Research Library Published Research

Published Research

Does the Covid vaccine affect your cycle?

Illustration of a vaccine vile inside a circle covered in tiny dots

Key takeaways

  • On average, there was no significant changes to cycle length for those who received the vaccine across two or more cycles 
  • Those who received both doses of the vaccine within one cycle saw an average increase in cycle length of two days

When the Covid vaccines became available, one of the questions that kept coming up was: do Covid vaccines have an effect on the menstrual cycle?

We asked our users to log in the app if they had been vaccinated or not, which type of vaccine they got and when they received their vaccinations. We then compared the cycle length and period length for vaccinated and unvaccinated, to see if there was a difference between cycles before and after vaccination, and if there was a difference in cycle length between the two groups.

This study, which included 4000 users in the US, found that, on average, cycle length changed by less than one day after vaccination. Interestingly, if a user got two vaccine doses within the same cycle, the average change in cycle length increased to two days. However, this change was temporary and went away in the following cycles. There was no change in average period length.

These results are reassuring to those getting vaccinated. When looking at a large group, there will always be outliers – individuals that do experience larger changes – but they are in the minority. Changes to cycles are common, but if you are worried about changes to your health after vaccination, you should always speak to your healthcare professional.

In conclusion, there was a small change in cycle length found within this group as a whole, but because the difference was less than one day, the majority of users didn’t experience any noticeable change in their cycle.

We are continuing our research into the Covid vaccines and the menstrual cycle, and we are now in the process of analyzing data from the rest of the world. We will also be going into more detail to investigate if there is any difference in results depending on vaccine type or when in the cycle the vaccine is given.