Vaccines 22 May

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The Highlights

Post-exposure SARS-CoV-2 vaccination

Muller CP. Can integrated post-exposure vaccination against SARS-COV2 mitigate severe disease? Lancet Regional Health 2021, published 17 May. Full text:

Chances might be good that post-exposure vaccination with SARS-CoV-2 vaccines could mitigate COVID-19 disease. Claude Muller of the Luxembourg Institute of Health recently pointed out that there is enough time for protective vaccine effects to set in (Muller 2021):

  1. The time from SARS-CoV-2 infection to hospitalization is around two weeks:
    • Incubation time of SARS-CoV-2 infection: 5 days (Elias 2021)
    • Time from symptom onset to hospitalization: around 7 to 10 days
  2. Partial protection from mRNA vaccines has been shown as early as two weeks after the first vaccine dose (Polack 2020, Dagan 2021)

Individuals with a long incubation period would benefit most from post-exposure vaccination. While a large randomized control trial would need to demonstrate the efficacy of this approach, post-exposure SARS-CoV-2 vaccination would cause no harm and could only benefit the vaccine recipients (Muller 2021). Post-exposure vaccination is not new – protection is quite high in a number of infectious diseases (hepatitis A, 85%; hepatitis B, 85%; measles, 83%; varicella, 67%; smallpox, 45%; and mumps, 38%) (Gallagher 2019).



Low JS, Vaqueirinho D, Mele F, et al. Clonal analysis of immunodominance and cross-reactivity of the CD4 T cell response to SARS-CoV-2. Science 2021, published 18 May. Full text:

One paper, three highlights, and the prospect of a pan-coronavirus RBD subunit vaccine (Low 2021):

  • COVID-19-recovered individuals have a robust CD4+ T cell response to naturally processed SARS-CoV-2 spike and nucleoprotein, including effector (TEM), helper (cTfh), and memory (TCM) T cells.
  • A characterization of almost 3,000 S-reactive T cell clones from 34 COVID-19 individuals revealed an immunodominant S346-365 region within the receptor-binding domain (RBD) that is highly conserved among zoonotic and human sarbecoviruses, including SARS-CoV-2 and its variants of concern. The S346-365 region was recognized by 94% of individuals and by 33% of the clones.
  • An analysis of both pre- and post-COVID-19 samples from one individual showed that T cell clonotypes against the highly conserved fusion peptide could be tracked back to a sample of 2014 and was found to be expanded in the post-COVID-19 sample, thus providing evidence for the recall of preexisting cross-reactive memory T cells upon SARS-CoV-2 infection.


19 May

Protection of the unvaccinated

White EM, Yang X, Blackman C, Feifer RA, Gravenstein S, Mor V. Incident SARS-CoV-2 Infection among mRNA-Vaccinated and Unvaccinated Nursing Home Residents. N Engl J Med 2021, published 19 May. Full text:

If you vaccinate 82% of a vulnerable nursing home population – and continue to use face masks and other infection-control measures – the remaining 18% of unvaccinated residents may be highly protected. This is the result of a study of 22,232 residents of 280 nursing homes across 21 US states, 18,242 (82%) of whom received at least one dose of mRNA vaccine (80.4% BioNTech/Pfizer, 19.6% Moderna) and 13,048 (71.5%) also received the second dose (White 2021). Most infections were asymptomatic, both in vaccinated and unvaccinated residents.


Table xxx. Incident SARS-CoV-2 infection among 3990 unvaccinated nursing home residents
Total Asymptomatic SARS-CoV-2 infection Symptomatic SARS-CoV-2 infection Percent of infected residents who were asymptomatic
Positive test
at 0-14 days* 173 (4.3%) 115 (2.9%) 58 (1.5%) 66.5
at 15-28 days* 69 (1.7%) 42 (1.1%) 27 (0.7%) 60.9
at 29-42 days* 16 (0.4%) 13 (0.3%) 3 (0.1%) 81.2
at >42 days 12 (0.3%) 10 (0.3%) 2 (0.1%) 83.3

* After first vaccination at the nursing home


18 May

BioNTech/Pfizer vaccine after AZ

ISCIII 20210518. El uso combinado de las vacunas de AstraZeneca y Pfizer contra el SARS-CoV-2 ofrece una potente respuesta inmunitaria. Instituto de Salud Carlos III 2021, published 18 May. Full text:

Giving a booster dose of the BioNTech/Pfizer vaccine to people who already received a first injection of the AstraZeneca vaccine is safe and highly effective. This is the result of the Spanish CombivacS study which enrolled 673 volunteers who had received a first dose of the AstraZeneca vaccine (ISCIII 20210518). After 8 to 12 weeks, 441 individuals received the BioNTech/Pfizer vaccine for their second dose (the BioNTech/Pfizer group) and 232 a second AstraZeneca injection (the AstraZeneca group). In the BioNTech/Pfizer group, the neutralizing antibody titers rose seven-fold, as compared with three-fold in the AstraZeneca group. Less than 2% of study participants reported severe side effects, mostly headaches, general malaise and muscle pain. In the future, such a vaccination strategy, also known as ‘heterologous prime and boost’, may simplify vaccination campaigns in countries with fluctuating vaccine supplies.


16 May

Bailly B, Guilpain L, Bouiller K, et al. BNT162b2 mRNA vaccination did not prevent an outbreak of SARS COV-2 variant 501Y.V2 in an elderly nursing home but reduced transmission and disease severity. Clin Infect Dis 2021, published 16 May. Full text:

The BioNTech/Pfizer vaccine did not prevent an outbreak of the B.1.351 variant (first detected in South Africa) in a French nursing home, but it reduced transmission: all unvaccinated residents (5/5), but only half of the vaccinated residents (13/26) were infected (Bailly 2021). The SARS CoV-2 viral load was significantly higher in non-vaccinated residents (mean cycle threshold (Ct) value: 15, range 12-17) than in vaccinated residents (mean Ct: 21, range: 13-32). The vaccine also reduced disease severity. Among the vaccinated residents who were infected, 2 (15.4%) were asymptomatic and 9 (69.2%) had mild to moderate disease; two individuals (15.4%) had severe disease and died. Among the 5 non-vaccinated residents, 4 progressed to severe disease; one of them died.


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