Finally We Know How COVID Vaccines Impacted Periods

Since the start of the pandemic there has been much speculation about how COVID-19 impacts menstruation. From the stress of the lockdowns to catching the virus to the vaccines, women have reported noticing changes to their monthly cycles. 

Until very recently there was little to no data available on the subject but now research is starting to emerge. A new study from the fertility tracking app Natural Cycles has looked at whether or not the COVID-19 vaccines impacted people’s periods. 

This is a much-discussed topic. In September 2021 it was revealed that more than 30,000 people had reported to the MHRA Yellow Card scheme (where you can report the side effects of medicines) that they have had changes to their menstrual cycle after having the COVID-19 vaccine. These included heavier than usual periods, delayed periods and unexpected bleeding after all three major vaccines (Moderna, AstraZeneca and Pfizer). 

After reviewing the reports, the Medicines and Healthcare products Regulatory Agency (MHRA) said it "does not support a link" between COVID vaccines and the symptoms. In response, the Royal College of Obstetricians and Gynaecologists called for more research. 

Today, that research comes from Natural Cycles (which is approved by the FDA (Food and Drug Administration) as contraception in the United States and costs £49.99 a year in the UK). It will doubtless be the first of many published studies but as with many aspects of women’s health, there is a lack of data on menstruation because in the UK the NHS does not collect information about women’s periods in a centralised way. The most easily available and large-scale numbers come from period tracker apps, which is why they are first to report back. 

Published today in the journal Obstetrics & Gynecology (known as The Green Journal), Natural Cycles research has analysed the menstrual cycles of nearly 4,000 US-based Natural Cycles users to determine if the COVID-19 vaccines changed cycle length or period length. It did not look at the suspected and reported side effects mentioned above. 

Researchers compared cycles of vaccinated (2,403) individuals and unvaccinated (1,556) individuals. Those who were vaccinated received mostly Pfizer-BioNTech (55%, Moderna 35%, Johnson & Johnson 7%) and all who participated in the study gave consent to use their de-identified data. 

The results found that people did experience changes. There was an average change of less than one day in overall cycle duration following vaccination and no changes to period length. In the rare instance that a user received two vaccine doses within the same menstrual cycle, Natural Cycles found that the average change in cycle length may increase to two days but these changes are temporary and appear to resolve quickly in subsequent cycles. 

"This research is reassuring given the many questions around the impact of the COVID-19 vaccines on the menstrual cycle and the lack of evidence to answer these questions prior to our study," said Dr Jack Pearson, Natural Cycles’ lead medical researcher and on-staff medical expert. "The menstrual cycle is often overlooked and under-researched, especially when considering what a key indicator it is for a woman’s overall health."

There are caveats here, of course. Firstly, this is only a survey of Natural Cycles users in the United States. Secondly, an average calculation flattens the numbers. Natural Cycles told Refinery29 that there was a subgroup within the 4,000 users of about 400 people who did experience "a more significant change".

As for why the vaccine has impacted menstruation, Dr Pearson told Refinery29: "Changes in menstrual cycles are common and are often a natural indicator of stress, sickness or changes in lifestyle and environment. While we can’t say for sure why minor changes occurred as a result of getting the vaccine, it’s very reassuring to see that the minor changes were temporary. It also emphasises something we hear from Natural Cycles users a lot, which is that being able to track their cycle so closely is a powerful tool."

This piece of research follows another published by Natural Cycles at the end of 2021 which found that pandemic-related stress did not significantly impact menstruation for the majority of their users. Ultimately, the fact remains that more data is needed on periods and more research is required to understand the full impact of all aspects of the pandemic on women and people who menstruate.