Covid-19: They rule out the relationship between the vaccine against covid-19 and the absence of a period

There appears to be no causal relationship between the Pfizer and Moderna covid-19 vaccines and cases of amenorrhea (absence of menstruation). This has been determined by the Pharmacovigilance Risk Assessment Committee (PRAC) of the European Medicines Agency (EMA). In Spain alone, 4,712 notifications of gynecological disorders have been registered.

After evaluating all available data, including results from the scientific literature and cases of amenorrhea reported to the European Database of Adverse Events Following Administration of Moderna or Pfizer Vaccines, the PRAC has determined that ‘the available data do not support a causal association and an update to the product information for either of the two vaccines.” In any case, the EMA has announced that the PRAC “will continue to monitor this issue carefully” and has asked Moderna and Pfizer to include this issue in their next periodic safety update reports for their vaccines. On the other hand, the PRAC has advanced that it “continues to evaluate” cases of intense menstrual bleeding (heavy periods) with Moderna and Pfizer vaccines.

Vallivana Rodrigo Casares, a researcher in Clinical Trials in the Vaccine Research Area, points out the issues that can cause the absence of menstruation. Before determining the causes, we must take into account that menstrual cycles are not always the same. Although the average cycle is 28 days, the International Federation of Gynecology and Obstetrics considers normal cycles between 24 and 38 days, as well as bleeding up to 8 days.

The female reproductive system is regulated by hormonal interaction between the hypothalamus, pituitary gland, and ovaries. This interaction is known as the HHG axis. Stressors from daily life, the environment, and health (for example, very extreme diets, intense exercise, infections, or medications) can influence this system and disrupt the release of hormones.

Thus, menstrual changes can be due to various reasons. In the case of vaccines, messenger RNA vaccines (Comirnaty-Pfizer and Spikevax-Moderna) produce a lot of immunological activity and create a very robust immune response. This can be a stressful factor that causes transient disturbances.

There is little literature on the matter and studies are still lacking to ensure an association between both variables. Unfortunately, menstruation is not usually a factor considered in clinical trials. One of the reasons is that most studies, to prevent pregnancy from occurring during trials (with the risks that this entails), require women to take highly effective contraceptives, usually hormonal. Therefore, it is very difficult to detect menstrual variations during the development of a drug, since its cycles are artificial.

Until now, many women have noticed variations in their cycles, even if they had previously had similar episodes regardless of their vaccination status, although all of them have been temporary and not very serious. However, more time is needed to observe short-term and long-term phenomena in a larger population.