Does someone who has been vaccinated still need to wear a mask and take other precautions?

Infection and Spread Vaccines

A: Yes. We don’t yet know whether the vaccines prevent someone from being infectious.

The existing COVID-19 vaccine trials focused on a specific endpoint – symptomatic COVID-19 disease. We know that both the Pfizer and Moderna vaccines were close to 95% efficacious in preventing disease (YAY!!).
But wouldn’t we expect the vaccine to also stop transmission? Most experts think it’s highly likely that the vaccine will at least *reduce* infectiousness in vaccinated people who happen to encounter the infection, but we don’t know for sure or by how much. It could be that the virus still has time to replicate and shed before the vaccine-induced immunity kicks in and prevents progression to symptomatic illness. Unfortunately, we currently have no data on whether the vaccines reduce duration of infectiousness or how infectious individuals are, thus it remains possible that vaccinated people could be silent spreaders.

(Quick aside that vaccinated people CANNOT SHED VIRUS from the vaccine itself, as none of the vaccines contain live virus. This post only refers to the situation where they are exposed to the infection after vaccination.)


Oxford/Astrazeneca was the only trial thus far to explicitly include testing of asymptomatic infection in a portion of trial participants. This subset was sent swab testing kits each week to self-administer and send back to test for asymptomatic infection. Results were decidedly mixed. In the group that received the standard full doses, there were as many asymptomatic infections in the vaccine as control group, which would suggest *no protection* against asymptomatic infection. In the group with the low first dose, there were 59% fewer asymptomatic infections in the vaccine group, but this result was imprecise due to low numbers (95% CI (1% to 82.9%) and suggests the vaccine at best provides *imperfect* protection against asymptomatic infection.

Moderna did swab tests at the visit for the 2nd dose (so only measuring protection after 1 dose) and found a 67% lower rate of asymptomatic infection in the vaccine group. This analysis was preliminary, and more data are being collected. This definitely counts as *promising* evidence that the Moderna vaccine reduces the transmission even after the first dose, but not completely.


➡️ All 3 leading vaccine candidates reduce symptomatic cases and are likely to reduce transmission but not completely. We just don’t know yet, but more data should be on the way.

➡️ Since we don’t yet have data on vaccine protection against infectiousness and the vaccine isn’t 100% even against disease, those vaccinated should continue to take preventative measures to protect themselves and those around them.

➡️ So when will life get back to normal? I like to think of the next few months as a slow fade into normal rather than a dramatic flipping of a switch. As more people get vaccinated, even imperfect reductions in transmission will start to slow the spread of the virus by creating a firewall and dead end for the virus. The better the vaccines are at reducing transmission as opposed to just disease, the faster we should see this happen. This is that magical and ultimate goal of *vaccine-induced* herd immunity.

If in the worst case scenario the vaccines prevent disease but not infectiousness at all (which seems unlikely), this would still be a triumph because the vaccine would prevent most of the serious illnesses, hospitalizations, and deaths associated with COVID-19 in those who take up the vaccine. But this would mean that unvaccinated people would still be at risk of catching it from vaccinated people and the virus could continue circulating much longer.

So if you or a family member have been vaccinated, that is amazing news! Once past the second dose, they will have MUCH lower risk of symptomatic or severe COVID-19. But especially while there is SO much virus circulating around and most remain unvaccinated, there is a chance vaccinated people can still spread to susceptible people, so we must remain vigilant in our precautions.

When we start to turn the corner on these huge case numbers and get more data on vaccines and transmission, the Nerdy Girls will be the first to re-evaluate the risks and rejoice at a return to more normal life. That day IS coming!

And Happy Winter Solstice—the daylight only gets longer from today.

The Nerdy Girls

Here’s Why Vaccinated People Still Need to Wear A Mask

Does the Astrazeneca Vaccine Also Stop Covid Transmission?

Helpful twitter thread on current state of knowledge on vaccines and transmission

Link to original FB post