COVID-19 vaccination is estimated to have saved millions of lives globally in its first year of rollout. However, reports of irregular menstruation after vaccination have been a major source of vaccine hesitancy among young women.
At the time, such reports were often dismissed as natural cycle variation. But new data suggests that, while the effects are only temporary, vaccination can indeed have a short-term impact on the cycles of some people who menstruate.
How did the COVID vaccines affect periods?
Menstrual cycle lengths vary naturally from month to month. Therefore, it is hard to determine whether unusual menstrual patterns are due to internal or external factors. However, several studies have found consistent associations between COVID-19 vaccination and small, temporary changes in menstruation.
“Three large studies using data from thousands of people on menstrual cycle tracking apps have found that COVID vaccination is associated with a delay to the next period,” Victoria Male, a Senior Lecturer in Reproductive Immunology at Imperial College London, told News week.
In an article published by the journal Science, Male summarized the findings of these studies and the possible mechanisms underlying their results.
“Most months, about 5 percent of people will have a very late period–more than 8 days–but after vaccination this number goes up to about 6 percent,” she said.
There is also some evidence that people may experience heavier periods immediately after vaccination. “One study, which asked people to recall their experiences rather than using app data, found that 7 percent had a heavier than usual period the month before they were vaccinated, compared to about 14 percent the month after they were vaccinated,” Male said.
However, in all cases these effects were only temporary. “Importantly, periods go back to normal within one or two cycles,” Male said. “In most people who experience a change, periods return to normal the next cycle and in these studies, everyone was back to normal within two cycles.”
How can a vaccine affect menstruation?
Menstruation can be affected by everything from stress to flu, and SARS-CoV-2 infection itself has been associated with unusual menstrual patterns.
The COVID-19 vaccine is by no means unique in this regard, and as early as 1549, smallpox inoculation was associated with temporary changes to menstruation in some women. Similar variation has been reported with vaccines for typhoid, hepatitis B and human papillomavirus.
“All kinds of COVID vaccines–and some non-COVID vaccines–had menstrual side effects, suggesting that any changes are caused by the immune response, rather than by a particular vaccine ingredient,” Male said.
Two plausible biological mechanisms have been put forward to describe how these immune responses could bring about changes in menstruation. The first is that the innate immune response could temporarily interfere with the reproductive hormones that drive the menstrual cycle.
“One percent more people than usual will have a very late period following COVID vaccination, but this is less likely to happen among people taking combined hormonal contraception,” Male said. “This suggests that the immune response may be causing late periods by affecting the hormones that drive the menstrual cycle: when we keep hormones stable by taking contraception, we are less likely to experience a late period.”
To support this theory, the timing of vaccination within the menstrual cycle also seemed to affect the likelihood of experiencing cycle elongation. One of the studies in Male’s article found that cycle elongation was only seen in women who had received the vaccine during the first half of their cycle, before ovulation.
The second hypothesis is that vaccination could affect the activity of immune cells in the lining of the uterus, which control the breakdown and regeneration of uterine tissue throughout the cycle.
“Seven percent more people than usual have a heavy period following COVID vaccination, but this is less likely to happen among younger people,” Male said. “This might suggest that the immune response is causing changes to repair in the lining of the uterus, and this might affect older people, who are less good at tissue repair, more.”
Evidence for both theories suggests that the observed changes in menstruation could be a result of both reproductive hormone variation–which may be responsible for cycle elongation–and repair of the lining of the uterus–which could cause heavier bleeding.
Did COVID vaccines impact fertility?
Despite temporary changes to menstruation, there is no evidence that COVID-19 vaccination affects fertility.
“We have looked for evidence of long-term ramifications, [but] we can’t find any,” Male said. “In terms of fertility, there don’t even seem to be short-term problems: people who are actively trying to become pregnant are just as likely to conceive in the cycle they receive the COVID vaccine as in any other cycle.
“Although there was never any scientific reason to suspect that COVID vaccination would harm fertility, a lot of ordinary people were worried about this, and so research into it has been done.
“Among more than 2000 couples trying to conceive by having sex, vaccination did not make a difference to fertility. There are also at least seven studies done in IVF clinics that show that COVID vaccination doesn’t reduce the chance of conception through IVF.”
There is, by contrast, some evidence that SARS-CoV-2 infection can temporarily affect male fertility. Infection has also been shown to affect the later stages of pregnancy.
“COVID infection in pregnancy can cause preterm birth and stillbirth, which is why doctors and midwives recommend getting vaccinated or boosted if you are pregnant,” Male said. “But of course it is important to be sure that COVID vaccination is safe in pregnancy, so a lot of work has been done on this, with 30 studies in 8 countries looking at almost 360,000 people vaccinated in pregnancy.
“None of these studies has found any increased risk of pregnancy problems following vaccination, and there is some evidence that vaccination can reduce the risk of stillbirth.”
What does this mean for women’s health?
There is ample data to support that COVID-19 vaccination is safe for the vast majority of people. However, like any medication, this vaccination can cause side effects. In most cases, these side effects are mild and short-lived, as is the case for changes in menstruation.
While thousands of people who menstruate have reported this temporary cycle variation, it has been largely understudied and is still not listed as a formal vaccine side effect. This lack of attention to women’s health follows a historical trend of neglect for issues that affect about half of the world’s population.
Even before the development of the COVID vaccine, it had been observed that women tend to develop higher antibody responses and report more side effects following vaccination compared to men. Despite this, the effects of COVID-19 vaccination on the menstrual cycle were largely neglected during clinical trials.
More generally, women are still often underrepresented in clinical trials for new medications.
Changes in menstruation can be concerning and have led many to worry about the effects of vaccination on fertility. Highlighting menstrual irregularities as a potential side effect and confirming that these changes are temporary and not a risk to reproductive health would have curbed the spread of misinformation about the effects of COVID-19 vaccination on fertility and likely reduced vaccine hesitancy among young women.
Is there a health issue that’s worrying you? Do you have a question about COVID vaccination? Let us know at firstname.lastname@example.org. We can ask experts for advice, and your story could be featured on News week.
Male V, Covid-19 Vaccination and Menstruation, Science, Nov 18 2022, DOI: 10.1126/science.ade1051
Wesselink AK, et al., A Prospective Cohort Study of COVID-19 Vaccination, SARS-CoV-2 Infection, and Fertility, American Journal of Epidemiology, January 20 2022, https://doi.org/10.1093/aje/kwac011
Jensen A, et al., COVID-19 Vaccines: Considering Sex Differences in Efficacy and Safety, Contemporary Clinical Trials, February 8 2022, https://doi.org/10.1016/j.cct.2022.106700