There is a new paper, published last week, on vaccines and menstrual periods. This paper follows up on some previous work and addresses the question of whether periods were heavier/earlier/disrupted by the COVID mRNA vaccines.
The study covers about 39,000 people who were surveyed about their post-vaccination periods. The headline statement is that 42% of people reported heavier menstrual flow after vaccination, 14% reported lighter or no change in flow, and 44% reported no change. The authors also reported breakthrough bleeding among a substantial share (about two-thirds) of the 1,800 people who do not typically menstruate (mostly those who are on long-acting contraceptives like IUDs).
The authors in this paper are very straightforward about what the paper can and cannot accomplish. What they make clear at the end, and I agree is important, is that the sample here is not randomly selected. The authors found their sample through various word of mouth, Twitter, media coverage, etc. (It is possible I advertised it in the newsletter, although I cannot find any evidence!) All of this is to say that the approach to recruitment isn’t random, and as a result it is difficult to draw any conclusions about magnitudes.
Bottom line: it does not seem at all likely to me (or, I think, to the authors) that 42% of women who get the COVID vaccine will have heavier menstrual bleeding. The authors make a strong point about the value of this analysis, noting that while there is no reason to think any menstrual cycle disruptions are a risk, studying it carefully will normalize and reassure women who do experience that. The remaining missing piece is a more systematic understanding of the size of the risks. For that, we’ll need a different sample and likely more questions on standard variability in cycle.