Revisiting whether COVID jabs affect periods

COVID vaccination effects on menstrual cycle appear minor and short lived...
14 February 2022

Interview with 

Viki Male, Imperial College London


COVID vaccine in gloved hand


Covid jabs do affect women’s periods, but only to a minor degree and only temporarily and with no long-term impact; that’s the conclusion of a number of studies that were set up last year to investigate reports from some women of menstrual changes in the immediate wake of receiving covid vaccines. We reported last year that this investigation was underway, so it’s a pleasure to circle back and speak again with Imperial College’s Viki Male, who works on the interactions between the immune and reproductive systems. She explained what was going to happen. Now she’s here to tell Chris Smith what’s been found…

Vicki - As the vaccines were rolled out last year, particularly as they were rolled out to younger age groups, people started talking about noticing changes to their periods after they'd had the vaccine and quite a few reports about 35,000 by last autumn had been made to our vaccine safety monitor scheme, the yellow card, and the same sort of thing was happening in the United States of America as a result of which the national institutes of health put aside, um, 1.6, 7 million to do more research into this, to find out if there was a real link. And, um, some of those studies have now started to report,

Chris - How did they investigate?

Vicki - One of the NIH funded studies that reported just a couple of weeks ago, used data that people were inputting into a menstrual cycle tracking app. So this is really nice, because people are inputting their data in real time and also, these people were using the app to help track and control their fertility. So they were really motivated to put in correct data because it would have a real impact on them. There were about 4,000 people in this study of whom 2,400 were vaccinated and the rest acted as unvaccinated controls. After the first dose, they saw no effect on the subsequent period, but after the second dose, there was on average about half a day delay to the subsequent period. So a change, but a small one, and definitely very small compared to natural variation. But the people who really noticed something were those who had both doses in the same cycle. And of course this is something that doesn't happen in the UK because our inter-dose interval, but they had more than a two day delay on their next period. But it's really important to know that even these people who had a two-day delay on the period after they'd had both vaccines in the same cycle, their periods were back to normal within two months.

Chris - What about the other study?

Vicki - So there was another study in Norway, which looked at almost 6,000 people, all of whom had been vaccinated, and they asked them to recall their periods before and after the vaccine. They did notice that about seven and a half percent of people in an unvaccinated cycle would call their period heavier than usual. But about 13.5 percent of people in a vaccinated cycle would call their period heavier than usual. And that was a significant difference. So again, here we have, a change, which in this case is heavier than normal periods, but it's reasonably small compared to normal variation.

Chris - I suppose one very important comparison would be to say, well, what happened if a person in that sit duration actually caught COVID. What does that do to your menstrual cycle?

Vicki - Yes. I think this is also a really important point. So studies that have looked at how COVID affects the menstrual cycle haven't been designed in the same way as these, so we can't compare them exactly. But there have been two, which find that change in menstrual cycles after COVID is between 15 and 25% of people whodone caught COVID. So actually that's quite a large possibility that you'll experience a change if you get COVID.

Chris - Why do we think this is happening and should a person be worried about this?

Vicki - So neither of these studies have really done anything to address the potential mechanism, although as an immunologist, I do think it's quite interesting that you don't see anything after the first dose, but you do after the second dose. When something happens more the second time than the first time, that's so often a sign that the immune system is involved, which of course probably shouldn't be surprising here. Vaccines are designed to work with the immune system. So to me, that suggests that this is an immune mediated mechanism, and we do know that the immune system can crosstalk with sex hormones. And of course it's sex hormones that are driving the menstrual cycle. So that could be one way that this is happening. Another way that this could be happening is because we know that there's a really rich immune system in the lining of the uterus, which helps to control the buildup and the breakdown of the lining of the uterus. So it's possible that a big stimulation of the immune system could affect what's going on temporarily there too. These, to me, are the most likely hypotheses, but definitely in terms of the take home message, the changes that we are seeing are really small compared to natural variation. They reverse quite quickly. So I wouldn't want this to put anyone off getting their COVID vaccine.



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