COVID-19 Vaccines Can Make Periods Longer, Study Says

Side effects from vaccines are not unusual, and in fact are expected. But when the COVID-19 shots were first authorized in the U.S., the effect these vaccines might have on the reproductive system weren’t known.

In a study published Sept. 27 in BMJ Medicine, researchers provide more information on this question, documenting how COVID-19 vaccines can affect menstrual cycles, as well as how long the impact lasts.

Dr. Alison Edelman, professor of obstetrics and gynecology at Oregon Health & Science University, and her team conducted the largest analysis to date on the effect that the vaccines have on menstruation. It included nearly 20,000 vaccinated and nearly 5,000 unvaccinated people around the world. The work is an extension of their first study into the issue, which was focused on data from the U.S.

In the latest study, Edelman found that any COVID-19 vaccine can extend the menstrual cycle—the time between periods—by less than a day on average, although it didn’t have much effect on how long bleeding lasts. The team also found that this change tended to only last for one cycle after vaccination, resolving by the next period.

Edelman began looking into the issue after people began reporting changes in their cycles after vaccination to U.S. government databases that track vaccine side effects. Surveys also documented changes in cycles. “Before, there was no data around this,” she says. “Now we have information to know that the vaccine does change the menstrual cycle, at least on a population level. It looks like a brief change, and it goes back to normal pretty quickly. But it’s important information to have.”

The latest data add to the existing data gathered from the U.S. because they include a larger number of people as well as a broader variety of COVID-19 vaccines. While three shots (from Moderna, Pfizer-BioNTech, and Johnson & Johnson-Janssen) have been approved or authorized in the U.S., other vaccines that use different technologies (like AstraZeneca’s) are also available around the world. Edelman and her team found that the effect on menstrual cycle length was similar with all of the COVID-19 vaccines. That means that the newer mRNA-based shots don’t seem to be associated with any menstrual changes than the other vaccines, which should allay concerns about the novel technology.

Exactly how the vaccines can prompt changes in periods isn’t clear, but previous studies have hinted that the effect is likely related to cross-talk between the immune system—which is activated after vaccination—and the reproductive system. Temporary inflammatory reactions after immunization, similar to those generated after getting natural infections, could affect processes like ovulation, and the extent of the effect could depend on when during the cycle people get vaccinated. “At this point we don’t know the exact mechanism, but there are a lot of hypotheses based on established research that has come before,” says Edelman. “We need more studies to understand this.”

COVID-19 may provide a good opportunity to launch such research. Edelman and her team are also continuing to mine the data to answer other questions about how the COVID-19 vaccines might affect menstruation, including whether vaccination affects menses itself. They are also exploring how getting infected with COVID-19 might affect periods, since infections of any kind are known to affect menstruation. Data from U.S. and global populations collected in studies so far were gathered in the first year after the vaccines were authorized, from late 2020 to late 2021, when fewer people were infected compared to 2022, when widely circulating and highly contagious Omicron variants have circulated.

The studies also do not account for the potential effect of booster shots, which were not authorized in the U.S. until fall of 2021, so the scientists are also investigating whether additional vaccine doses affect cycles in the same way.

While a cycle-length increase of less than a day may seem small, Edelman says that it’s important to acknowledge that vaccines can have an effect on periods. Building scientific knowledge around the topic can help people better track their fertility or know what to expect after getting vaccinated. “Hopefully this will create a foundation for information about menstrual cycles and future vaccines as well,” she says. “Menstrual cycles have been woefully understudied for so long, and we didn’t recognize the need for foundational information. Whether the cycles change or not is incredibly important to know for reassuring people and building trust in something like vaccines.”