Czech data doesn’t show COVID-19 mRNA vaccines reduce fertility, contrary to John Campbell video

On 11 May 2025, YouTuber and retired nurse educator John Campbell posted a YouTube video claiming data from Czechia (more widely known as the Czech Republic) suggested COVID-19 vaccines reduce women’s fertility. The video was viewed more than 120,000 times.

Campbell, who has spread misinformation about COVID-19 and vaccines, has more than 3.2 million subscribers to his YouTube channel.

Claims that COVID-19 vaccines pose a threat to human fertility aren’t new. Science Feedback debunked some of these claims in previous reviews.

This time around, the claim is based on a preprint (a study that has yet to be peer-reviewed and published) that first appeared in April 2025. The first author of the preprint is Vibeke Manniche, a physician who previously co-authored a letter that allegedly found a “batch-dependent safety signal” for the Pfizer COVID-19 mRNA vaccine

A review from Science Feedback explained that the batch analysis by Manniche et al. was highly flawed and that the data didn’t support the conclusion in the letter. In addition, Manniche is another figure who spread misinformation about COVID-19, as fact-checks from Danish fact-checking group TjekDet explained. In December 2024, Manniche was found guilty of fraud by a Danish court for issuing COVID-19 test exemptions that she wasn’t authorized to issue.

The preprint was also promoted by other outlets known to publish misinformation, such as Slay News and Natural News. The writer of the Slay News article is listed as Frank Bergman, who isn’t a real person, as our recent investigation of the website showed. Natural News has published health misinformation and conspiracy theories for years. Cardiologist Peter McCullough, another source of COVID-19 and vaccine misinformation, also made a similar claim on the website America Out Loud.

But Campbell’s videos and these articles fail to inform users about the limitations of the preprint, and that published studies don’t support the claim. We explain below.

What the preprint reported

The preprint analyzed national data from the Czech Republic to estimate the number of what it called “successful conceptions” in women aged 18 to 39 years old who were either unvaccinated or vaccinated prior to conception. The data analyzed spanned the period from January 2021 to December 2023. “Successful conceptions” were defined as conceptions that led to live births nine months later.

Based on its analysis, the preprint reported that successful conceptions were “considerably lower” for women who were vaccinated compared to unvaccinated women. It concluded by saying that its results “call for further studies of the potential influence of COVID-19 vaccination on human fecundability and fertility”.

It should be noted that there are two versions of the preprint. The first one incorrectly compared the lower bound for successful conceptions in vaccinated women with the upper bound for unvaccinated women, thereby inflating the difference in successful conceptions between the two.

The second version of the preprint corrected this error. This is most visually evident in Figure 1B, which reported the range of estimated values for successful conception in unvaccinated and vaccinated groups, rather than point values. It’s worth noting that in his video, Campbell used the incorrect Figure 1B from the first version of the preprint, rather than the corrected Figure 1B in the second version (posted on 7 May 2025), even though the second version was already available before he published his video on 11 May 2025.

Preprint didn’t account for confounding factors that affect childbearing, like age and education level

To better understand the scientific credibility of the preprint, Science Feedback reached out to Victoria Male, a senior lecturer in reproductive immunology at Imperial College London.

In an email, Male explained that while unvaccinated women may have been more likely to have a baby compared to vaccinated women, this in itself doesn’t mean vaccination harms fertility. “To find out if that’s the case, we need to take account of other ways in which the vaccinated and unvaccinated groups differ,” she wrote.

Indeed, these differences may constitute what researchers call confounders or confounding factors. Confounders are variables that influence the outcome being studied (in the case of the preprint, “successful conception”) but are unrelated to the variables being studied (in the preprint’s case, the effect of COVID-19 vaccines on fertility).

One potential confounder is age. Male pointed out that older individuals were more likely to get vaccinated and were also less likely to have children, in part because people generally become less fertile with age, but also “because older people are more likely to have completed their families”.

“This is something that can be accounted for in national cohort studies, but the authors have not done so here,” Male wrote.

Another potential confounding factor is educational attainment. In a YouTube video, scientist Susan Oliver underscored how women’s education levels are correlated with the number of children they have. She cited a study conducted in Czechia that looked at what factors were correlated with COVID-19 vaccine refusal^[1].

That study found, among other things, that one of the differences between people who tended to get vaccinated and people who chose not to get vaccinated was education level: those who had a university education were more likely to get vaccinated compared to those with lower levels of education.

This is coupled with the fact that women who have a university degree tend to have fewer children. Indeed, a correlation between higher educational attainment in women and a decline in fertility rate has generally been observed across the world, not just in Czechia. This isn’t because university education affects women’s biology and thereby causes women to become less fertile, but because having a higher education level translates to various socioeconomic changes that influence women’s childbearing decisions.

Male also raised questions about the unusual methods that the preprint authors used to estimate successful conception rates.

“In a national cohort study of pregnancy outcomes, the authors would usually provide a detailed description of how vaccination and birth records were linked for individuals, steps taken to ensure that vaccination happened before conception for each individual in the dataset and some kind of mathematical treatment to take account of individuals moving from the ‘unvaccinated’ to the ‘vaccinated’ group as they get vaccinated. None of this is done here, so I don’t have a lot of confidence that the rates reported are even correct.”

The methods used in the preprint to estimate successful conception rates were also dissected in an analysis by mathematician Peter Hegarty. Hegarty stated that “there is no attempt made in the paper to match cohorts and there may be all kinds of statistically signficant(sic) differences between the vaccinated and unvaccinated populations”.

Both Male and Hegarty acknowledged the preprint’s main finding that vaccinated women generally have fewer children compared to unvaccinated women could be a genuine trend. However, both found no credible evidence showing that this trend is because the COVID-19 mRNA vaccines reduce fertility.

Published studies haven’t found that COVID-19 vaccination reduces fertility

The preprint stated that its results “call for further studies of the influence of COVID-19 vaccination on human fecundability and fertility”.

There have been numerous studies on this subject. Male pointed to multiple studies that looked at whether COVID-19 vaccination is associated with a difference in the ability to conceive in patients undertaking in vitro fertilization. These studies found no association between COVID-19 vaccination and biological markers related to fertility and ability to conceive^[2-8].

Science Feedback also addressed earlier iterations of the infertility narrative in several reviews, such as inaccurate claims about the placental protein syncytin-1 and the alleged accumulation of spike protein in ovaries.

Conclusion

A preprint by Manniche et al. reported finding fewer births in vaccinated women compared to unvaccinated women in Czechia. However, articles and social media posts portraying this as evidence that COVID-19 vaccines reduce fertility are misleading. The preprint didn’t account for various factors, like age and education, that also correlate with childbearing and COVID-19 vaccination status. These articles and posts contradict published studies, which didn’t find an association between COVID-19 mRNA vaccination and reduced fertility.

REFERENCES

• 1 – Zidkova et al. (2023) COVID-19 Vaccination Refusal—Which Factors are Related in the Czech Republic, One of the Most Affected Countries in the World? International Journal of Public Health.
• 2 – Aharon et al. (2022) In Vitro Fertilization and Early Pregnancy Outcomes After Coronavirus Disease 2019 (COVID-19) Vaccination. Obstetrics and Gynecology.
• 3 – Morris and Morris. (2021) Exposure of ovaries to COVID-19 vaccination does not impair fertility. Fertility and Sterility.
• 4 – Avraham et al. (2022) Coronavirus disease 2019 vaccination and infertility treatment outcomes. Fertility and Sterility.
• 5 – Jacobs et al. (2022) Fresh Embryo Transfer Cycle Characteristics and Outcomes Following In Vitro Fertilization via Intracytoplasmic Sperm Injection Among Patients With and Without COVID-19 Vaccination. JAMA Network Open.
• 6 – Albeitawi et al. (2022) COVID-19 infection and vaccine have no impact on in-vitro fertilization (IVF) outcome. Scientific Reports.
• 7 – Aizer et al. (2022) The effect of coronavirus disease 2019 immunity on frozen-thawed embryo transfer cycles outcome. Fertility and Sterility.
• 8 – Miller et al. (2024) Does the SARS-CoV-2 mRNA vaccine and its serum IgG levels affect fertility treatments and obstetric outcomes? An observational cohort study. Clinical and Experimental Medicine.