Single dose of Pfizer vaccine ‘can reduce asymptomatic infection by four-fold’

Single Dose Of Pfizer Vaccine ‘Can Reduce Asymptomatic Infection By Four-Fold’ Single Dose Of Pfizer Vaccine ‘Can Reduce Asymptomatic Infection By Four-Fold’
The BioNTech/Pfizer Covid-19 vaccine is prepared (Nick Potts/PA), © PA Wire/PA Images
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By Thomas Hornall, PA

A single jab of the Pfizer/BioNTech vaccine can reduce by four-fold the number of asymptomatic coronavirus infections, a new study has indicated.

The new data, from Addenbrooke’s Hospital in Cambridge, suggests the “dramatic reduction” in positive tests among asymptomatic healthcare workers who had received one dose of the vaccine can make “hospitals even safer places to be”.

The study, which is awaiting peer review, analysed results from thousands of Covid-19 tests carried out weekly on hospital staff displaying no coronavirus symptoms.

It was led by a team at Cambridge University Hospitals NHS Foundation Trust (CUH) and the University of Cambridge.

A University of Cambridge statement said: “During a two-week period between January 18 and 31 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).”


Results showed that 26 out of 3,252 (0.80 per cent) tests from unvaccinated healthcare workers were positive.

This compared to 13 out of 3,535 (0.37 per cent) tests from healthcare workers who had their vaccines less than 12 days beforehand and four out of 1,989 (0.20 per cent) tests from staff at 12 days or more post-vaccination, the university said.

A statement added: “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 and 75 per cent protection.

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

Vials of Pfizer-BioNTech coronavirus vaccine (Nick Potts/PA)

Dr Nick Jones, first author on the study and an infectious diseases expert, 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.”


Dr Mike Weekes, an infectious disease specialist at CUH, who led the study, said the findings were “great news”.

He said: “This will be welcome news as we begin to plot a road map 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.”

Researchers release the figures ahead of peer review because of “the urgent need to share information relating to the pandemic”, a statement added.

Earlier this week, real world data from Public Health England (PHE) showed that a single shot of the Pfizer vaccine cuts the chance of hospital admission and death from Covid-19 by more than 75 per cent among the over-80s.

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