Can a vaccinated person spread Sars-CoV-2?

What vaccines can and can't do

We’ve heard a lot of good news regarding vaccines lately: it seems that we have at least two vaccines, and potentially more on the way, that might lead us towards the end of this pandemic. Both Pfizer and Moderna vaccines demonstrate greater than 90% efficacy, and this is huge. 

As game-changing as the recent efficacy data suggests that the Pfizer and Moderna vaccines are, it’s important to note what “90% efficacy” means. In Pfizer and Moderna’s vaccine trials, vaccine efficacy was measured as the ability of the vaccine to prevent COVID-19, the illness caused by SARS-CoV-2. These trials did not directly measure the ability of a vaccine to prevent infection in the absence of illness—namely, asymptomatic infections. This means that infection—and therefore transmission to others—might still be possible if you’ve been vaccinated.

In order to completely block infection, vaccines need to exhibit what we call “sterilizing immunity.” Sterilizing immunity is actually pretty rare, and it usually isn’t a huge concern for vaccine developers—vaccines are typically designed to prevent disease, and transmission is less of a concern when there are high levels of vaccination and low levels of disease prevalence.

In the current pandemic, though, we care a lot about whether vaccines prevent someone from becoming infectious. So, sterilizing immunity would be the best-case scenario. In order for this to be achieved, a vaccine would need to trigger a specific immunologic response that basically kills the virus before it can spread at all. 

There is some data to support the efficacy of the current vaccines against asymptomatic infections, and some researchers think that vaccine candidates in the pipeline show promise for achieving sterilizing immunity. However, other experts think that achieving sterilizing immunity with a vaccine won’t be possible for COVID-19. At the very least, it’s likely that people vaccinated for COVID-19 will clear the virus more quickly and have a smaller viral load if they do become infected. 

The bottom line: We know that the vaccine reduces symptomatic infection, and this should reduce the overall prevalence of COVID-19 death. The vaccine will be vital to ending the pandemic, but until large enough sectors of the population are vaccinated and we have more data, it’s important that we don’t let our guard when it comes to social distancing and face masks any time soon.  

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