A thread on vaccine efficacy and why you should get whichever vaccine you're able to access, without stressing about "70% efficacy" or "95% efficacy". 1/10

"Vaccine efficacy" in these phase 3 studies is roughly translated as "ability to prevent symptomatic COVID-19", an endpoint that was in part chosen to make the studies fast and efficient. However, it's probably not the most important endpoint to you, the vaccine recipient. 2/10

The reason is that "symptomatic" may just mean a sore throat that goes away. YOU probably care more about not getting seriously ill, being hospitalized, having long-hauler symptoms, etc. But those are difficult endpoints for a clinical trial. 3/10

Because those outcomes are less common, you have to enroll more volunteers to demonstrate a statistical benefit, so the studies would take longer, perhaps much longer. It's much easier to show a benefit in preventing *any* symptoms because "cold" type symptoms are common. 4/10

The other end of the spectrum would be trying to show a decrease in *any* infection, including asymptomatic infection. This is extremely useful from a public health viewpoint, but tricky because you have to get all volunteers tested periodically, adding to complexity. 5/10

Given these tradeoffs, the top-line number for "vaccine efficacy" you see basically represents how well you'll be protected from any symptoms, even mild ones. If you look at severe infections, the vaccines appear highly protective even though "vaccine efficacy" may be lower. 6/10

Because there are fewer severe infections overall, the degree of statistical certainty is lower, but it's likely to be true, and makes sense biologically. 7/10

I bring this up because from the JNJ results just published, some are proposing numbers in the 70-90% vaccine efficacy range, and a couple of people have said to me that they'd wait until they could get the "better" vaccines. 8/10

Consider though, that what you'd be waiting for might just be the difference between a bad case of COVID-19 and *any symptoms at all*. The relevant question is: would you wait an extra few months for that? 9/10

To bring it full circle: we'll know more about these vaccines as time goes on. Whatever public perceptions are of the quoted numbers, all the vaccines in play right now will likely be good enough to protect you. And you could always get a "better" one in the future. 10/end