Thread by @VikiLovesFACS

I’m getting a lot of questions about #COVID19 vaccination and the menstrual cycle. Does the #vaccine affect your #period? And does that mean there will be an effect on #fertility? 💉🩸 🧵

(Short answer upfront: Maybe, and no.)

Does the #COVID19 #vaccine affect #periods? Well, a lot of people are reporting this and I like to start from a position of believing what people say about their own bodies. So I think it is likely that this is a side-effect in some people. 2/

But research is being done to find this out! I know at least 2 studies - the biggest is run by @KateClancy. You can read about her study, and participate if you want, here. Ppl who used to have periods but no longer do are also invited to participate! 3/

https://twitter.com/KateClancy/status/1379847815636135941?s=20

This study will tell us two important things. Firstly, is this a common side-effect? Secondly, the kinds of people who report these effects may start to give us an idea of the mechanism by which any effect is occurring. 4/

If people are interested, I will go into potential mechanisms in a follow-up thread. But for now, let’s answer that second question: if there *is* an effect of #COVID19 #vaccination on the menstrual cycle, does that tell us anything about its effects on #fertility? 5/

To answer that question, I want to look in detail at two studies on how other vaccines affect the menstrual cycle and fertility.... 6/

Let’s start with this study, which surveyed almost 30,000 teenagers about their experiences of HPV vaccination. It’s a retrospective study, so there may be recall bias. But a strength is that there is an unvaccinated comparison group. 7/

ncbi.nlm.nih.gov/pmc/articles/P…

In this study, teenagers who had the HPV vaccine were more likely to say that they had experienced heavy periods or irregular periods than those who had not received the vaccine. 8/

So there may be an effect on the menstrual cycle, but is there an effect on fertility? Well, part of the reason I have chosen to focus on this study is because we have *lots of evidence* that HPV vaccination does not harm fertility… 9/

https://twitter.com/VikiLovesFACS/status/1363474421403570180?s=20

And to highlight just one paper from the linked thread, in people with a history of sexually transmitted disease, the HPV vaccine actually *improves* fertility (almost certainly because it stops you getting HPV!) 10/

ncbi.nlm.nih.gov/pmc/articles/P…

Now let’s talk about the second study. This one looked at the effect of flu vaccination on hormonal fluctuations during the menstrual cycle, rather than people’s own experiences of their periods. 11/

clinicaltrials.gov/ct2/show/resul…

It found a slight reduction in progesterone levels in the second half of the menstrual cycle in the month women received the vaccine, compared to a month in which they did not. 12/

So flu vaccination potentially impacts the menstrual cycle (and similar to COVID19 vaccination, anecdotally people have told me about the flu vaccine having a short term effect on their periods).

But does it affect fertility? 13/

Again, no. In this prospective study of 8654 women who were trying to conceive, having received the flu vaccine made no difference to the likelihood of getting pregnant in the next 12 months. 14/

sciencedirect.com/science/articl…

So… getting the #COVID19 #vaccine might affect your #periods – work is being done to find that out. But other vaccines can also affect the menstrual cycle in the short-term and have no later impact on fertility. 15/

For this reason, even if COVID19 vaccination does affect periods, we don’t expect it to affect fertility. \thread

But more resources on #COVID19 #vaccines and #fertility follow for the curious....

My explainer covers questions about #COVID19 #vaccination and #fertility, but also #pregnancy and #breastfeeding….

drive.google.com/file/d/1_wHIYX…

And this thread goes on a deeper dive into fertility specifically….

(Okay, that's actually the end of the thread!)

https://twitter.com/VikiLovesFACS/status/1366478659683569666?s=20