Nobody in the covid vaccine trials asked about periods

They found their answer in Natural Cycles, an app where women who don't use hormonal contraception track their cycle. The study took in 19,622 users aged 18 to 45, from the UK, Europe, the US, Canada and elsewhere. Of these, 14,936 had been vaccinated and 4,686 had not.

In the cycle with the first dose, length grew on average 0.71 days more than in unvaccinated women over the same stretch; with the second, 0.56 days. Less than a day. The type of vaccine barely mattered. A shift of eight days or more, the size that falls outside normal variation, happened to 6.2% of vaccinated women and 5% of unvaccinated ones. Cycles stretched most for the 743 women who got both doses in a single cycle: by 3.7 days compared with unvaccinated women. For women with one dose per cycle, the next cycle was back on time. Periods themselves did not get longer.

The data come from 2020–21 and cover the first two doses; the study says nothing about boosters. Women with irregular cycles or on any hormonal contraception weren't in the sample, covid infections couldn't be accounted for, and flow, pain and spotting weren't measured. Nor did the study find out why cycles shift; the authors suspect a passing immune response. A cycle answers to what happens in the whole body, and that answer deserved measuring from day one.