A study by Clalit, Israel's largest HMO, shows a 33% drop in detectable Sars-CoV-2 virus by PCR in over-60s 14 days after their first dose of the Pfizer/BioNTech vaccine. Study based on 200k virus tests.

Best I can tell from media reporting, this is a retrospective analysis of the PCR tests. Israelis require these tests when exposed to a Covid case. Most tests are given due to known or suspected exposure to a Covid-19 case, though some are due to suspect symptoms.

From days 5-12 after the first shot, there is no difference between vaccinated and unvaccinated people in test positivity. Day 13 a slight drop was observed. From day 14 onwards, a 33% decrease was seen with no further dropping.

This is, I believe, the largest data set so far about whether coronavirus vaccines also protect against transmission. It suggests that at *minimum*, they cut transmission by 33%, but there's reason to hope it's more.

1. This analysis didn't look at *how much* virus people who tested positive were emitting in each group, and didn't try to see if the virus could replicate. Maybe vaccinated people had lower viral loads or inactive virus.