Thread by @sailorrooscout

Ready for some wonderful news? Here we go! A recent study has found that a single dose of Pfizer’s vaccine can reduce the number of asymptomatic SARS-CoV-2 infections by 75%. This demonstrates these vaccines can greatly reduce the risk of transmission. 🧵
authorea.com/users/332778/a…

Cambridge University Hospitals NHS Foundation Trust and researchers at the University of Cambridge analyzed their results from thousands of COVID-19 tests carried out each week as part of their screening program on asymptomatic HCWs who showed no signs of infection.

Their vaccination program for HCWs began on December 8th of last year, with mass vaccination efforts continuing til January 8th of this year. During a two-week period between the 18th and 31st of January, researchers screened similar numbers of vaccinated and unvaccinated staff.

They averaged around 4,400 PCR tests per week and their results were then separated out to identify unvaccinated staff, and staff who had been vaccinated >12 days prior to testing (when protection against symptomatic infection is thought to occur).

Researchers found that 26 out of 3,252 (0.80%) tests from unvaccinated healthcare workers were positive. They compared this to the 13 out of 3,535 (0.37%) tests from healthcare workers <12 days post-vaccination and 4 out of 1,989 (0.20%) tests from healthcare workers at

12 days or more post-vaccination. Their data suggests a four-fold decrease in the risk of asymptomatic COVID-19 infection amongst HCWs who had been vaccinated for more than 12 days (that’s 75% protection!). The level of asymptomatic infection was also halved in individuals who

had been vaccinated for <12 days. When researchers included symptomatic HCWs, their analysis show similar reductions: 56 out of 3,282 (1.71%) unvaccinated HCWs tested positive. Compared to 8 out of 1,997 (0.40%) HCWs at 12 or more days post-vaccination, a 4.3-fold reduction.

Proportion of HCWs testing positive for SARS-CoV-2 in the CUHNHFT asymptomatic screening program (grey bars; Week 1, 1/18/2021-1/24/2021; Week 2, 1/25/2021-1/31/2021) and Ct values of positive tests (CT<36; blue dots; both weeks).

This provides real-world evidence for a high level of protection against asymptomatic SARS-CoV-2 infection after a single dose of Pfizer’s vaccine, especially at a time of predominant transmission of variant B.1.1.7. This is extremely good news I expect to see in other vaccines!

Wanted to boost this study again! I don’t think it’s getting nearly the attention it deserves.