Link Between Unusual Menstruation Patterns and the COVID-19 Vaccine

As more and more people have received COVID-19 vaccines and subsequent booster shots, there has been a wide range of reports concerning possible side effects certain people or groups of people experience in the weeks and months after their shots. One such complaint or report concerns unusual menstrual patterns in women after getting the COVID-19 vaccine. In recent months, several studies have been conducted to investigate if there is a link between the vaccine and changes in menstruation.

Social media and other news outlets have reported alterations in menstrual cycles since the COVID-19 vaccine became widely available in 2021. But what do the outcomes of scientific studies show?

They indicate there may in fact be truth to the reports so commonly seen online. Before examining a few research studies, it is first necessary to mention that women's cycles vary widely. While a 28-day cycle with five days of bleeding is considered the most common and the most predictable, it is not the case for everyone.

Cycles will also vary for several reasons, including weight changes, stress, excessive exercise, medications, and undetected pregnancy. Therefore, potential changes in menstrual cycles related explicitly to the COVID-19 vaccine may be difficult to pin down.

COVID vaccine and menstruation

Up until recently, questions about how vaccines affect menstrual cycles have been largely excluded from vaccine research. Sadly, there are not a lot of large-scale scientific studies on the topic. That said, a few smaller studies have shown a potential link.

One study of 4,000 people reported that the first dose of the COVID-19 vaccine had little or no impact on the timing of the next menstrual cycle and, more specifically, arrival of the respondent's period. However, after receiving the second dose (and/or boosters) experienced a slight delay in the onset of bleeding. This difference remained in the first and second cycle post vaccine but was resolved by the third cycle. Interestingly, test participants who received both vaccine doses during their 28-day cycle noticed an increase in cycle length of two days, which was also resolved by the third cycle post-vaccine.

Another study conducted in Norway of over 5,500 participants showed approximately 41% of participants reported changes in their menstrual cycles after receiving their second vaccine dose. However, 38% of study participants reported intermittent changes and disturbances in their cycles before ever receiving the COVID-19 vaccine. Therefore, it is difficult to determine if their cycle changes were directly connected to receiving the vaccine.

A third global study, including participants from Europe, the UK, the US, and Canada, collected self-reported data from approximately 15,000 vaccinated individuals. This study also incorporated data from nearly 4,700 unvaccinated respondents to compare the potential effects of the COVID-19 vaccine on menstrual cycles. This study also showed that overall, women who received the COVID-19 vaccine had a less than one day increase in cycle length compared to their three pre-vaccination cycles. The unvaccinated respondents reported no changes to their cycle length. Respondents who received a second dose reported a change in cycle length of just over ½ a day. As with other study results, the reported changes in cycle length reported by study respondents resolved within two to three cycles after their last vaccine dose.

A recent, large-scale study of more than 35,000 people conducted in cooperation with Tulane University found that 42% of study participants who normally had regular and predictable menstrual cycles reported heavier than usual bleeding. 44% of respondents in the same study reported no change after receiving their vaccination. Among study participants who usually do not menstruate, 71% on long-acting contraceptives, 39% on gender-affirming hormones, and 66% of post-menopausal participants reported breakthrough bleeding.

Although the Tulane University study is by far the most extensive study to date, it is essential to remember that the data conducted for it was gathered through self-reporting of personal experiences. Therefore, the data results cannot be used to establish a direct link between COVID-19 vaccines and changes in menstruation patterns. The study (and others like it that rely on self-reporting) should not be considered predictive of the effects within the general population.

Results from these and other studies suggest there could be a connection between the COVID-19 vaccine and changes in the menstrual cycle.

The most recent data indicates small changes, including the below, can occur:

• Heavier than usual bleeding.
• Breakthrough bleeding.
• Longer or shorter menstrual cycles.

However, it is also crucial to remember that these changes and others that can affect your menstrual cycle can also occur for several other reasons. Therefore, if you experience a difference in your cycle, regardless of whether you have received a COVID-19 vaccine or booster, it is important to talk to your healthcare provider for more information and to explore other potential reasons for cycle changes.