The growing evidence that COVID vaccines reduce transmission, explained

In their initial clinical trials, Moderna and Pfizer didn’t study whether vaccinated people got asymptomatic cases of Covid-19 — that is, people who tested positive for the coronavirus but did not suffer any symptoms. However, when people went in for their second shot, Moderna did give them a nasal swab test for Covid-19. In a supplement to its submission to the FDA, Moderna says that 14 of the 14,134 vaccinated people had Covid-19 (with no symptoms at the time) and 38 of the 14,073 people in the control group had Covid-19 (with no symptoms at the time).

That rules out one big worry about the vaccines: that they might make Covid-19 mild in vaccinated people — so mild they don’t experience any symptoms — without actually preventing it. Instead, it was clear from back in December that the vaccines reduce asymptomatic infection as well as reducing symptomatic infection.

Using Moderna’s nasal swab test data, infectious disease biologist Marm Kilpatrick at UCSC estimated that the vaccine, after a single shot, reduces a person’s odds of infection with Covid-19 by up to 90 percent. (When I emailed him, we determined that with some more pessimistic assumptions, the reduction might be more like 78 to 88 percent.) Of course, the overall efficacy of the vaccine after both doses will almost certainly be higher.

The new data on the Pfizer/BioNTech vaccine in Israel and in the UK backs up that finding. It suggests that after two shots, the vaccine is 85 to 90 percent effective at preventing infection with Covid-19.