We are seeing a growing body of evidence to support the view that the coronavirus vaccines reduce both symptomatic and asymptomatic infection in those vaccinated.  This is important since it extends the level of protection from those vaccinated to those unvaccinated as a result of reduced transmission. Public health pandemic modelling is very sensitive to different assumptions around the transmission rate so quantification of the effect is key and the Cambridge Health Care Workers’ (HCW) data notably point to a large impact on infection prevalence 12 days after vaccination with the Pfizer-BioNTech vaccine.  This reduction in infection prevalence is around 75% in a group of HCW who had a low frequency of prior infection (7.2% antibody positive).  As with all observational data, there is some potential for bias in the comparison since individuals were not randomised to vaccination, so some confounding with other causes is a possibility.  Nonetheless, these results are extremely encouraging and point to a large reduction in asymptomatic infection due to vaccination with a likely correspondingly large reduction in transmission.  The implication for public health policy is that as vaccination continues at pace, the options to soften the lockdown measures increase.

This excellent study analysed COVID-19 tests and disease as part a screening programme in Cambridge healthcare workers (HCWs). In this group, vaccination began on 8 December 2020, scaled up from 8 January 2021. Staff were screened 18-31 Jan using around 4,400 PCR tests/week.

The study found asymptomatic positive tests in 26/3,252 (0·80%) tests from unvaccinated HCWs, 13/3,535 (0.37%) tests from HCWs <12 4="" days="" post-vaccine="" and="" 1,989="" (0·20%)="" tests="" from="" hcws="">12 days post-vaccine. This indicates that vaccination reduces asymptomatic SARS-CoV-2 infections four-fold, from 1:125 to 1:500.</12>

With respect to symptomatic disease in HCWs, they found 56/3,282 (1·71%) unvaccinated HCWs, 8/1,997 (0·40%) HCWs at >12 days post-vaccination. This suggests a 4·3-fold reduction in COVID, from 1:58 to 1:250.

Notably, one-third of infections were asymptomatic regardless of vaccination status.

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The data included are limited, and this is a short pre-print, not a full and peer-reviewed paper; but it appears to a solid and reliable study.

The number of cases and infections was relatively small, so the confidence intervals may be large. As we get more data, we will become increasingly certain of our findings. It may already be possible to combine the information from this paper with, for example, the data from the SIREN study published earlier this week – certainly the data seem reassuringly consistent.[2-4]

These data are based on a population – healthcare workers – who are tested routinely and regularly. This means that the data on reduction of asymptomatic infection is much more robust than it would otherwise be.

These preliminary data are insufficient in themselves for us to be confident that vaccination will prevent infection and transmission – and thus could contribute to herd immunity; but they are strongly suggestive that this might be the case, giving us cause for hope that vaccination might indeed – eventually – reduce or even eliminate the need for social distancing, mask-wearing, and other behavioural control measures. In the meantime, such measures remain essential.

These data apply only to the Pfizer-BioNTech vaccine. This is because we started to use that vaccine sooner, so we have sufficient data to be able to comment on its efficacy. We should not assume from this that Pfizer-BioNTech is any better or worse than other vaccines. The pre-authorisation studies all used different protocols, in different populations, at different times, when the range of variants circulating was different, so we cannot compare vaccine efficacy based on those studies. The data from Scotland earlier this week suggest, for example, that the Astra-Zeneca vaccine is at least as effective at preventing hospital admission.[5,6]

1. Weekes M, Jones NK, Rivett L, Workman C, Ferris M, Shaw A, et al. Single-dose BNT162b2 vaccine protects against asymptomatic SARS-CoV-2 infection [pre-print]. Authorea 2021; Updated 24 Feb 2021; Accessed: 2021 (26 Feb): (https://www.authorea.com/users/332778/articles/509881-single-dose-bnt162b2-vaccine-protects-against-asymptomatic-sars-cov-2-infection).
2. English PMB. Expert reaction to new reports from PHE on Pfizer-BioNTech vaccine effectiveness data and the SIREN study. Science Media Centre 2021; Updated 22 Feb 2021; Accessed: 2021 (22 Feb): (https://www.sciencemediacentre.org/expert-reaction-to-new-reports-from-phe-on-pfizer-biontech-vaccine-effectiveness-data-and-the-siren-study/).
3. Public Health England. First real-world UK data show Pfizer-BioNTech vaccine provides high levels of protection against COVID-19 from the first dose. London: Public Health England, 2021 (22 Feb); 1-3.
4. Ramsay M. COVID-19: analysing first vaccine effectiveness in the UK. Public Health England Blogs 2021; Updated 23 Feb 2021; Accessed: 2021 (23 Feb):

(https://publichealthmatters.blog.gov.uk/2021/02/23/covid-19-analysing-first-vaccine-effectiveness-in-the-uk/).

5. Public Health Scotland. Vaccine linked to reduction in risk of COVID-19 admissions to hospitals. Press release 2021; Updated 22 Feb 2021; Accessed: 2021 (22 Feb).
6. Bird S, Akbar A, Gilbert S, Lambe T, Pollard A, Finn A, et al. Expert reaction to preprint from Scotland looking at hospitalisations in Scotland and vaccinations. Science Media Centre 2021; Updated 22 Feb 2021; Accessed: 2021 (26 Feb): (https://www.sciencemediacentre.org/expert-reaction-to-preprint-from-scotland-looking-at-hospitalisations-in-scotland-and-vaccinations/).

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This study focussing on asymptomatic COVID-19 infection in healthcare workers demonstrates clearly the effectiveness of the vaccine in preventing infection, in addition to the known effect it has on protecting against symptomatic and severe disease.  This means that vaccination will lead to a substantial reduction in transmission of the virus.  This is especially important within hospital settings where staff have, in the past, inadvertently passed on infection to vulnerable patients.

More broadly, it emphasises the importance of everyone taking up the offer of a vaccine in order to protect both themselves and others in the community.

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Confirmation that a single dose of the Pfizer/BioNTech vaccine is very likely to protect from SARS-CoV-2 transmission. This study in healthcare workers from Cambridge shows that one dose of the vaccine is 75% protective against asymptomatic infection at 12 days post-vaccination. Earlier this week a similar studies from PHE and from Israel also showed high levels of protection from asymptomatic infection particularly after the second dose of the Pfizer/BioNTech vaccine – in the PHE study the first vaccine dose reduced the risk of infection by more than 70% and this was increased to 85% after the second dose. These studies are very encouraging because they suggest that the vaccines will prevent the spread of the virus. You can’t spread the virus if you’re not infected and these studies show that the vaccine blocks infection in individuals who don’t have symptoms but could pass on the infection.

To see such a reduction in infection rates after a single dose of the Pfizer vaccine is very impressive and shows that vaccination truly does offer a way out of the current restrictions and a much brighter future.

It will be important to understand whether the reduced risk of infection played out across all the exposure risk groups included in the study, but nonetheless, this is still excellent news.

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This piece of research corroborates recent findings of Public Health England’s SIREN study1. In working age adults, the Pfizer-BioNTech vaccine has good efficacy for preventing asymptomatic infection by the Covid-19 coronavirus.  This is really important if we are to decrease the amount of spread from people who are unaware that they are infected.  While this is very encouraging, no data were provided to show how long the effect will last for and continued surveillance is required in case this protection dwindles.

While this research is encouraging, like the SIREN findings, it relies on studying NHS staff and there appears to have been no assessment made of their pre-existing immune status, which might have been higher than in the general population given healthcare workers’ likelihood of exposure at work.  Pre-exposure to the coronavirus before receiving a dose seems to cause a more potent immune response to the vaccine.

It’s also important to remember that these findings cannot automatically be extended to every other vaccines.  According to the Oxford Vaccine group, the Oxford/AstraZeneca Covid-19 vaccine “did not provide protection against asymptomatic infection” in their trials2.

1: https://papers.ssrn.com/sol3/papers.cfm?abstract_id=3790399

2: https://papers.ssrn.com/sol3/papers.cfm?abstract_id=3777268

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