Since it’s been available, women have been reporting changes in their period after the COVID-19 vaccine. A new study just confirmed at least one of these changes is normal.

Anecdotal evidence about changes in menstrual cycles for those who got the COVID-19 vaccine has been piling up on social media since the shot became widely available in 2021. People with periods have been reporting longer periods, missed periods, heavier periods, and even, for some post-menopausal women, the return of a period, per the New York Times. But until now, there’s been a lack of large-scale clinical research examining what happens to your period after the COVID-19 vaccine. 

A new study published Thursday reports that, confirming at least one post-vaccine period change. Published in the journal Obstetrics & Gynecology, the study found that after getting the COVID-19 vaccine, people with periods had a slightly longer menstrual cycle, with periods coming an average of one day later than normal. (The length of the actual period remained the same.) 

Importantly, the change isn’t harmful, according to health experts, and it isn’t permanent. The study found that the menstrual cycle returned to normal one or two periods after getting the vaccine. This falls in line with existing research on the safety of the vaccine for fertility: The COVID-19 vaccine has been proven to have no negative side effects on fertility or pregnancy. The American College of Obstetricians and Gynecologists recommends all people who are pregnant, or plan to become pregnant get the vaccine. 

The data on changes in menstrual cycles came from nearly 4,000 women using the Natural Cycles fertility app, about 60% of whom had received the COVID-19 vaccine. That data has one very important drawback, however. As the Times points out, the set of women using the Natural Cycles app isn’t nationally representative—these users tend to be white, educated, thinner than the average American woman, and are not using hormonal contraception, making it difficult to draw generalizable conclusions. 

Alison Edelman, M.D., a professor of obstetrics and gynecology at Oregon Health & Science University and the lead author of the study, also noted that there’s lots of room for individual variation in the data. For example, a large factor influencing the findings was a group of 380 women who experienced a two-day delay in the start of their period, she told the Times. A smaller group of vaccinated women—about 5%, per CNN—experienced cycles eight days longer than normal. (However it’s also worth noting that this number was the same among unvaccinated women in the study.) “Though the cycle length was less than one day different at the population level, for an individual, depending on their perspective and what they’re relying on menses for, that could be a big deal,” she said. “You might be expecting a pregnancy, you might be worrying about a pregnancy, you might be wearing white pants.”

The study also doesn’t address reports from post-menopausal women (who aren’t likely to be using the app). Experiencing spotting or bleeding post-menopause can be a cause for concern, whether you’ve received the vaccine or not, and should be evaluated by a doctor. 

The study is a step in the direction of understanding how the COVID-19 vaccine—and vaccines in general—impact periods. “It validates that there is something real here,” Hugh Taylor, M.D.,  chair of the department of obstetrics, gynecology, and reproductive sciences at Yale School of Medicine, who was not involved in the study, told the Times. Adding, “I want to make sure we dissuade people from those untrue myths out there about fertility effects. A cycle or two where periods are thrown off may be annoying, but it’s not going to be harmful in a medical way.”

This post originally appeared on SELF.

The U.S. reached a dark milestone in early November: 10 million reported coronavirus cases. But there is also big news for a potential coronavirus vaccine. On November 9, Pfizer and German drugmaker BioNTech announced that data from their ongoing coronavirus vaccine trial shows the vaccine is more than 90% effective in preventing COVID-19, per The New York Times. A late-stage trial conducted a few weeks later boasted even better numbers. The NYT reported on November 18 that the Pfizer vaccine is actually 95% effective and safe, and causes no serious side effects. 

The Pfizer-BioNTech vaccine is just one of 11 potential coronavirus vaccines currently in late-stage trials. “If Pfizer already has promising results, other trials may have some good news soon as well,” says Purvi Parikh, M.D., an allergist and immunologist with the Allergy and Asthma Network and co-investigator on the vaccine trials. 

On November 16, a second vaccine, from Moderna, which uses similar technology, was reported to be 94.5% effective in preventing COVID-19. “These are obviously very exciting results,” Dr. Anthony Fauci told CNN. “It’s just as good as it gets—94.5% is truly outstanding.” One difference between the Pfizer and Moderna vaccines is that the Moderna version does not have to be stored at as cold a temperature, which could make distribution easier. (In a delightful twist, news broke on November 17 that the $1 million Dolly Parton donated to coronavirus research at Vanderbilt University actually helped fund Moderna’s vaccine.) 

So what does this mean about when you’ll actually be able to get a safe, reliable coronavirus vaccine? Here’s everything you need to know.

When will a coronavirus vaccine be approved?

Before the vaccine can be approved by the Food and Drug Administration, Pfizer (and any other drugmakers with a promising vaccine trial) will have to finish collecting two months of safety data. Pfizer will be done collecting this information in late November. Assuming everything checks out, the company plans to ask for fast-tracked emergency authorization from the FDA, according to The New York Times.

On November 22, Pfizer submitted its application for emergency approval from the FDA and Moderna announced it would be asking for the same approval on November 30. According to CNN, the Vaccines and Related Biological Products Advisory Committee is scheduled to meet on December 10 to review Pfizer’s application and on December 17 to review Moderna’s application. CNN also reports Moderna will be asking the committee to review “an expanded data set showing the vaccine is 94.1% effective at preventing COVID-19 and 100% effective at preventing severe cases of the disease.”

On December 2 it was announced that the U.K. approved the Pfizer vaccine. The vaccine will be given “according to clinical need,” health secretary Matt Hancock said, adding, “The goal is to save as many lives as possible and stop hospitalizations.”

The first coronavirus vaccines were administered in the U.K. on Tuesday, December 8. 

Will the coronavirus vaccine be safe?

The FDA is responsible for making sure that any fast-tracked vaccine not only works but is safe. “The vaccine must go through a minimum of three phases before approval,” Parikh says, “the last being the most thorough with a minimum of 30,000 individuals tested, especially those in high-risk groups such as the elderly, those with diabetes, heart disease, and obesity, and racial groups with health disparities.” 

There are still some unknowns, like whether or not a vaccine will be safe for kids or pregnant women. Pfizer announced in October that it would start testing its vaccine in kids, but currently there are no trials that include pregnant women—who are more likely to suffer from serious complications of the coronavirus. “We will need that data to determine safety, as every group is different,” says Parikh.