Shortly after the coronavirus vaccine was released last year, women across the country began posting on social media about what they believed to be a strange side effect of changes in their periods.

Now, new research shows that many of those complaints were justified. A study of nearly 20,000 people around the world found that getting the covid vaccine could alter the timing of the menstrual cycle. On average, unvaccinated people experienced a one-day delay in their periods.

The information for the studyThe 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 “anonymized” data from the app to compare the menstrual cycles of 14,936 participants who were vaccinated and 4,686 who were not.

Because app users track their menstrual cycles every month, the researchers were able to analyze three menstrual cycles before vaccination and at least one cycle after, and compared them to four menstrual cycles in the unvaccinated group.

The data showed that vaccinated subjects delayed their periods by 0.71 days, on average, after the first dose of vaccine. However, people who received two vaccinations in one menstrual cycle experienced more disruption. The average increase in cycle length in this group was four days, and 13 percent experienced a delay of eight days or more compared to 5 percent in the control group.

Alison Edelman, professor of obstetrics and gynecology at Oregon Health & Science University, who led the study, said that for most people, the discomfort is temporary and lasts for one cycle before returning to normal. She said there were no indications that menstrual damage had any effect on fertility.

“Now we can give people information about what to expect from the menstrual cycle,” Edelman said. “So I hope that’s comforting to individuals in general.”

Researchers don’t know exactly why the vaccines affect the menstrual cycle, but Edelman says the immune and reproductive systems are linked, and inflammation or a strong immune response can cause menstrual fluctuations.

Any change in your period can be stressful, an unplanned pregnancy or illness can cause anxiety, and people complain that public health officials didn’t warn about the vaccine’s side effects or do more research before releasing it.

A major limitation of the study was that it included only those who were not on birth control, had regular cycles before receiving the vaccine, and were between the ages of 18 and 45.

The study also did not provide all the answers. Questions people ask about vaccines and periodsIncluding how the shots affect trans men and non-binary individuals. Ever since the vaccines came out, many people on social media have complained that they feel longer, harder and more sick after taking the vaccine. This study did not look at the severity of periods or side effects such as cramps, but researchers said that, on average, taking the vaccine did not seem to last longer.

Edelman said. Preliminary findings A separate study found that taking the coronavirus vaccine can sometimes make periods worse. Data from nearly 10,000 people are still being peer-reviewed, but vaccination appears to slightly increase the risk of serious bleeding.

However, she acknowledges that her research only looked at people with regular menstrual cycles who were not using hormonal contraceptives, and that individual experiences can vary greatly.

Kaithia Pillai, 21, of Berkeley, California, said that for two months after her March 2021 shooting, her normally light periods were painful and twice as long.

“The pain was not like normal pain. It was until I was crying and could not get out of bed,” she said.

Pili said she was stressed and thought something else might be wrong, but after two cycles, her period returned to normal. In the year When they took a second dose in July 2021, her periods got worse again, but she said she felt calmer about it because she saw similar stories shared online.

Other studies have shown that vaccines have different effects on menstruation. A survey Published last fall, it collected information about periods and vaccinations from 16,000 people — including transgender and postmenopausal women — and found that thousands reported heavier bleeding or spotting than usual.

Although these observations are not of medical concern, Kathryn Lee, an assistant professor at Tulane University who helped conduct the survey, said the data is useful for planning and helping trans men get more support if their menstrual flow is due to gender dysphoria. People make decisions about stocking up on tampons and pads.

Lorena Grundy, 27, used an IUD and hadn’t had a period in more than three years before she took her first Pfizer shot in February 2021. She got her period the next day at work.

“The vaccine didn’t move my period earlier or later — it made one,” says Grundy, who lives in Somerville, Mass.

She said that if she had been made aware of the side effects, she would have prepared and brought a pad for work. Her period lasted three or four days – and came back three weeks later when she took a second dose of the vaccine. But it didn’t happen again when she got a boost last November.

“I think it’s good to make sure we listen to women about their bodies,” she says. “I’m still happy to have a vaccine, but I think this is a sign that maybe we should prepare for them so they don’t panic.”

While Edelman’s research suggests that menstrual changes are temporary, some people report permanent changes in their menstrual cycles long after the shot.

Sammy Beechan, 32, of Hammond, Ore., had a “blessed, beautiful cycle” that came “like clockwork” every 28 days and had light cramping and only four days of light-to-moderate bleeding.

After Johnson and Johnson’s shot in April 2021, nothing changed, but after receiving Moderna’s boost in October, Beechan noticed that her period began to come every 24 days, with heavy bleeding for more than four days, more painful cramps and extreme mood swings. Doctors ruled out endometriosis and other possible medical conditions as the cause.

Bichan said vaccinating against covid is beneficial, but they want more information about current side effects before the vaccine is introduced. “I went from being very consistent and now every month I’m like, OK, I guess this is it,” Beechan said.

Diana Bianchi, director of the Eunice Kennedy Shriver National Institute of Child Health and Human Development, which funded Edelman’s research, said the finding of a very late period after vaccination is not surprising.

“I don’t recommend going to the doctor after the first time, because all the evidence indicates that the change will resolve, it’s only temporary,” she said. “If it’s a constant change between menstrual cycles, that’s probably a reason to see your primary care doctor or OB/GYN.”

The National Institutes of Health has funded at least four other research projects on coronavirus vaccines and the menstrual cycle — some involving adolescents and those with endometriosis — in hopes of providing better information and boosting public confidence in the vaccines.

Olivia Rodriguez, 26, said she didn’t plan on getting a booster shot after her second shot in March 2021. Although she finished her period, she started another one within a few days of receiving the vaccine. . She said he had been bleeding for 10 days instead of the normal four or five days. She also experienced more painful cramps.

At first, she was shocked, but soon found stories online of other women who had been through the same situation. It was comforting, she said, but still wary of taking another shot.

Rodriguez, a member of the Osage Nation, said medical researchers need to gain the trust of Native Americans and people of color by first providing more information about side effects.

“I never really got an explanation for why or what happened,” she said.

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