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Pfizer’s Covid-19 vaccine leads to fourfold decrease in asymptomatic infection, Cambridge University Hospitals study shows

A single dose of the Covid-19 vaccine from Pfizer/BioNTech Covid can lead to a fourfold reduction in the number of asymptomatic infections, data from Addenbrooke’s Hospital suggests.

The study by the team at Cambridge University Hospitals NHS Foundation Trust (CUH) and the University of Cambridge analysed the results from thousands of Covid-19 tests carried out each week under its screening programme for hospital staff with no signs of infection.

A Covid-19 vaccine
A Covid-19 vaccine

CUH began vaccinating staff on December 8, with mass vaccination beginning on January 8.

Between January 18 and 31, a team screened similar numbers of vaccinated and unvaccinated staff, using about 4,400 PCR tests per week.

Analysis of the results showed 26 out of 3,252 (0.8 per cent) of unvaccinated healthcare workers had tested positive for the SARS-CoV-2 virus.

Thirteen out of 3,535 (0.37 per cent) of tests were found to be positive among healthcare workers who had received the jab within the last 12 days - meaning the protection it confers against symptomatic infection would not yet have fully kicked in for them.

Only four out of 1,989 (0.2 per cent) of tests from health workers who had received the jab 12 days or more earlier came back positive.

This suggests a fourfold decrease in the risk of asymptomatic Covid-19 infection 12 days or more past the first jab, and implies 75 per cent protection.

The level of asymptomatic infection was also halved in those vaccinated for fewer than 12 days.

Dr Mike Weekes, an infectious disease specialist at CUH and the University of Cambridge’s Department of Medicine, who led the study, said: “This is great news – the Pfizer vaccine not only provides protection against becoming ill from SARS-CoV-2 but also helps prevent infection, reducing the potential for the virus to be passed on to others.

“This will be welcome news as we begin to plot a roadmap out of the lockdown, but we have to remember that the vaccine doesn’t give complete protection for everyone. We still need social distancing, masks, hand hygiene and regular testing until the pandemic is under much better control.”

Dr Nick Jones, first author on the study and an infectious diseases/microbiology registrar at CUH, said: “Our findings show a dramatic reduction in the rate of positive screening tests among asymptomatic healthcare workers after a single dose of the Pfizer-BioNTech vaccine.

“This is fantastic news for both hospital staff and patients, who can be reassured that the current mass vaccination strategy is protecting against asymptomatic carriage of the virus in addition to symptomatic disease, thereby making hospitals even safer places to be.”

Among symptomatic healthcare workers, there were similar reductions.

How the Pfizer/BioNTech vaccine works. Graphic: PA
How the Pfizer/BioNTech vaccine works. Graphic: PA

While 56 out of 3,282 (1.71 per cent) of unvaccinated healthcare workers tested positive, only eight out of 1,997 (0.4 per cent) of healthcare workers who had received their first jab 12 or more days earlier tested positive - a 4.3-fold reduction.

Giles Wright, programme director for the CUH vaccination hub said: “Throughout the pandemic so far, we have taken a systematic approach to keeping our staff safe and well. The huge efforts of all those involved in the testing, tracing and vaccination programmes at CUH are making the plan a reality. We are very encouraged by the findings of our research. It gives further hope for the near future.”

The data has yet to be peer-reviewed, but has been released due to the urgency of sharing information on the pandemic.

The research was supported by Wellcome, the Medical Research Council, NHS Blood & Transplant, Addenbrooke’s Charitable Trust and the NIHR Cambridge Biomedical Research Centre.

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