It's one of the first indications from UK scientists that the Pfizer vaccine reduces the transmission of SARS-CoV-2, the virus that causes Covid-19, as well as protecting people from getting ill.
The study by Cambridge University Hospitals NHS Foundation Trust (CUH) and the University of Cambridge analysed data from thousands of Covid-19 tests carried out each week on hospital staff who showed no signs of infection.
The findings show a dramatic drop in the rate of positive tests amongst asymptomatic staff following vaccination.
Dr Michael Weekes led the study and is an honorary consultant in infectious diseases at CUH. He said: "Two weeks after a single dose of the Pfizer Covid-19 vaccine, hospital staff became four time less likely to become infected with Coronavirus."
Vaccination for health care workers on the CUH site began on 8 December 2020, with mass vaccination of staff from 8 January 2021.
During two weeks between 18 to 31 January this year, the study team screened similar numbers of vaccinated and unvaccinated staff using around 4,400 PCR tests per week.
The results were then separated out into staff who were unvaccinated, and staff who had been vaccinated for more than 12 days (when it's thought protection against infection occurs).
Analysis of the data suggests a four-fold decrease in the risk of asymptomatic Covid-19 infection amongst health care workers who have been vaccinated for more than 12 days (75 per cent protection). The level of asymptomatic infection was also reduced in those vaccinated for less than 12 days.
Dr Nick Jones, who is first author on the study and an infectious diseases/microbiology registrar at CUH, said: “Our findings show that the current mass vaccination strategy is protecting against asymptomatic carriage of the virus in addition to symptomatic disease.
"This is fantastic news for both staff and patients, making hospitals even safer places to be."
The study is yet to be peer-reviewed but has been published here.
This work was supported by Wellcome, the Medical Research Council, NHS Blood & Transplant, Addenbrooke’s Charitable Trust and the NIHR Cambridge Biomedical Research Centre.
Cambridge University Hospitals NHS Foundation Trust expressed thanks to all the staff from CUH who had been involved in setting up, delivering and taking part in the testing and vaccination programmes.