Single dose of Pfizer BioNTech vaccine can reduce asymptomatic SARS-CoV-2 infections

New data from Addenbrooke's Hospital in Cambridge suggests that a single dose of the Pfizer BioNTech vaccine can reduce by four-fold the number of asymptomatic SARS-CoV-2 infections.

This implies that the vaccine could significantly reduce the risk of transmission of the virus from people who are asymptomatic, as well as protecting others from getting ill.

The study by a team at Cambridge University Hospitals NHS Foundation Trust (CUH) and the University of Cambridge analyzed results from thousands of COVID-19 tests carried out each week as part of its screening programs on hospital staff who showed no signs of infection.

Vaccination for health care workers on the CUH site began on 8 December 2020, with mass vaccination from 8 January 2021. During a two-week period between 18 and 31 January 2021, the team screened similar numbers of vaccinated and unvaccinated staff using around 4,400 PCR tests per week.

The results were then separated out to identify unvaccinated staff, and staff who had been vaccinated more than 12 days prior to testing (when protection against symptomatic infection is thought to occur).

The study, which is yet to be peer-reviewed, found that 26 out of 3,252 (0·80%) tests from unvaccinated healthcare workers were positive.

This compared to 13 out of 3,535 (0.37%) tests from healthcare workers less than 12 days post-vaccination and 4 out of 1,989 (0·20%) tests from healthcare workers at 12 days or more post-vaccination.

This suggests a four-fold decrease in the risk of asymptomatic COVID-19 infection amongst healthcare workers who have been vaccinated for more than 12 days (75 percent protection). The level of asymptomatic infection was also halved in those vaccinated for less 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."

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."

Dr Nick Jones, Study First Author and Infectious Diseases/Microbiology Registrar, Cambridge University Hospitals

Giles Wright, program director for the CUH Vaccination Hub said: "Throughout the pandemic so far, we have taken a systematic approach to keep our staff safe and well. The huge efforts of all those involved in the testing, tracing, and vaccination programs 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."

When the team included symptomatic healthcare workers, their analyses showed similar reductions. 56 out of 3,282 (1·71%) unvaccinated healthcare workers tested positive. This compared to 8 out of 1,997 (0·40%) healthcare workers at 12 or more days post-vaccination, a 4·3-fold reduction.

The researchers have released their data ahead of peer review because of the urgent need to share information relating to the pandemic.