When people talk about the effectiveness or otherwise of vaccines they often mean one of two things. There is the extent to which a vaccine can protect you against infection: Will it stop you catching COVID? And there’s the protection a vaccine gives you against serious harm: If I catch COVID will it prevent me going to hospital or dying?
These two types of protection – against transmission and against severe disease – are both very important, but they are also quite different. Just because a particular vaccine is good at one thing doesn’t necessarily mean it’s equally good at the other thing. Just because a particular vaccine’s effectiveness at preventing transmission is waning doesn’t necessarily mean its effectiveness at preventing serious disease is diminishing at the same rate.
I say all of this not just because the two types of protection are often conflated (though they are) but because keeping this distinction under your hat is quite important when navigating all the data around COVID and vaccines.
And to be absolutely clear, the majority of what’s below is about that first type of protection: Protection against transmission.
As you’ll probably be aware, the big picture here is that immunity against transmission has been waning recently. Here’s one way of looking at it. If you got the Pfizer vaccine, its “vaccine effectiveness” (an epidemiological term meaning the extent of protection it grants you vs someone who’s unvaccinated) was 92% straight after your second dose. But over the following weeks that immunity wanes, down to 69.7% by week 20.
The protection granted by AstraZeneca was lower to start with at 63% straight after dose two, falling to 47.3% by week 20. Now, these numbers are specifically about the Delta variant. From what we know, these vaccines were a touch better (a lot better in AstraZeneca’s case) at protecting against, for instance, the Alpha/Kent variant. Even so, it’s clear both that they are not as high as everyone would like and they are heading downwards.
Which brings us to the new data published today by the UK Health and Security Agency (better known as the agency formerly known as Public Health England) on the early evidence from the UK’s booster programme. The news here is very promising. It suggests that two weeks after a booster dose of Pfizer, those who originally had two doses of AstraZeneca are up to 93.1% protection and those who originally had two Pfizer jabs are up to 94% protection.
In other words, it’s not just that the booster jabs replenish that immunity, they lift it up beyond where it was at two doses – a little bit higher for team Pfizer and a lot higher for team AstraZeneca.
