Is there a Better Time in the Menstrual Cycle to get Vaccinated against COVID-19?

Plus! Some old research on vaccine side effects and the menstrual cycle

I have been asked a lot about the best time of the menstrual cycle to get vaccinated against COVID-19. The motivation here seems to be about avoiding side effects, specifically menstrual irregularities. Meaning, if the vaccine does cause menstrual irregularities, is there a time in the cycle that will lessen those effects

Here’s the answer. The best time of your menstrual cycle to get vaccinated is today. The second best time is tomorrow. The third best time is the day after tomorrow. 

You get my point. Just look at the rates of positive tests and hospitalizations in the United States in this image from The New York Times COVID-19 Tracker (August 22, 2021). 

While there have been many people who have reported they have had a heavy period or an early period or a more painful period post vaccination against COVID-19, we don’t yet know if that is a true cause and effect. I explore all the possibilities in this post I wrote back in April. The European Medicine Agency released an updated report on the vaccines on August 6 and their current conclusion is no cause and effect between the vaccines and menstrual disturbances has been established. You can read what they had to say here

I am open to the possibility that the vaccine may cause temporary menstrual irregularities with no long-term consequences, but until it is studied we will not know if there is a cause and effect. What I do know for sure is hospitals in many states are overwhelmed with COVID-19. The Delta strain is highly infectious. Vaccines reduce the risk of catching SARS-CoV-2 if exposed, of getting very sick with COVID-19 if you do catch the virus, and reduce the risk of spreading it to other people. Basically, the vaccine is going to keep you alive and help keep other people alive.

This is an urgent situation, so the best time to get vaccinated is simply now.

In addition, the vaccine is safe in pregnancy and it does not cause infertility, so there is no reason to delay. There is information about what what we know about fertility and pregnancy here. More than 357 million doses of COVID-19 vaccines have been administered so far in the United States alone, so if there were serious health repercussions, we’d know. The only medical reason for someone not to get vaccinated is a true allergy to one of the ingredients. The next step if you have a history of allergic reaction to some of the ingredients is to consult an allergist who can assess your risk and almost certainly help you get vaccinated safely. 

It is a fair question to ask about the timing of the COVID-19 vaccine and the menstrual cycle. After all, the menstrual cycle is intertwined with the immune system. The endometrium (lining of the uterus) is part of the immune system (read more about that here) and regulatory T cells, a specific type of T cells, vary throughout the menstrual cycle (they are highest in the first part of the cycle before ovulation, or the follicular phase) and lowest after ovulation (during the luteal phase). Other immune cells, such as B cells and monocytes, also vary during the menstrual cycle. In addition, some people report flares in autoimmune conditions depending on the time in their menstrual cycle, although it's important to point out this mechanism may not be immune mediated. For example, it’s possible that the fluctuations in hormones could affect pain receptors or muscle spasm.

The point is it is biologically plausible that menstrual timing of vaccination could impact side effects and/or the robustness of the response to any vaccine. In fact, one vaccine has been shown to have more side effects based on menstrual cycle timing, the rubella vaccine. After the vaccine was introduced there were reports of more side effects among women versus men, specifically joint pain.

The rubella vaccine is a live attenuated vaccine, meaning the virus has been weakened in such a way so it doesn’t cause an infection, but it still stimulates the immune system to produce antibodies. Vaccine researchers took the reports of joint pain seriously as there is always the possibility with a live attenuated vaccine that a mild form of the disease could happen, and arthritis is a known complication of rubella. You can find one of the original articles on rubella vaccination and side effects from 1970 here

As there was a suggestion that the menstrual cycle might be related to side effects, several studies addressed not just the risk of joint symptoms, but when those symptoms might develop depending on the menstrual timing of vaccination. I find it amazing that researchers were thinking about the menstrual cycle and vaccines in the 1970s and 1980s and then this research seemed to stop. I wonder if there was more interest with the rubella vaccine as a live attenuated vaccine has potential to cause disease and at that time the risk to the fetus of inadvertent vaccination during an early pregnancy was largely unknown (although we need to move beyond this the menstrual cycle only matters because of pregnancy thinking!). It’s also possible the researchers considered joint symptoms real symptoms and menstrual irregularities something to suck up. 

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What did these papers find? Overall, it seems some of the rubella vaccines were associated with joint pains as a side effect and others not as much. You can read two of the papers that addressed the menstrual cycle with regards to side effects here and here. This difference between vaccines was attributed to the degree of attenuation (basically, how weakened the virus was, meaning those that weren’t quite as weak caused more rubella-like symptoms, which intuitively makes sense). Several of the studies showed joint pain was more common with vaccination in the luteal phase (after ovulation). Some of the studies suggested birth control might blunt this effect, meaning side effects were more common on days 16-28 for those women not taking the birth control pill. In all of these studies the robustness of the response to the vaccines, meaning how well the vaccine stimulated the immune system, was the same regardless of menstrual timing of vaccination.

One study from Japan took evaluating the menstrual cycle very seriously. The women completed menstrual calendars for 3 months before and after vaccination and also measured basal body temperature for one month before and after vaccination (that means charting temperature daily to monitor for the spike in temperature seen with ovulation). In addition, progesterone levels were measured weekly for 6 weeks after vaccination (progesterone levels rise with ovulation). Unfortunately, there is no mention in the study if the vaccine impacted menstrual regularity. While 18% of the women did not ovulate the cycle after the vaccine, the researchers didn’t report what percentage of cycles before vaccination weren’t ovulatory. 


What does this mean for COVID-19 vaccination? Nothing, because the rubella vaccine is very different from the COVID-19 vaccine. However, it's nice to know that researchers took reports of joint pain from women seriously. And the idea that timing a vaccine during certain phases of the menstrual cycle to lessen some side effects is perhaps an avenue to explore for vaccines going forward. Certainly, menstrual timing of vaccination should be recorded in studies going forward.  

Why haven’t researchers looked at menstrual timing and menstrual irregularities with the COVID-19 vaccine? It is possible that some data has been collected and it is just unpublished. It’s also possible they didn’t think about it because they didn’t feel it was a “significant” side effect. It’s possible that so many people were on contraception that measuring a true effect on the menstrual cycle simply wasn’t possible within the study population. And it’s also possible that the researchers only viewed the reproductive cycle from a pregnancy perspective as opposed to many people live with this every month for thirty or more years perspective.

While menstrual cycle timing and vaccinations opens up many intriguing paths for research, until we have more data, the best time to get vaccinated is simply now.