Study Finds Lower Conception Rates Among Women Who Got mRNA COVID Shots
- by Amber Baker
- Published
- Risk & Failure Reports
A recent large-scale population study conducted in the Czech Republic and published this month in the International Journal of Risk & Safety in Medicine has found that women who received a COVID-19 biologic prior to conception were significantly less likely to achieve a successful pregnancy compared to their unvaccinated peers. According to authors of the study, “the Comirnaty (Pfizer/BioNTech) and Spikevax (Moderna/NIAID) mRNA vaccines comprised 96% of all administered COVID-19 vaccine doses, with an 11:1 ratio in the use of these two vaccines.” Using nationwide health data from the Czech Republic between 2021 and 2023, the researchers tracked conception rates, defined as pregnancies that resulted in live births, among women aged 18 to 39 who did and did not receive the mRNA biologic prior to conception.1
From mid-2021 onward, monthly conception rates among women who received the mRNA biological were consistently and substantially lower—by as much as 1.5 times—than those of women who did not get the injection, despite women who got the mRNA shots comprising roughly 70 percent of the reproductive-age population. Notably, the contribution of the group of women who got the mRNA biological to the nation’s monthly birth totals was far below what would be expected based on their population share.1
Earlier studies that found no impact on fertility primarily focused on women who were actively trying to conceive and often relied on self-reported data—a method that overlooks women not planning pregnancy or those who experienced subtle reproductive disruptions. In contrast, the Czech study used comprehensive national health data, including official vaccination records and birth statistics for all women aged 18 to 39 across the country. This allowed researchers to analyze real-world outcomes drawn from national health records, rather than self-reported data.1
As a result, this study offers a more complete and accurate picture of conception trends in the general population, not just among those actively planning pregnancy. The researchers also addressed the possibility of self-selection bias: the idea that women hoping to become pregnant may have delayed or avoided getting the mRNA COVID shots. However, if that were the only explanation, the country’s overall fertility rate should have stayed the same. Instead, the national fertility rate declined steadily, falling from 1.83 births per woman in 2021 to 1.62 in 2022 and 1.45 in 2023. As the authors explain, “self-selection bias does not seem to fully explain the observed association between vaccination status and SC [successful conception] rates.”1
Years of Research Has Linked mRNA COVID Shots to Menstrual Irregularities and Lower Birth Rates
Although most regulatory agencies and public health experts have consistently asserted that mRNA COVID shots do not affect fertility, this new data adds weight to a growing body of research exploring the potential links. A 2023 Global Research Data analysis found that birth rates declined across 19 European countries following peak COVID shot uptake, with particularly steep drops occurring several months after mass rollout campaigns. In Switzerland, for example, the decline was described by researchers as “unprecedented in modern history,” exceeding even drops seen during World Wars and the introduction of birth control.2
Separately, a 2024 study published in NPJ Vaccines demonstrated direct effects of mRNA COVID biologics on human ovarian granulosa cells, which play a critical role in hormone regulation and egg development. The authors of that study proposed this mechanism as a possible explanation for the widespread reports of menstrual irregularities following receipt of the mRNA COVID shots.3
Menstrual changes became common enough in the years following the rollout of COVID shots that BMJ Sexual & Reproductive Health published a 2024 review confirming a consistent pattern of cycle length changes, increased bleeding, and pain—symptoms widely reported by women that were not systematically tracked in early public health data or vaccine studies. While some researchers have described these effects as temporary, the review acknowledged that such changes “point to potential transient disruption of hormonal signaling pathways,” which may plausibly influence fertility outcomes on a broader scale.4
Czech Study Limitations Include Untracked Miscarriages and Early Losses
The Czech study only measured successful conceptions that resulted in live births, meaning that miscarriages and other forms of pregnancy loss were not captured. This is a key limitation, as any potential fertility impact linked to vaccination might also manifest in other outcomes, such as implantation failure, early loss, or miscarriage—factors the study was not designed to detect.1
Although authors of formal studies to date have concluded that there is no causal link between COVID shots and miscarriage, most research has focused on pregnancies that were already clinically confirmed. As such, early pregnancy losses that occur before a pregnancy is medically confirmed, such as through a clinical test or ultrasound, remain underexplored in COVID shot safety data.1
Clinical Trials to Authorize COVID Shots Did Not Assess Fertility-Related Outcomes
Fertility-related outcomes were not thoroughly evaluated in the clinical trials used to authorize mRNA COVID biologicals. The trials did not monitor menstrual health, sperm quality, hormone levels, or actual conception rates, leaving a major gap in the early safety profil6e. Similarly, preclinical (animal) testing was limited and did not include comprehensive reproductive toxicity assessments.5 6
This omission is acknowledged in Section 13.1 of the respective COVID shot package inserts, which uniformly state that the product has “not been evaluated for the potential to cause carcinogenicity, genotoxicity, or impairment of fertility.” No COVID shot package insert explicitly states that fertility has been thoroughly studied or confirmed safe in either male or female subjects. As a result, potential impacts on human reproduction were not part of early evaluations of COVID shot safety, fueling calls for independent, post-marketing investigations—as noted by the study authors in the recent nationwide investigation from the Czech Republic.5 6
The lack of early fertility-related safety data for COVID shots reflects a broader pattern in biomedical research, where women’s reproductive health has often received limited attention in trial design and data reporting. Despite comprising more than half the population, women—particularly pregnant women—have historically been excluded from clinical research.
A 2022 study by Harvard Medical School found that women made up only about 40% of participants in clinical trials for major diseases that disproportionately affect them, including cancer, cardiovascular disease, and psychiatric disorders. As Harvard’s Dr. Barbara Bierer explains, “These are issues that are very important that do affect a product’s safety and effectiveness.”7
Online Searches For COVID Shot-Related Fertility Concerns Increased in 2021
Beyond general concerns about side effects, fertility fears have ranked among the top reasons for vaccine hesitancy among adults of reproductive age. A 2021 Andrologia study documented a massive surge in internet searches related to fertility after COVID shots became available. Search terms like “COVID vaccine fertility” rose by 710 percent, “COVID vaccine infertility” by 264 percent, and “COVID vaccine fertility CDC” by an astounding 2,943 percent in the weeks immediately following the rollout of the shots.8 9
These sharp increases in public concern emerged at a time when official data on reproductive outcomes were scarce. The authors of the Andrologia study concluded that the search trends likely reflected widespread anxiety about vaccine safety concerning fertility, particularly among younger adults.9
Calls for Further Investigation Into the Impact of mRNA COVID Shots on Fertility
The authors of the Czech study state that their findings do not prove a causal relationship between the receipt of the COVID biological and lower successful conception rates in women who have gotten the shot. However, they point out that there is enough evidence to raise serious, data-driven questions about the potential negative effects of mRNA COVID shots on fertility, questions that have remained largely unaddressed in large-scale, population-level research – even as global birth rates decline and public skepticism toward recommendations of health officials grows.
The authors emphasize that the observed association is meaningful enough to justify further investigation, writing: “These hypothesis-generating and preliminary results call for further studies of the influence of COVID vaccination on human fecundability and fertility.” Fecundability refers to the likelihood of conceiving within a given menstrual cycle—a critical metric that has been largely absent from early vaccine research.1
While regulatory agencies have consistently recommended that all healthy women and men get COVID shots, including pregnant women, U.S. Department of Health and Human Services (DHHS) Secretary Robert F. Kennedy, Jr. proposed a federal ban in June 2025 on administering the shots to pregnant women, citing the absence of long-term fertility safety data and calling for a more precautionary approach.10
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