Now researchers at five institutions, backed by funding from the National Institutes of Health, will be conducting yearlong studies to examine any possible connections between vaccination and irregular menstruation, and to help allay concerns that might prevent women from getting their shots.

The evidence around abnormal periods is so far purely anecdotal. There is no known link between vaccination and changes in menstruation, and public health experts reiterate that vaccines are safe, effective and necessary to end the pandemic.

But the stories underpin a persistent data gap about reproductive health and women’s menstrual cycles that is not collected during clinical trials, including during trials of the Covid vaccines. There have also been no scientific studies published examining a potential relationship between the two.