Thread by @DearPandemic

1/ Q: Does someone who has been #vaccinated still need to wear a mask & take other precautions?

A: Yes. We don’t yet know whether the vaccines prevent someone from being infectious.
nytimes.com/.../health/cov…

2/ The existing #Covid_19 #vaccine trials focused on a specific endpoint – symptomatic COVID-19 disease. We know that both the @pfizer & @moderna_tx vaccines were close to 95% efficacious in preventing disease (YAY!!).

3/ 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.

4/ It could be that the virus still has time to replicate & shed before the vaccine-induced immunity kicks in & prevents progression to symptomatic illness. Unfortunately, we currently have no data on whether the vaccines reduce duration of infectiousness.

5/ (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.)

6/ ➡️ WHAT DO WE KNOW?
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 & send back to test for asymptomatic infection.

7/ 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.

8/ 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%) & suggests the vaccine at best provides *imperfect* protection against asymptomatic infection.

9/ @moderna_tx did swab tests at the visit for the 2nd dose (so only measuring protection after 1 dose) & found a 67% lower rate of asymptomatic infection in the vaccine group. This analysis was preliminary, & more data are being collected. This is definitely *promising* evidence

10/ BOTTOM LINE:

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

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

12/ ➡️ 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.

13/ The better the vaccines are at reducing transmission as opposed to just disease, the faster we should see this happen. This is that magical & ultimate goal of *vaccine-induced* herd immunity.

14/ Sorry that link in first tweet is broken; see great explainer by @apoorva_nyc: nytimes.com/2020/12/08/wor…

Also see nice summary in @WIRED: wired.com/story/does-the…

& this very helpful thread by @DiseaseEcology:

https://twitter.com/DiseaseEcology/status/1339093209796890625/