Small, temporary change, study finds

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Do COVID-19 vaccines affect the length of a person’s menstrual cycle? Gerardo Vieyra / NurPhoto via Getty Images
  • Researchers recently investigated the link between COVID-19 vaccines and menstrual cycle length.
  • They found that COVID-19 vaccines cause small, temporary increases in cycle length.
  • The authors did not find an association between COVID-19 vaccines and clinically significant changes in menstrual cycle length.

Some may be reluctant to get the vaccine because of claims of a possible link between COVID-19 vaccines and abnormal menstrual cycles.

So far, clinical trials of COVID-19 vaccines have not collected data on the post-vaccine menstrual cycle. As of May 2021, however, a small number of people had reported problems with the menstrual cycle.

The authors of a new study write that “[t]The International Federation of Gynecology and Obstetrics classifies a variation in cycle length as normal if it is less than 8 days. In addition, stress can cause cycle lengths to vary or cause a person to skip a cycle.

This normal variability can be of concern when taken in conjunction with COVID-19 vaccines.

A lack of population-level evidence of the relationship between COVID-19 vaccines and menstrual cycles limits healthcare professionals to addressing concerns about possible menstrual side effects.

Researchers funded by the National Institutes of Health (NIH) recently set out to study the relationship between menstrual cycle length and COVID-19 vaccination.

Medical News Today spoke to Dr Victoria Male, senior lecturer in reproductive immunology at Imperial College London in the UK, who was not involved in the study.

She said: “The main finding of the study is that people who receive both doses of the vaccine in the same cycle are, on average, 2 days behind their next period.”

“However, their periods returned to normal within two cycles. People receiving their first dose do not experience any delay in their periods, and those receiving their second dose are on average less than half a day late, ”she added.

“The results were reassuring and reinforced the overall safety of the COVID-19 vaccines, as they described a slight and temporary impact on the menstrual cycle length of the vaccinated study participants. The authors found the change in cycle length to be less than 1 day and temporary, resolving into two post-vaccination cycles, ”said Dr Amy Roskin, chief medical officer of The Pill Club – a digital health company – who was not involved in studying it.

The study appears in Obstetrics and Gynecology.

Researchers collected menstrual cycle data collected between October 2020 and September 2021 from the fertility awareness app Natural Cycles.

All participants were residents of the United States between the ages of 18 and 45 and had undergone at least three cycles after pregnancy or after using hormonal contraception.

The researchers analyzed data from participants from six consecutive cycles – three before the vaccine and three post-vaccine, or six cycles for those who were not vaccinated.

In total, the researchers included 2,403 vaccinated individuals and 1,556 unvaccinated individuals. Of the vaccinated cohort, 55% took the Pfizer-BioNTech vaccine, 35% the Moderna vaccine and 7% the Johnson and Johnson vaccine.

After adjusting for confounding factors, the researchers found that vaccinated individuals had an average increase in cycle length of 0.64 days after an initial vaccine compared to unvaccinated individuals.

This average increase in cycle length was slightly larger at 0.79 days for those who received the second dose of the vaccine compared to those who were not vaccinated.

The researchers added that the increase in cycle length for the first and second rounds of vaccination may be largely due to the fact that 358 people received two doses of the vaccine in the same cycle.

They note that this group experienced an average 2.38 day increase in cycle length compared to unvaccinated individuals.

In cycle six, the last of the cycles recorded, the researchers noted no significant difference in cycle length between the vaccinated and unvaccinated groups.

The researchers say their findings cannot be explained by generalized pandemic stress because their control group saw no change over a similar length of time.

Instead, they noted that mRNA vaccines create a robust immune response or stressor, which could have a temporary effect on the hypothalamic-pituitary-ovarian axis. This bodily system regulates the menstrual calendar.

They say the results among individuals who received two doses of the vaccine in the same cycle support this possibility.

Professor Alison Edelman, professor of obstetrics and gynecology at Oregon Health and Science University in Portland, and lead author of the study, said MNT:

“More research is needed to determine the causal relationship between COVID-19 vaccines and changes in menstrual cycle length. “

“However, based on previous research that confirms a biological link between the immune system and the reproductive system, we can hypothesize that vaccine-related menstrual disorders may be related to the body’s acute immune response to the vaccine,” she added.

“We know that the COVID-19 vaccines currently available are effective in activating the immune system. This immune response creates a temporary increase in small proteins – called cytokines – that can have a brief impact on a person’s ability to regulate the menstrual cycle, potentially causing temporary changes in cycle timing.

– Teacher. Alison edelman

Dr Mike Armor, senior reproductive health researcher at the University of Western Sydney in Australia, who was not involved in the study, developed the effects of stress on the menstrual cycle. He said MNT:

“First, an increase in stress levels leads to a decrease in [the] release of follicle stimulating hormone and luteinizing hormone, lowering progesterone levels in the luteal phase and subsequently increasing the production of prostaglandins. Second, cortisol and adrenaline levels are increased during times of stress, mediated by an increase in the adrenocorticotropic hormone. The increase in cortisol and the increase in adrenaline affect the synthesis of prostaglandins.

“This can affect the length of the cycle but also symptoms such as dysmenorrhea [or period pain] and other premenstrual symptoms. The fact that the changes in cycle length were most pronounced in people receiving two doses of the vaccine in a cycle supports this theory, ”he explained.

Researchers have found no clinically significant changes in menstrual cycle length associated with COVID-19 vaccines.

The researchers note that there are some limitations to their study. Professor Edelman said: “The users of the menstrual cycle tracker apps do not necessarily reflect the general population. We know from this dataset that this population is more educated, white and has a [body mass index (BMI)] than national distribution.

“BMI is a factor that can influence the variability of the menstrual cycle. However, this may have been compensated by the inclusion criteria of our dataset – requiring three pre-vaccination cycles meeting the criteria for normal cycle length, ”she added.

The researchers further wrote that their data did not include a sufficient number of cycles to understand the long-term effects of vaccinations on cycle length.

“The most important [limitation] when we look at it from an international point of view, it is because this study [mainly] looked at those who received mRNA vaccines such as [Pfizer-BioNTech] and Moderna and specifically excluded those using the Oxford-AstraZeneca, so we cannot say whether those receiving the Oxford-AstraZeneca would have the same changes in their cycle as those reported, ”explained Dr Armor.

Despite these limitations, the results are likely to be reliable due to the design of the study. Dr Male said:

“While there have already been a handful of studies on how COVID vaccination can affect the menstrual cycle, all of the ones we have so far have been retrospective. This means that they suffer from recall and reporting bias. “

“A great strength of this new study is that it uses data collected prospectively from people who were already recording their cycles using an app,” continued Dr. Male. “Other strengths are the inclusion of an unvaccinated control group and the large number of participants (3,959 in total). In short, we can be quite confident about these results.

“These results are reassuring and validating. At the population level, the changes we are seeing do not indicate any reason to worry about long-term physical or reproductive health, nor do they indicate any reason to avoid vaccination. On a personal level, these results provide the information people want to know what to expect when they get vaccinated, ”concluded Dr. Edelman.

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