The data for the study, published Tuesday in the British Medical Journal, was taken from a popular period-tracking app called Natural Cycles and included people from around the world, but most were from North America, Britain and Europe. The researchers used “de-identified” data from the app to compare menstrual cycles among 14,936 participants who were vaccinated and 4,686 who were not.
Because app users tracked their menstrual cycles each month, the researchers were able to analyze three menstrual cycles before vaccination and at least one cycle after, and compare them with four menstrual cycles in the unvaccinated group.
The data showed that vaccinated people got their periods 0.71 days late, on average, after the first dose of vaccine. However, people who received two vaccinations within one menstrual cycle experienced greater disruptions. In this group, the average increase in cycle length was four days, and 13 percent experienced a delay of eight days or more, compared with 5 percent in the control group.
Alison Edelman, a professor of obstetrics and gynecology at Oregon Health & Science University, who led the study, said that for most people the effects were temporary, lasting for one cycle before returning to normal. She said there were no indications that the period side effects had any impact on fertility.
“Now we can give people information about possibly what to expect with menstrual cycles,” Edelman said. “So I hope that’s overall really reassuring to individuals.”
Researchers don’t know exactly why the vaccines seem to affect menstrual cycles, but Edelman said that the immune and reproductive systems are linked and that inflammation or a strong immune response could trigger menstrual fluctuation.
Any change in getting your period can be stressful, triggering worries about an unplanned pregnancy or an illness, and people have expressed frustration that public health officials didn’t warn them about the possible side effect or do more research before rolling out the vaccines.
One major limitation of the study is the fact that it included only those who were not on birth control, had regular cycles before getting vaccinated and were between the ages of 18 and 45.
The study also didn’t answer all of the questions raised by people about vaccines and periods, including how the shots affect trans men and nonbinary individuals. Ever since the vaccines were rolled out, many people on social media have complained of longer, heavier and more-painful periods after getting vaccinated. This study did not look at the heaviness of periods or other side effects such as cramps, but researchers said it did show that, on average, getting vaccinated did not appear to cause longer periods.
Edelman said preliminary findings from a different study suggest that getting a coronavirus vaccine sometimes may cause heavier periods. The data, collected from nearly 10,000 people, is still undergoing peer review, but it showed that getting vaccinated increased slightly the probability of having heavier bleeding.
However, she acknowledged that her studies have looked only at people with normal menstrual cycles who aren’t using hormonal contraceptives, and that individual experiences may vary widely.
Caiityya Pillai, 21, who lives in Berkeley, Calif., said that for two months after her March 2021 shot, her normally light period became extremely painful and lasted twice as long.
“The pain…
Read More: Do covid vaccines affect periods? A new study says they do.
