Do COVID vaccines cause 42% of menstruators to experience heavier bleeding?

The short answer is: it’s hard to tell from the survey data from Lee at al.

The actual survey can be found here (under Survey Materials) and the paper can be found here.

One thing that I noticed is that the paper doesn’t really describe the survey accurately. The survey asked participants if their period was “Lighter than usual”, “Heavier than usual”, or “About the same”.

For some reason, the paper reported “Lighter than usual” as “Not heavier”. It’s also weird that a lighter period isn’t considered to be some sort of bizarre vaccine side effect but heavier menstruation is.

The second issue is that we don’t know what the baseline rates are. If for example a survey asked participants about their last 10 periods, how many of them would have been rated as heavier, lighter, etc.? I’m not sure what we’re supposed to compare the data to.

A third issue is that Moderna doesn’t have a dose effect compared to Pfizer (Moderna generally causes more side effects and contains a higher dose of active ingredient). Perhaps Moderna doesn’t have a dose effect when it comes to menstruation issues. However, it seems weird that Moderna did not cause more side effects.

I’m not sure if there are reliable findings to be gleaned from this survey.