Objective. Clinicians and regulators are receiving reports of changes to
menstrual periods following COVID-19 vaccination. However, it is unclear
if the two are biologically linked. If they are, people using hormonal
contraception are predicted to be less likely to report a change and
spontaneously cycling people vaccinated prior to ovulation more likely.
The objective was to test these hypotheses. Design. Retrospective cohort
study. Setting. UK. Population. 1273 people who had received at least
one dose of a COVID-19 vaccination, have periods or withdrawal bleeds
and keep a record of the dates of these. Methods. Participants reported
whether they use any hormonal contraception and, for each dose of the
vaccine, on which day of their menstrual cycle they were vaccinated and
details of how the timing and flow of their next period compared to
their normal experience. Main outcome measures. Association between 1.
the use of hormonal contraception and reported changes to timing or flow
of the next menstrual period, and 2. the timing of vaccination within
the menstrual cycle and reported changes to timing or flow of the next
menstrual period. Results. The data from this cohort did not support the
pre-specified hypotheses that people using hormonal contraception would
be less likely to report a change, or that spontaneously cycling people
vaccinated prior to ovulation would be more likely to report a change.
Conclusions. This study did not detect strong signals supporting the
idea that COVID-19 vaccination is linked to menstrual changes in most
people. Funding. No specific funding.