Question: I was wondering if there were any updates on the effectiveness of vaccines on preventing transmission.  If a breakthrough infection occurs, is a vaccinated person just as likely as an unvaccinated person to transmit the virus on to others?

Answer: In good news, evidence is accumulating that vaccinated people who experience breakthrough infections are much less likely than unvaccinated people to transmit the virus.  One of the best sources of data we currently have is from an observational study published by New England Journal of Medicine late last month, Effect of Vaccination on Household Transmission of SARS-CoV-2 in England, which found that the risk of household transmission was 40-50% lower in households where the index case had received at least one vaccination shot at least 21 days before having tested positive.  Study authors used individuals’ addresses to link England’s database of all lab-confirmed COVID cases with other data on vaccinations, and were thereby able to trace household transmission by index case vaccination status for hundreds of thousands of individuals.  Of course, vaccinated people already have a much lower risk of infection, so we’re talking very, very low risk of onward transmission (e.g. vaccinated person is unlikely to become infected and once infected, far less likely to transmit infection).  Unfortunately, we don’t have data specific to the Delta variant yet, and there are of course other limitations in the data we do have.  If you’d like to read more, this June 2021 report from Public Health Ontario offers a nice summation of current evidence. 

Why a lower risk of transmission? One of the big reasons we’d expect to see reduced transmission from breakthrough cases is that we’d expect lower viral load in breakthrough cases since the immune system is primed to detect and fight the SARS-CoV-2 virus before it has the opportunity to replicate.  As a reminder, hIgher viral loads are associated with increased disease severity and with onward transmission risk.   Data from a study in Israel and a study of nursing home residents in the US indicate that those who experience breakthrough infections (or infections after one dose of a two dose vaccination) have lower viral loads.