People have been asking me if we might expect the #COVID19 #vaccine to affect female #fertility. The short answer is no. The long answer is... also no, but with more details...

There are both theoretical and practical reasons to think the COVID19 vaccine will not impact fertility. I’m going to start with the theoretical ones, since the reasons for thinking it might are *also* theoretical. 2/15

The vaccine works by training the immune system to recognise a protein of the virus called “Spike”. Like all proteins, this is made up of a string of amino acids – like beads on a string. The string then gets folded up into a 3D shape so that the protein can do its work. 3/15

The vaccine trains the immune system to make antibodies, which attach to the Spike protein in its 3D shape. The virus needs Spike to help it attach to cells and infect them. So blocking Spike with antibodies stops the virus infecting cells. 4/15

The reason some people have brought up fertility is that there is a protein in the placenta, called Syncytin1, which has some short regions in which some of the amino acids (the beads) match up with those on the viral Spike protein. 5/15

(As a side-note, the reason for this is pretty cool. It’s because about 30 million years ago, this gene became incorporated in the genome *from a virus*. As mammals evolved, they found a use for this gene in making the placenta.) 6/15

So the idea was that antibodies that can bind to Spike might also bind to Syncytin1, and this could affect the placenta. 7/15

BUT the matching regions are short and don’t match perfectly. For the curious, here is one of the matching regions. Wuhan is the virus and HERVW is Syncytin1. Each letter is an amino acid and the matches are highlighted with *s. 8/15

Furthermore, the antibodies are not looking at the order of the amino acids (beads), they look at the shape of the whole protein, which differs between Spike and Syncytin1. 9/15

For this reason, we do not expect that antibodies produced in response to #COVID19 Spike protein will bind to Syncytin1, or affect the placenta. 10/15

But now for some practical reasons to be reassured that the #COVID19 #vaccine will not affect fertility... 11/15

The immune response to natural #COVID19 infection *also* involves making antibodies to the Spike protein. So if these antibodies were a bad thing for pregnancy, we would expect natural infection to be associated with miscarriages. 12/15

Enough people have now been infected that we can be confident that this is not the case. For evidence, see:

https://jamanetwork.com/journals/jamanetworkopen/fullarticle/2773105
https://link.springer.com/article/10.1007/s00404-020-05848-0
https://www.ajog.org/article/S0002-9378(20)31177-7/pdf

13/15

So in short, there are no convincing reasons to believe that #COVID19 #vaccination will affect female fertility, and a lot of reasons to believe that it will not. 14/15

I am a reproductive immunologist and I will be getting the vaccine as soon as it is offered to me - I would recommend that everyone else do the same! 15/15

PS. If anyone wants to play with the alignments themselves, @andrew_croxford gives you the tools to do so here: https://twitter.com/andrew_croxford/status/1334593606196187136?s=20