Key Findings: One reason for vaccine hesitancy is concern about potential negative effects on fertility. Since reproductive toxicity was not assessed in clinical trials for COVID-19 vaccines, and SARS-CoV-2 has been associated with reduced sperm parameters, a research team evaluated pre- and post-vaccination sperm parameters following mRNA vaccination. The study results were recently published in the latest issue of JAMA.

Currently, two mRNA vaccines—BNT162b2 (Pfizer-BioNTech) and mRNA-1273 (Moderna)—have received Emergency Use Authorization from the U.S. Food and Drug Administration. Despite demonstrating high efficacy and few adverse effects in clinical trials, only 56% of Americans express willingness to receive vaccination.

One reason for vaccine hesitancy is concern about potential negative effects on fertility. Since reproductive toxicity was not assessed in clinical trials for COVID-19 vaccines, and SARS-CoV-2 has been associated with reduced sperm parameters, a research team evaluated pre- and post-vaccination sperm parameters following mRNA vaccination. The study results were recently published in the latest issue of JAMA.

This single-center prospective study by the University of Miami recruited healthy volunteers aged 18 to 50 who planned to receive the COVID-19 mRNA vaccine. The study was approved by the University of Miami Institutional Review Board, and written informed consent was obtained from all participants.

Researchers pre-screened male participants to ensure they had no underlying fertility issues. Those with COVID-19 symptoms or positive test results within 90 days were excluded. Participants provided semen samples after 2 to 7 days of abstinence—before the first vaccine dose and approximately 70 days after the second dose. Semen analysis was conducted by trained andrologists following World Health Organization guidelines, assessing semen volume, sperm concentration, sperm motility, and total motile sperm count (TMSC), including four participants with oligospermia (sperm concentration <15 million/mL).

Between December 17, 2020, and January 12, 2021, 45 men volunteered for the study, with follow-up samples collected at a median of 75 days after the second dose. The study concluded on April 24, 2021. Baseline samples were obtained after a median abstinence period of 2.8 days, while follow-up samples were collected after a median abstinence of 3 days. Among the 45 men, 21 (46.7%) received BNT162b2 vaccination, and 24 (53.3%) received mRNA-1273. Baseline median sperm concentration was 26 million/mL, and TMSC was 36 million/mL. After the second vaccine dose, median sperm concentration significantly increased to 30 million/mL, and median TMSC rose to 44 million. Semen volume and sperm motility also showed significant improvements.

Eight of the 45 men had oligospermia before vaccination (median concentration: 8.5 million/mL [IQR, 5.1–12]). Among these eight, seven achieved normal sperm concentration ranges at follow-up, while one remained oligospermic. No participant developed azoospermia post-vaccination.

In this study of sperm parameters before and after COVID-19 mRNA vaccination, none of the sperm parameters showed a significant decline in this small group of healthy men. Since the vaccine contains mRNA rather than live virus, it is unlikely to affect sperm parameters.

While these results showed statistically significant increases in all sperm parameters, the magnitude of change fell within the range of normal individual variability and may be influenced by regression to the mean. Additionally, this increase could be attributed to prolonged abstinence before the second sample collection. Men with asthenospermia did not experience further decline.