The Covid-19 vaccine was given to my colleagues at the hospital. Most of us had a sore arm, some of us felt feverish, but one comment stood out. One of the women said that her period was heavier than usual. Every year after we get our flu vaccine, that happens to me as well. Isn't it a vaccine side-effect?

Is it true? This was the first time that I had heard of such an effect, but it didn't make sense to me. Those of us who were complaining of feeling under the weather could attest to the fact that vaccinations can be a physical stressor. I wondered if the effect was as run of the mill as my colleague thought.

Pieces of evidence were found here and there. In 1549, the Chinese doctor Wan Chhan casually mentioned that an early procedure similar to vaccination could cause menstruation to occur unexpectedly. According to a report from 1913, a number of the nurses at the New York hospital noticed a difference in their menstrual cycles after they received the vaccine. Seven out of 16 women in a phase I trial of a Hepatitis B vaccine reported menstrual changes, while a survey of almost 30,000 Japanese teenagers found no correlation between the two.

My colleague might have been right about the vaccine side-effect. This wasn't something that was studied or discussed. It was more of a curious thing.

For a short time, it was a curiosity. As the programme was rolled out to younger people, social media was alight with people saying they had noticed a change to their menstrual cycle after the vaccine. At a time when vaccine hesitancy among young women was being driven by false claims that Covid-19 vaccination could harm their fertility, the possibility that vaccination might cause changes to the menstrual cycle was worrying. Failing to investigate the link would make these fears worse.

Without formal studies, it was not possible to know if the Covid-19 vaccine caused any changes, or if people were just becoming more aware of the changes that would have happened. The US National Institutes of Health allocated $1.67 million to investigate a possible link between US residents and diseases. A group of young adults was recruited by the Norwegian Institute of Public Health to look into the effects of other Covid-19 vaccine effects. In the UK, there wasn't a lot of funding for small studies.

This week the results from those studies have been published. The timing of periods returned to normal after we found that getting a Covid-19 vaccine could delay it by a day or two. The US study found the same thing. The effect may be caused by temporary changes to sex hormones, as we found evidence that this delay was less likely in people taking combined contraceptives. The Norwegian study found that about 8% of people had a heavier than usual period before they got their vaccine, but this went up to 14% after they got their vaccine.

We know from other studies that Covid-19 vaccine does not affect female fertility and the results of these studies are reassuring. People who noticed a change in their periods were right to tell doctors and scientists and we were right to listen. This story shows the benefits of asking patients and the public to help us frame research questions that are relevant to them.

There is more to learn. The ideal setting to differentiate between menstrual changes associated with Covid-19 vaccine and those that occur in the placebo group was provided by the clinical trials. The opportunity was missed because participants weren't asked about their menstruation. menstrual and reproductive health has traditionally been ignored in medical research In the future, we should do better.

  • Viki Male works at Imperial College London.