I sometimes see people making the mistaken assumption that once a group that make up X% of COVID hospitalisations/deaths are vaccinated, it will reduce hospitalisations/deaths by the same %, even if control measures are lifted. There are two main problems with this... 1/

First, there is a trade off between level of infection in the population and risk reduction through vaccination. Disease outcomes (e.g. hospitalisations/deaths) can broken down into the following: new infections x average-risk-per-infection... 2/

If we remove 50% of the hospitalisation risk within a population through vaccination, for example, but have a large increase in level of infection, it could mean no reduction (or even an increase) in overall hospitalisations... 3/

Second, vaccination won't necessarily reduce risk of severe outcomes by 100% if infected – we all hope effectiveness is high, of course, but it's still unclear exactly what the level of protection will be several months down the line. 4/

When discussing potential impact of vaccines, important to remember just how much control measures are current reducing infection, and that recent hospitalisations/deaths only reflect fraction of people potentially at risk of that outcome if infection levels were much higher. 5/5