A good starting point is the measured efficacy of the vaccine, which for the Pfizer and Moderna vaccines, the only ones I’ll be talking about in this post, is ~95% (1/20th) two weeks after the second shot. This predicts that an interaction between two vaccinated people is ~0.25% (~1/400th) as likely to transmit covid as if they were both unvaccinated.
Efficacy at presenting symptoms is not the same thing as efficacy at preventing infection.
The FDA decided to allow vaccines to get approved without measuring infectiousness. That was a good decision because it allowed approving them faster (and not taking up important PCR capacity) but it comes at the cost of not knowing how protective it is against spreading the virus.
Given that nonsymptomatic spread is a thing and the way the underlying biology works it makes sense to expect less then 95% protection but significantly more then 0% protection for spreading the virus.
In recent news, the Oxford vaccine seems to have a 67% effect on transmission. Even if you expect the Biontech/Moderna vaccines to be better, eye-balling them for 75-80% would make more sense then 95%.
Also, all of these numbers are presenting efficacy in preventing the particular strains that were circulating in the times and places of the corresponding studies. I’d personally discount transmission prevention a bit further due to uncertainty about whether these numbers fully reflect the virus strains that are circulating in my local community — our only data for when new strains spread to more communities are lagging indicators, and the effectiveness numbers for some newer strains are lower or less certain than the numbers we’re using in this discussion. (But then I’m also quite far to the cautious end of the spectrum.)
Efficacy at presenting symptoms is not the same thing as efficacy at preventing infection.
The FDA decided to allow vaccines to get approved without measuring infectiousness. That was a good decision because it allowed approving them faster (and not taking up important PCR capacity) but it comes at the cost of not knowing how protective it is against spreading the virus.
Given that nonsymptomatic spread is a thing and the way the underlying biology works it makes sense to expect less then 95% protection but significantly more then 0% protection for spreading the virus.
In recent news, the Oxford vaccine seems to have a 67% effect on transmission. Even if you expect the Biontech/Moderna vaccines to be better, eye-balling them for 75-80% would make more sense then 95%.
Also, all of these numbers are presenting efficacy in preventing the particular strains that were circulating in the times and places of the corresponding studies. I’d personally discount transmission prevention a bit further due to uncertainty about whether these numbers fully reflect the virus strains that are circulating in my local community — our only data for when new strains spread to more communities are lagging indicators, and the effectiveness numbers for some newer strains are lower or less certain than the numbers we’re using in this discussion. (But then I’m also quite far to the cautious end of the spectrum.)