Earlier in the pandemic, stories circulated about people noticing changes to their period after getting a COVID-19 vaccine.
Back when COVID-19 vaccines were still new, there was little information about how they impacted menstrual cycles, but now medical research is catching up.
The latest offering is a new study published by British Medical Journal Medicine this week which looked at menstrual cycle data from nearly 20,000 participants across multiple countries.
It's an expansion of an earlier study of nearly 4,000 participants in the US.
In essence, this newer and larger study found that COVID-19 vaccines were associated with slight changes to menstrual cycles.
"These changes are small compared with normal variation and resolve in the cycle after vaccination, except in people who received both doses in one menstrual cycle," the researchers said.
How did the study work?
Researchers looked at the data from consenting users of period tracking app Natural Cycles.
The app has users log temperature readings to determine their fertility status based on where they are in their menstrual cycle.
The study had 19,622 participants all up, with 14,936 being vaccinated and 4,686 being unvaccinated.
Each participant continued data from at least four menstrual cycles.
For the vaccinated group, researchers looked at the three menstrual cycles prior to getting their first vaccine dose and subsequent consecutive cycles through to the cycle after their second shot.
For the unvaccinated group, the researchers looked at the four to six consecutive cycles over a similar time period.
What happened to periods after getting the vaccine?
The study found that, on average, participants' cycle length increased by less than a day — technically 0.71 days — after the first dose.
After the second dose, the increase was 0.56 days.
Participants who had both shots within a single cycle had a 3.91 day increase.
But that was just the average — researchers said 6.2 per cent of vaccinated participants experienced a change in cycle length for eight or more days.
For the cycle after vaccination, cycle length had increased by an average of 0.02 days for participants who had one dose per cycle and and 0.85 days for two doses in one cycle.
While the average menstrual cycle is around 28 or 29 days, changes aren't uncommon.
"Teenagers might have cycles that last 45 days, whereas women in their 20s to 30s might have cycles that last 21 to 38 days," a Victorian Department of Health fact sheet says.
In fact, the International Federation of Gynaecology and Obstetrics says a variation in cycle length of less than eight days is normal.
"Our research was not designed to determine why these changes might happen; these changes are probably due to temporary vaccine-related activation of immune response but more research is needed," the researchers said.
Did the vaccine type matter?
It doesn't look like it.
"Cycle length changes due to COVID-19 vaccination appear similar across the different vaccine types," researchers said.
They included nine vaccine types in the study, inlcuding mRNA and vector vaccines.
- Pfizer-BioNTech
- Moderna
- Oxford-AstraZeneca
- Covishield
- Johnson & Johnson
- Sputnik V
- Covaxin
- Sinopharm
- Sinovac
Did they find anything about heavier periods?
This particular research doesn't go into that.
But it did say that, while cycle lengths increased, the number of bleeding days didn't.
"We found no differences in menses length in any group of vaccinated individuals, compared with the unvaccinated cohort," researchers said.
A different study published in July showed 42 per cent of its participants said they bled more heavily than usual after receiving a vaccine — but 44 per cent reported no changes.
The study, which was based on a survey of nearly 40,000 participants, also looked at post-vaccination breakthrough bleeding for respondents who don't typically menstruate.
Of those, 71 per cent of participants on long-acting reversible contraceptives had breakthrough bleeding, as did 66 per cent of postmenopausal respondents and 39 per cent of people on gender-affirming hormones.
"We emphasise that menstrual bleeding changes of this nature are generally not indicative of changes to fertility," researchers said.
"Generally, changes to menstrual bleeding are not uncommon or dangerous, yet attention to these experiences is necessary to build trust in medicine."
But both studies only show an association between the vaccine and menstrual changes — not that the shots cause changes.
In a piece for The Conversation, PhD candidate in biological anthropology at University of Oxford Gabriella Kountourides said we needed clinical trials that followed people a longer period of time to make those kinds of conclusions.
What about period pain?
The Natural Cycles study didn't look into that either.
The researchers said that's what studies should focus on next.
"Future work should assess other aspects of changes to menstrual cycles, such as unexpected vaginal bleeding, menstrual flow and pain, and define the mechanism by which the post-vaccination menstrual changes described here occur," they said.
Is there going to be more research about this?
The scientists behind the Natural Cycles study hope so.
"Although small changes in menstrual characteristics might not be meaningful to clinicians and scientists, any perceived effect to a routine bodily function linked to fertility can cause alarm for those experiencing it, and can contribute to vaccine hesitancy," the said.
"Even small changes, when unanticipated, can have a large adverse impact on the quality of life of people who menstruate and who experience episodes of social embarrassment, anxiety related to uncontained bleeding or fertility planning or prevention, and worry about what bleeding changes mean for their overall health.
"The absence of evidence about vaccines and menstrual health coupled with the long standing sex specific research inequities can also be interpreted by the public as a dismissal from the scientific and medical community."
Researchers in the July study also want work done in this area.
They want people to be armed with more knowledge about how their menstrual cycles could change when they get vaccinated so they're prepared.
"Menstruation is seldom considered a variable during vaccine trials aside from determining last menstrual period as part of established protections against volunteers being or getting pregnant," they said.
"Recognising and attending to this emerging phenomenon of bleeding changes can help bolster trust between people who menstruate and medical providers, which is an area that has a long history of medical misogyny and gaslighting.
"Current and historic focus on fertility and reproduction in research and clinical trials is insufficient for addressing the changes in bleeding patterns that cause concern in many people.
"We urge other researchers and funding bodies to increase investment in understanding queer, trans, and nonbinary menstrual experiences."
Editor's note (October 4, 2022): This article has been changed to clarify that the report was from BMJ Medicine.
