Thread by @VincentRK

Headlines about a study in @TheLancetInfDis stating that vaccinated people are just as likely to spread Covid as unvaccinated are misleading. Here's why.

1) Vaccination has lower risk of transmission even in this study (see table) & severe disease.

2) This study is primarily on household contacts. Household contacts live with each other. So the exposure is prolonged and sustained. So differences between vaccinated and unvaccinated groups will narrow in household settings.

3) What the study shows is that transmission from vaccinated people to household contacts can occur but at lower rate than unvaccinated. It also basically explains why the delta wave has occurred in even in some well vaccinated countries.

4) We knew from India's second wave that unlike the wild type variant once people had delta, in the household it spread to almost everybody. That is why India had the devastating crisis. So it is not surprising that delta variant can spread at a lower rate after vaccination.

5) Finally and most importantly this study is conditioned on a breakthrough infection after vaccination occurring. Vaccinated people are much less likely to get infection in the first place.

6) In this CDC funded study in the US, unvaccinated people who had prior COVID had 5 times greater risk of getting COVID compared to fully vaccinated people. This risk will be far higher if you compare to unvaccinated people who haven't had COVID. cdc.gov/mmwr/volumes/7…

7) Transmission depends on

a) how likely you are to get Covid; and b) if you get Covid, then how likely are you going to transmit.

8) Vaccinated people are far less likely to get Covid. That's the main variable.

9/ In the @TheLancetInfDis study, fully vaccinated individuals with delta variant infection had a faster drop in viral load than unvaccinated individuals.

10) Bottomline. @DrEricDing

https://twitter.com/drericding/status/1454154831673937924