Prasanna Simha Mohan Rao visualize the flow fields of coughs under various mouth covering scenarios. The results:

  1. N95 masks are the most effective at reducing the horizontal spread of a cough (spread: 0.1 and 0.25 meters).
  2. A simple disposable mask can reduce the spread to 0.5 meters, while an uncovered cough can travel up to 3 meters.
  3. Coughing into the elbow? Not very effective! Unless covered by a sleeve, a bare arm cannot form the proper seal against the nose necessary to obstruct airflow and a cough is able to leak through any openings and propagate in many directions (Simha 2020).


Transmission setting

Gebrekidan S, Bennhold K, Apuzzo M, Kirkpatrick DD. Ski, Party, Seed a Pandemic: The Travel Rules That Let Covid-19 Take Flight. The New York Times 2020 published 1 October. Full-text:

ISCHGL, Austria — They came from across the world to ski in the most famous resorts of the Austrian alps… (Gebrekidan 2020).


Lednicky JA, Lauzardo M, Hugh Fan Z, et al. Viable SARS-CoV-2 in the air of a hospital room with COVID-19 patients. Int J Infect Dis. 2020 Sep 16:S1201-9712(20)30739-6. PubMed: Full-text:

John A. Lednicky and colleagues isolated viable virus from air samples collected 2 to 4.8 meters away from two COVID-19 patients (Lednicky 2020). The genome sequence of the SARS-CoV-2 strain isolated was identical to that isolated from the NP swab from the patient with an active infection. Estimates of viable viral concentrations ranged from 6 to 74 TCID50 units/L of air.


Freedman DO, Wilder-Smith A. In-flight Transmission of SARS-CoV-2: a review of the attack rates and available data on the efficacy of face masks. Journal of Travel Medicine September 25. Full-text:

Review of outbreaks during flights. According to the authors, the absence of large numbers of confirmed and published in-flight transmissions of SARS-CoV is encouraging but not definitive evidence that fliers are safe. At present, based on circumstantial data, strict use of masks appears to be protective (Freedman 2020). Structured prospective studies to quantitate transmission risk on flight with rigid masking protocols are now most pressing.


Khanh NC, Thai PQ, Quach H-L, Thi NA-H, Dinh PC, Duong TN, et al. Transmission of severe acute respiratory syndrome coronavirus 2 during long flight. Emerg Infect Dis 2020, published 18 September. Full-text:

The authors report a cluster of cases among passengers on VN54 (Vietnam Airlines), a 10-hour commercial flight from London to Hanoi on March 2, 2020. Among the 16 persons in whom SARS-CoV-2 infection was detected, 12 (75%) were passengers seated in business class along with the only symptomatic person (attack rate 62%) (Khanh 2020). The authors find that blocking middle seats, currently recommended by the airline industry, may in theory prevent some in-flight transmission events but seems to be insufficient to prevent superspreading events. They conclude that the risk for on-board transmission of SARS-CoV-2 during long flights is real and has the potential to cause COVID-19 clusters of substantial size, even in business class–like settings with spacious seating arrangements well beyond the established distance used to define close contact on airplanes.  (Note that at the time, March 2, the use of face masks was not mandatory on airplanes or at airports, and there was no social distancing on the aircraft.)


Chen J, He H, Cheng W, et al. Potential transmission of SARS-CoV-2 on a flight from Singapore to Hanghzou, China: An epidemiological investigation. J Trav Med 2020, Jul 6, 2020. Full-text:

Among 335 passengers on a flight from Singapore to Hangzhou in China (a Boeing 787, 5-hour flight, seat occupancy 89%), a total of 16 COVID-19 patients were diagnosed among all passengers, yielding an attack rate of 4.8%. However, after careful investigation, only one case was identified who appears to have become infected during the flight (Chen J 2020). He was seated near four infected passengers from Wuhan for approximately an hour (he had moved a seat) and did not wear his facemask correctly during the flight. The sources of infection in the other 15 passengers were complex and the passengers could have acquired their infections in Wuhan before the tour, or during the group tour before boarding.


Hoehl S, Karaca O, Kohmer M, et al. Assessment of SARS-CoV-2 Transmission on an International

Flight and Among a Tourist Group. JAMA Netw Open August 18, 2020, 3(8). Full-text:

Two likely SARS-CoV-2 transmissions on a 4.5-hour flight from Tel Aviv to Frankfurt, with 7 index cases. Both passengers were seated within two rows of an index case (Hoehl 2020). According to the authors, it could be speculated that the rate may have been reduced further had the passengers worn masks.


Plautz J. Is it safe to strike up the band in a time of coronavirus? Science, 17 July 2020. Full-text:

Is keeping 2 meters away enough to stay safe from a trumpet at full blast? Try it, find out! Introduce five student musicians – a soprano singer and clarinet, flute, French horn, and trumpet players — in a clean room one at a time and let them perform a short solo piece (Plautz 2020).


Hu M, Lin H, Wang J, et al. The risk of COVID-19 transmission in train passengers: an epidemiological and modelling study. Clin Infect Dis 2020, published 29 July. Full-text:

How risky is train traveling in the COVID-19 era? To answer this question, analyze passengers in Chinese high-speed trains. Jinfeng Wang and colleagues quantified the transmission risk using data from 2,334 index patients and 72,093 close contacts who had co-travel times of 0–8 hours from 19 December 2019 through 6 March 2020. Unsurprisingly, travelers adjacent to an index patient had the highest attack rate (3.5%) and the attack rate decreased with increasing distance, but increased with increasing co-travel time. The overall attack rate of passengers with close contact with index patients was 0.32% (Hu M 2020). The author’s conclusion: during COVID outbreaks, when travelling on public transportation in confined spaces such as trains, increase seat distance and reduce passenger density.


Shen Y, Li C, Dong H. Community Outbreak Investigation of SARS-CoV-2 Transmission Among Bus Riders in Eastern China. JAMA Intern Med, September 1, 2020. Full-text:

If you take the bus, choose seats near a window (and open it). On January 19, 2020, 68 individuals (including the source patient) took a bus on a 100-minute round trip to attend a worship event. In total, 24 (35%) received a diagnosis of COVID-19 after the event. The authors were able to identify seats for each passenger and divided bus seats into high-risk and low-risk zones (Shen Y 2020). Passengers in the high-risk zones had moderately but non-significantly higher risk of getting COVID-19 than those in the low-risk zones. On the 3-seat side of the bus, except for the passenger sitting next to the index patient, none of the passengers sitting in seats close to the bus window developed infection. In addition, the driver and passengers sitting close to the bus door also did not develop infection, and only 1 passenger sitting by an operable window developed infection. The absence of a significantly increased risk in the part of the bus closer to the index case suggested that airborne spread of the virus may at least partially explain the markedly high attack rate observed.

Luo K, Lei Z, Hai Z, et al. Transmission of SARS-CoV-2 in Public Transportation Vehicles: A Case Study in Hunan Province, China. Open Forum Infectious Diseases 13 September 2020, ofaa430. Full-text:

Transmission in a bus. The tour coach was 11.3 meters long and 2.5 meters wide with 49 seats, fully occupied with all windows closed and the ventilation system on during the 2.5-hour trip. Among the 49 passengers (including the driver) who shared the ride with the index person, eight tested positive and eight developed symptoms (Luo K 2020). The index person sat in the second-to-last row, and the infected passengers were distributed over the middle and rear rows.


Fisher KA, Tenforde MW, Feldstein LR, et al. Community and Close Contact Exposures Associated with COVID-19 Among Symptomatic Adults ≥18 Years in 11 Outpatient Health Care Facilities — United States, July 2020. MMWR Morb Mortal Wkly Rep 2020;69:1258–1264. Full-text:

Eating and drinking and socializing? Everything may well return to normal in about two years. In the meantime, note that adults with a positive SARS-CoV-2 test result were found to be twice as likely to have had dinner at a restaurant than those with negative test results (Fisher 2020). Kiva Fisher and colleagues conclude that eating and drinking on-site at locations that offer such options might be important risk factors associated with SARS-CoV-2 infection. Bars and restaurants are in for a rough autumn and winter season.


Riediker M, Tsai D. Estimation of Viral Aerosol Emissions From Simulated Individuals With Asymptomatic to Moderate Coronavirus Disease 2019. JAMA Netw Open 2020;3(7):e2013807. Full-text:

In this modeling study, Michael Riediker from the Swiss Centre for Occupational and Environmental Health in Winterthur and Dai-Hua Tsai from the University Hospital of Psychiatry in Zurich, Switzerland, it is estimated that viral load concentrations in a room with an individual who was coughing frequently were very high, with a maximum of 7.44 million copies/m3 from an individual who was a high emitter (Riediker 2020). However, regular breathing from an individual who was a high emitter was modeled to result in lower room concentrations of up to 1248 copies/m3. They conclude that the estimated infectious risk posed by a person with typical viral load who breathes normally was low and that only a few people with very high viral load posed an infection risk in the poorly ventilated closed environment simulated in this study.

In late March 2020, a large outbreak on the aircraft carrier USS Theodore Roosevelt was characterized by widespread transmission with relatively mild symptoms and asymptomatic infection among mostly young, healthy adults with close, congregate exposures. One fifth of infected participants reported no symptoms. Preventive measures, such as using face-coverings and observing social distancing, reduced risk for infection: among 382 service members, those who reported taking preventive measures had a lower infection rate than did those who did not report taking these measures (e.g., wearing a face-covering, 56% versus 81%; avoiding common areas, 54% versus 68%; and observing social distancing, 55% versus 70%, respectively) (Payne 2020).


Adam DC, Wu P, Wong JY, et al. Clustering and superspreading potential of SARS-CoV-2 infections in Hong Kong. Nat Med (2020). Full-text:

Dillon Adam, Peng Wu and colleagues identified 4–7 superspreading events (SSEs) across 51 clusters (n = 309 cases) and estimate that 19% (95% confidence interval, 15–24%) of cases seeded 80% of all local transmissions (Adam 2020). After controlling for age, transmission in social settings was associated with more secondary cases than households when controlling for age. Social settings are likely to become major battle grounds of coming SARS-CoV-2 waves.


Wang L, Didelot X, Yang J, et al. Inference of person-to-person transmission of COVID-19 reveals hidden super-spreading events during the early outbreak phase. Nat Commun 11, 5006 (2020). Full-text:

Super-spreading events are an important phenomenon in the transmission of many diseases (such as SARS-CoV-1, MERS-CoV, Ebola virus, etc.), in which certain individuals infect a disproportionately large number of people. Here Yuhai Bi, Liang Wang and colleagues show that super-spreading events played an important role in the early stage of the COVID-19 outbreak. They estimated the dispersion parameter to be 0.23 (95% CI: 0.13–0.39) (Wang L 2020). (What is the dispersion parameter? Check this FT article: To beat Covid-19, find today’s superspreading ‘Typhoid Marys’)


Tufekci Z. This Overlooked Variable Is the Key to the Pandemic. The Atlantic 2020, published 30 September. Full-text:

Even non-scientists have heard about R0 (pronounced as “r-naught”)—the basic reproductive number of a pathogen, a measure of its contagiousness on average. But even some scientists may have not yet encountered k, the measure of its dispersion. If you haven’t done it before, do it now: explore k. It’s simply a way of asking whether a virus spreads in a steady manner or in big bursts, whereby one person infects many, all at once (Tufekci 2020).

Updated: 22 October 2020