Single dose of Pfizer BioNTech vaccine reduces asymptomatic infections and potential for SARS-CoV-2 transmission

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 analysed results from thousands of COVID-19 tests carried out each week as part its screening programmes on hospital staff who showed no signs of infection.

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 found that 0·8% of tests from unvaccinated healthcare workers were positive, compared with 0.37% of tests from healthcare workers less than 12 days post-vaccination and 0·2% 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.

When the team included symptomatic healthcare workers, their analyses showed similar reductions. 1·71% unvaccinated healthcare workers tested positive, compared with 0·4% healthcare workers at 12 or more days post-vaccination.

This is an abridged version of the press release which was first published on our website on March 2, 2021.