Research: Covid Vaccines May Have Had A Significant Negative Impact On Women's Fertility
Data from the Czech Republic show a significant decline in successful conception rates, i.e., conceptions resulting in live births nine months later, among women vaccinated against Covid.
The authors of a study published in the scientific journal International Journal of Risk & Safety in Medicine point out that a decline in birth rates following the Covid crisis is a fact in many countries. They note that, although several studies have shown that Covid-19 vaccination has no significant effect on female fertility and that vaccines are safe during pregnancy, adverse effects on the menstrual cycle are well documented. According to the study’s authors, this suggests that Covid vaccines may have affected women’s fertility after all.
To test this hypothesis, the researchers examined official data from the Czech Republic and analyzed the rates of successful conceptions (SC), i.e., conceptions resulting in live births approximately nine months later, among women who were either vaccinated or unvaccinated against Covid-19. The data was collected for women aged 18-39 from January 2021 to December 2023. Based on this, the numbers of SCs per month per 1000 women were calculated for preconception-vaccinated and unvaccinated women, respectively, as well as the number of SCs per month per 1000 women for all women aged 18–39 years.
During the period in question, there were approximately 1.3 million women aged 18-39 in the Czech Republic, and the proportion of women vaccinated against Covid-19 increased steadily in 2021, reaching a stable level of 70% by the end of the year. The Pfizer-BioNTech and Moderna vaccines were used to vaccinate 96% of women in the Czech Republic, with the majority receiving the Pfizer vaccine (11:1).
The authors showed in their study that, from at least June 2021, SCs per 1000 women were considerably lower among women who were vaccinated compared with those who were unvaccinated. Furthermore, SC rates for the vaccinated group were much lower than expected based on their proportion of the total population. Throughout 2022, SC rates remained approximately 1.5 times higher for women who were unvaccinated compared with those who were vaccinated.
“We conclude that at least from June 2021, SC rates in the Czech Republic for women vaccinated against Covid-19 before SC were substantially lower than for those who were unvaccinated before SC. These hypothesis-generating and preliminary results call for further studies of the influence of Covid-19 vaccination on human fecundability and fertility,” the authors write in the conclusion of their study.
The authors note that, although their sample is representative and covers the entire country, the possibilities for drawing conclusions are limited because the Czech government did not share several important data points with them or because these data were unclear for other reasons.
“While the strength of our study is its nationwide unselected sample of fertile women, the observed association between decreased SC rates and COVID-19 vaccination is, of course, not proof of a causal relationship between vaccination and fecundability,” they write. “For example, it is possible that more women who wished to become pregnant, that is, achieve SC, chose not to be vaccinated, and/or that more women who did not plan to become pregnant opted for vaccination. Indeed, such self-selection bias is compatible with the increase in SCs of the women unvaccinated before SC in the second half of 2021,” the authors explain.
At the same time, the authors acknowledge that deferring vaccination was against sanctioned national public recommendations in the Czech Republic at the time. “If this type of self-selection bias had been the predominant underlying factor for the observed reduction in SC rates in vaccinated women, the total fertility rate for the entire population would have remained relatively constant throughout the study period. In contrast, the total fertility rate in the Czech Republic decreased from 1.83 births per 1000 women in 2021 to 1.62 in 2022, and 1.45 in 2023, and, therefore, self-selection bias does not seem to fully explain the observed association between vaccination status and SC rates. Notably, we and others have previously found a batch-dependent safety signal for the Comirnaty vaccine (Pfizer-BioNTech) in the Czech Republic, Denmark, Sweden, and the US, and the possibility of a disproportionate influence of early batches of this vaccine on SC rates remains to be investigated,” authors write.
The authors also point out other limitations. “For example, unmeasured confounders such as the age distribution of the 18- to 39-year-old women, socioeconomic and lifestyle factors, comorbidities and sexual health, effects of individual COVID-19 vaccine products or vaccine boosters, concurrent COVID-19 infection, preconception fertility, contraception use, pregnancy loss, stillbirths, and paternal health and vaccination status. Unfortunately, we were not granted access to this information. Also, the estimated SC rates for vaccinated women were uncertain, mainly in the early phase of the vaccination campaign. In addition, the Czech Republic population is almost exclusively Caucasian, and results may not apply to other ethnicities,” the authors write.