COVID jabs might be changing periods

Some people are reporting changes to their period after receiving a COVID vaccine, what could be going on?
19 October 2021

Interview with 

Viki Male, Imperial College London


Covid vaccine vials


Along with the more common side effects of getting a COVID jab, like a sore arm or fatigue, some women have reported a change to their periods, ranging from starting early or late, bleeding more heavily, or skipping it all together. Now, some scientists are now calling for a closer look at what might be going on, including Viki Male who’s from Imperial College London and spoke to Eva Higginbotham about how, and why, this might be happening...

Viki - A lot of people, and back in September it was just a little bit over 30,000 people, have reported that they've noticed a change to their period happening shortly after the vaccine, but the changes that people are reporting have been mixed. The most common one is your period being later than usual, and the next most common one is it being heavier than usual, but some other people have noticed other things like skipping your period altogether, or perhaps their period being lighter than usual. The way that this data is collected, we can't actually know for sure if these are changes that are being caused by the vaccine, or if they are natural changes or variations to the menstrual cycle that just happened after people have got the vaccine and so they've made a connection and made a report. I think what is very important to say is that most people who are reporting this find that their period goes back to normal within a month or two. So this is not a long-term change. And, also, we know from other evidence that the vaccines don't have any impact on female fertility.

Eva - Are people reporting this relating to all the different kinds of vaccines that are available in the UK? So Pfizer and AstraZeneca, for example, or is it just one vaccine type?

Viki - Yeah, so it's actually all the vaccines that are available in the UK. And, in my research, I sometimes talk to people who are being vaccinated in other countries as well, and people are also saying this in connection with other vaccines. So what that tells us is that if there is a link between being vaccinated and having a change to your period, which to be clear, we don't know yet, it tells us that it's not, for example, any particular ingredient of one kind of vaccine. That suggests to me that if there is a link it's probably to do with the immune system being activated. And, in support of this idea, we do have some evidence that some other vaccines like the HPV vaccine can be linked with changes in periods. And we do also have evidence that other things that activate your immune system, like COVID itself, for example, in about a quarter of people who have periods, who get COVID, they'll find that there's a change to their period. So all of this does link into the idea that there could be something going on that could be mediated by the immune system.

Eva - Do you have any thoughts on what that mechanism could be? How could the immune system affect your menstrual cycle?

Viki - Well, I think that there are two biologically plausible ways that this could happen. The first is that we know that the immune system can affect reproductive hormones and indeed reproductive hormones can affect the immune system. Being vaccinated could perhaps slightly alter the levels of sex hormones, and sex hormones are what's driving your periods. So if they're changed, you might find your periods coming perhaps a little bit later. The other possibility is that we know that the lining of the uterus is really rich in immune cells, and this is actually what I work on in my day job, and these immune cells have a role in the build-up and the breakdown of the lining of your uterus. And so we could imagine that if we activate these cells, which the vaccine might do, because it gives your immune system a broad, activatory stimulus, this could perhaps slightly change the way that they're acting to build up and break down the lining of the uterus. And that would in turn have a knock-on effect on when, or perhaps how heavily, you bleed.

Eva - And what sort of studies do you think need to be done to figure this all out?

Viki - Well, ideally we would have asked people - solicited this information - in the clinical trials, because if in the clinical trials we'd said to everyone who has periods, "did you notice a change to your periods?", we would have the comparison between the people who got the vaccine and the people who got the placebo, or the other vaccines, depending on the kind of trial. And that would have told us for sure if there was a link here. What we're now trying to do a little bit is play catch up. So one approach to this that I'm really excited about is tapping in to the data that we can get from menstrual cycle tracking apps, where people have tracked their cycles over many years. We know what's normal for them, and then asking them to log when they get the vaccine, and then we can see a change. And, if there is a change, how common is it? Once we've got that data, I think we'll be able to say with quite lot of certainty, whether this is really linked or whether it's just something that people happen to be noticing and making the connection.

Eva - Is it common in clinical trials to ask people if whatever the treatment, or vaccine, or whatever it's for, has had an effect on the menstrual cycle?

Viki - As far as I know, it's never been done in vaccine trials, and that's partly, I suspect, because so many vaccine trials are done on children, because with most vaccines we're developing them to protect children or to protect people as soon as we can, which usually means giving the vaccine to them as children. So I think this is one of the reasons that we haven't usually thought about this before in vaccine trials. But what I hope we'll take forward from this is that, actually, this is probably the sort of thing that we should solicit, because it's the kind of thing that people are interested in, and do worry about.


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