Thread by @VikiLovesFACS

@strawbale23 @CaulfieldTim @harrietsherwood @RCObsGyn Good question – I’m glad you asked!

But before we get into the nitty gritty of the #COVID19 #vaccine #fertility data, I think it’s useful to first talk a bit about fertility and vaccines in general, and mRNA vaccines in particular... 🧵🤰🏽💉

@strawbale23 @CaulfieldTim @harrietsherwood @RCObsGyn The first thing to say is that no vaccine has ever harmed fertility. Many vaccines improve fertility (by preventing infectious diseases that make people less fertile!) but there has never been one that harms it. 2/

@strawbale23 @CaulfieldTim @harrietsherwood @RCObsGyn But these mRNA vaccines are pretty new, right? So maybe some completely unexpected side effect could show up ten years from now?

People ask me this a lot, but actually they’re not as new as you might think... 3/

@strawbale23 @CaulfieldTim @harrietsherwood @RCObsGyn mRNA vaccines have been under development for decades now, with the first human trials beginning in 2006. So those first human volunteers have been kicking about, uneventfully, for fifteen years now. 4/

@strawbale23 @CaulfieldTim @harrietsherwood @RCObsGyn This is a nice history, for the interested. Though of course it’s out of date now, because it was written before the new COVID19 vaccines were developed! 5/

nature.com/articles/nrd.2…

@strawbale23 @CaulfieldTim @harrietsherwood @RCObsGyn And, for completeness, if anyone is interested in how long the chimp adenovirus vector platform on which the Oxford-AZ vaccine is based has been around, the first human trials began in 2012. 6/

clinicaltrials.gov/ct2/show/NCT01…

@strawbale23 @CaulfieldTim @harrietsherwood @RCObsGyn So vaccines are safe (or even beneficial) for #fertility. And the platforms on which our approved #COVID19 #vaccines are based have been used in humans for a decade, without any surprise effects on fertility later.

But what about the COVID19 vaccines themselves? 7/

@strawbale23 @CaulfieldTim @harrietsherwood @RCObsGyn To me, the best source of data on this comes from the clinical trials.

Participants in the trials were asked not to become pregnant, but accidents happen and, across all trials of the three vaccines approved in the UK, 53 people become pregnant nonetheless. 8/

@strawbale23 @CaulfieldTim @harrietsherwood @RCObsGyn People became pregnant at the same rate in the vaccinated and control groups. So the vaccines do not prevent pregnancy. And the miscarriage rate was the same (normal) in both groups, so the vaccines do not seem to be having an impact on early pregnancy. 9/

@strawbale23 @CaulfieldTim @harrietsherwood @RCObsGyn If you want more details, it’s all linked in my explainer, here: 10/

drive.google.com/file/d/1_wHIYX…

@strawbale23 @CaulfieldTim @harrietsherwood @RCObsGyn Another source of data – though you don’t have such a neat control group as in the trials – are the people who had the vaccine following its general release, and then got pregnant. 11/

@strawbale23 @CaulfieldTim @harrietsherwood @RCObsGyn We have official records of some of these through the Yellow Card reporting system in the UK (6 with the Pfizer vaccine and 4 with the AZ) and through the VAERS reporting system in the US. 12/

gov.uk/government/pub…

@strawbale23 @CaulfieldTim @harrietsherwood @RCObsGyn These are likely to be under-reports, since a wanted pregnancy is not an adverse event and most people will therefore not report these. 13/

@strawbale23 @CaulfieldTim @harrietsherwood @RCObsGyn Finally, if you were really worried about this you might ask: Okay, we know the vaccines don’t stop people getting pregnant *now*. But could something change in the years following the vaccine that would reduce fertility? 14/

@strawbale23 @CaulfieldTim @harrietsherwood @RCObsGyn To answer this question, we need to look at how vaccines work. 15/

@strawbale23 @CaulfieldTim @harrietsherwood @RCObsGyn The vaccine itself clears the body very quickly. But vaccination does make a long-term change to your body in that it raises a memory immune response. That’s the point. It’s what protects you against the infectious agent when you meet it for real. 16/

@strawbale23 @CaulfieldTim @harrietsherwood @RCObsGyn For the very best vaccines, this memory response stays with you your whole life. But for many, it fades over time and this is why we need boosters. So the immune response might fade away, but it won’t get stronger, or change over time to attack something else. 17/

@strawbale23 @CaulfieldTim @harrietsherwood @RCObsGyn For this reason, even the person who suggested that the vaccine might have an impact on female fertility only thought it might last a couple of months – not that it would be a permanent thing. 18/

@strawbale23 @CaulfieldTim @harrietsherwood @RCObsGyn I won’t get into why there were never good reasons to believe him – the evidence has proven him wrong. But if you want to know the theoretical reasons we don’t expect the COVID19 vaccine to be associated with infertility, there's this ooold thread: 19/

https://twitter.com/VikiLovesFACS/status/1339510379332988929?s=20

@strawbale23 @CaulfieldTim @harrietsherwood @RCObsGyn So TLDR... many strands of evidence tell us the #COVID19 #vaccine does not harm #fertility, and will not do so in the future. As always, happy to chat if people feel like they need more information in order to make an informed decision! \thread