Mailbox: Vaccines and boosters

What happens between the first COVID vaccine and the second?
23 March 2021

Interview with 

Chris Smith, University of Cambridge


A needle and bottle of the COVID-19 vaccine.


Stephanie has gotten in touch with a COVID question: Please could someone explain the mechanism of immune response over time to vaccines? Given that the booster needs to be at least a few weeks after the first dose... what happens in those intermediary weeks? Chris Smith has the answer...

Chris - What we don't know is what the perfect time to administer these two doses is. Because the only reason we used three weeks or four weeks initially was because that was the time set by the manufacturers in their trials, because they wanted to get the data quickly. And in fact, we've resorted to 12 weeks between doses because there's some data suggesting that actually the longer you leave it, the better the immune response is. And you might think, "well, why is that?" And the reason is that the immune response is not just a static thing. You don't just see something you want to respond to, make the response, and then call it a day. The immune system is continuously evolving, optimising, and maturing its response. So as more time goes by, the response that you make initially is slowly petered out and dissipates, and out of that emerges a much more focused, much better response, that's more finely tuned, but it takes time to get there. So waiting a bit longer between doses can in fact mean, when you do come back with the booster, you in fact then boost the really finely tuned, even better response you've now made - rather than a more broad, less finely tuned one you would have made if you stimulated with a booster too soon. And actually if you come back too quickly, with another booster too quickly after the first dose, you can in fact do the opposite and de-tune your immune response completely. It's finding that sweet spot that researchers are still trying to find at the moment.


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