- Researchers recently investigated the link between COVID-19 vaccines and menstrual cycle length.
- They found that COVID-19 vaccines lead to small, temporary increases in cycle length.
- The authors did not find an association between COVID-19 vaccines and clinically meaningful changes in menstrual cycle length.
All data and statistics are based on publicly available data at the time of publication. Some information may be out of date. Visit our coronavirus hub for the most recent information on COVID-19.
So far, clinical trials of COVID-19 vaccines have not collected menstrual cycle data post-vaccine. By May 2021, however, a small number of people had reported menstrual-cycle-related issues.
The authors of a new study write that “[t]he International Federation of Gynecology and Obstetrics classifies a variation in cycle length as normal if less than 8 days.” Also, stress can cause cycle lengths to
This normal variability may be concerning 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 in addressing concerns about possible menstrual side effects.
Researchers funded by the National Institutes of Health (NIH) recently set out to investigate the relationship between menstrual cycle length and COVID-19 vaccination.
Medical News Today spoke with Dr. Victoria Male, a lecturer in reproductive immunology at Imperial College London in the United Kingdom 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 experience, on average, a delay to their next period of 2 days.”
“However, their periods returned to normal within two cycles. People receiving their first dose do not experience a delay to their periods, and those receiving their second dose experience on average less than half a day’s delay,” she added.
“The findings were reassuring and reinforced the overall safety of COVID-19 vaccines, as it described a mild and temporary impact on the menstrual cycle length of vaccinated study participants. The authors found that the cycle length change was less than 1 day and temporary, resolving within two post-vaccination cycles,” said Dr. Amy Roskin, chief medical officer at The Pill Club — a digital health company — who was not involved in the study.
The study appears in Obstetrics and Gynecology.
The researchers collected menstrual cycle data gathered between October 2020 and September 2021 from the fertility-awareness application Natural Cycles.
The researchers analyzed participant data from six consecutive cycles — three prior to the vaccine and three post-vaccine, or six cycles for those who remained unvaccinated.
All in all, 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 cycle length increase of 0.64 days after an initial vaccine compared with 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 when compared with unvaccinated individuals.
The researchers added that the increase in cycle lengths for both the first and second vaccine cycles may be largely driven by 358 people who received two vaccine doses within the same cycle.
They note that this group experienced an average 2.38 days increase in cycle length compared with unvaccinated individuals.
By cycle six, the last of the recorded cycles, the researchers noted no significant differences in cycle length between the vaccinated and unvaccinated groups.
The researchers say that their results cannot be explained by generalized pandemic stress as their control group saw no changes 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 menstrual timing.
They say that the findings among individuals who received two vaccine doses in the same cycle support this possibility.
Prof. Alison Edelman, a professor of obstetrics and gynecology at Oregon Health and Science University in Portland, and lead author of the study, told MNT:
“Additional research is necessary to determine the causal relationship between COVID-19 vaccines and changes to menstrual cycle length.”
“However, based on previous research that confirms a biological connection between the immune system and the reproductive system, we can hypothesize that vaccine-related menstrual disturbances may be related to the body’s acute immune response to the vaccine,” she added.
“We know that currently available COVID-19 vaccines are effective at activating the immune system. This immune response creates a temporary increase in small proteins — called cytokines — which can briefly impact a person’s ability to regulate the menstrual cycle, potentially resulting in temporary changes to cycle timing.”
– Prof. Alison Edelman
Dr. Mike Armour, a senior research fellow in reproductive health at Western Sydney University in Australia, who was not involved in the study, expanded on the effects of stress on the menstrual cycle. He told MNT:
“Firstly, an increase in stress levels causes a decrease in [the] release of follicle-stimulating hormone and luteinizing hormone, decreasing progesterone levels in the luteal phase and subsequently increasing prostaglandin production. Secondly, cortisol and adrenaline levels are increased during periods of stress, mediated via an increase in adrenocorticotropic hormone. Both increased cortisol and increased adrenaline affect prostaglandin synthesis.”
“This can affect cycle length 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 those receiving two doses of the vaccine in one cycle supports this theory,” he explained.
The researchers found no clinically meaningful changes in menstrual cycle length associated with the COVID-19 vaccines.
The researchers note that there are some limitations to their study. Prof Edelman said: “Users of the menstrual cycle tracking applications are not necessarily reflective of a general population. We know in this dataset that this population is more college-educated, white, and has a lower [body mass index (BMI)] than the national distribution.”
“BMI is a factor that can influence menstrual cycle variability. However, that may have been offset by the inclusion criteria for our dataset — needing three cycles pre-vaccine that met the criteria for normal cycle length,” she added.
The researchers further wrote that their data does not include a sufficient number of cycles to understand the long-term effects of vaccinations on cycle length.
“The most important [limitation] when looking at this from an international perspective is that this study [mainly] looked at those who received mRNA vaccines such as [Pfizer-BioNTech] and Moderna and specifically excluded those using Oxford-AstraZeneca, therefore we can’t say if those receiving the Oxford-AstraZeneca would have the same changes in their cycle as those reported,” explained Dr. Armour.
Despite these limitations, the results are likely to be reliable due to the study design.
Dr. Male said: “Although there have already been a handful of studies on how COVID vaccination may affect the menstrual cycle, all the ones we have so far have been retrospective. This means 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 logging their cycles using an app,” she continued. “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 of these findings.”
Dr. Edelman concluded: “These findings are reassuring and validating. On a population level, the changes we are finding indicate no cause for concern for long-term physical or reproductive health, and they do not indicate any reason to avoid vaccination. On a personal level, these findings provide the information that people want to know about what to expect when they get vaccinated.”