A few tweets on a topic that keeps coming up in discussion. There are many different types of vaccine efficacy - efficacy against infection, against transmission, against disease, and against severe disease - and these can vary for a single vaccine. How are they related? 1/5

Efficacy against infection will by necessity be lowest, because if a vaccine protects you from infection, it also protects you from transmitting to others and getting symptoms. We have a little data on this from Moderna and Oxford, but will get more from antibody testing. 2/5

Even if a vaccine does not prevent infection, it could make you less infectious by reducing viral load, reducing duration of infectiousness, or by preventing symptoms like coughing/sneezing. This effect is hard to measure without contact tracing or cluster randomized studies. 3/5

Even if a vaccine does not prevent infection, it can still prime your immune system so that you don't develop symptoms, particularly severe symptoms. In general, vaccines work best against severe disease. Though we have less data here, we see this trend for COVID vaccines. 4/5

So while we think about vaccines as having 70% efficacy of 95% efficacy, this almost always refers to the ability of the vaccine to prevent symptomatic disease of any severity. It will take time to generate reliable estimates of the other types of efficacy. 5/5