What we know about how COVID-19 vaccines can affect the menstrual cycle

(The Hill) — Since the beginning of the pandemic, women have reported experiencing changes in their periods after receiving or being vaccinated against COVID-19.

Their cycles had lengthened, some said. Their bleeding was heavier.

Research has backed up those anecdotal reports, showing that COVID-19 vaccination has a temporary but noticeable impact on women’s periods and associated symptoms.

Here’s what we know.

Vaccination appears to temporarily lead to longer cycles

Several recent studies have shown that the length of human menstrual cycles can increase by up to a day immediately after vaccination.

A study of nearly 4,000 women in the US found that menstrual cycle length increased by approximately 0.7 days after the first dose and by 0.9 days after a second dose. Although cycles were generally longer, researchers found no change in the number of days women’s periods lasted.

An even larger study of nearly 20,000 women in the UK found a similar effect on total cycle length, but also noted that it lasted longer in people who received both doses of a vaccine within the same menstrual cycle. For these individuals, their cycle length increased by an average of 3.7 days.

A paper published Jan. 7 in the Journal of Infection and Chemotherapy bolstered that finding with new data. The authors calculated the difference between predicted and actual menstrual cycle lengths in women in Japan before and after COVID-19 vaccination. Before women were vaccinated, the average difference was about 1.9 days. After two doses of a vaccine, this can be up to 2.5 days. The change was more pronounced in people who received two vaccine doses within the same cycle, with that group seeing an average difference of 3.9 days.

The changes may also not affect everyone in the same way, beyond the differences seen with more or less doses. Some people are more likely to have cycle disruptions than others. A study using long-term data from the Nurses’ Health Study in the US and Canada found that this increase in cycle length was more common in women whose periods were short, long or irregular before vaccination.

The studies found that most people’s menstrual cycles returned to normal after one or two cycles.

Vaccinated women may also be more likely to see other menstrual-related symptoms

Another recent study indicates that women are more likely to experience a range of symptoms associated with their period after being vaccinated.

The study, published Dec. 28 in the journal Influenza and Other Respiratory Viruses, analyzed data from nearly 5,000 women in six Arab countries and found that vaccinated individuals were more likely to experience back pain, nausea, fatigue, pelvic pain, nonprescription analgesic use, and transit. of loose stools related to their menstruation compared to unvaccinated individuals.

Vaccinated people also reported a heavier flow and more days of bleeding, according to the paper.

The authors note that more data is needed to confirm these findings.

The possible effect of COVID-19 infections is less clear

The study, based on long-term data from the Nurses’ Health Study, noted that COVID-19 infection did not affect cycle length in its cohort.

However, other small sample studies have reported that a low percentage of people may experience cycle changes after infection.

What does all this mean?

Research suggests that the changes in menstrual cycle length may occur because of the immune system’s influence on sex hormones. Inflammatory reactions to the COVID-19 vaccine can also affect the ovaries and uterus.

However, beyond the apparent impact on menstrual cycles and symptoms, whether COVID-19 vaccines affect fertility and reproductive health is still uncertain. Initial studies suggest that COVID-19 vaccination may not affect fertility.

More studies with larger samples and longitudinal datasets, where researchers follow up individuals and link their data over time, such as the Nurses’ Health Study, would help to better understand how the vaccines affect the bodies of both men and women and the affect reproductive health.

In general, research suggests that the benefits of vaccination outweigh the risks when it comes to reproductive health.

For example, pregnant people who have not been vaccinated may be at greater risk of poor outcomes, as medical experts point out in a review article published Jan. 12 in the journal Archives of Gynecology and Obstetrics. The experts indicated that such people appeared to have higher hospitalizations, intensive care unit admissions and morbidity rates than their vaccinated counterparts.

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