New research published in JAMA found there was no difference in sperm parameters in 45 men who received mRNA vaccines against COVID-19. The two authorized mRNA vaccines, which were developed by Pfizer Inc. /zigman2/quotes/202877789/composite PFE -0.15% and Moderna Inc. /zigman2/quotes/205619834/composite MRNA +8.73% , are the most commonly utilized COVID-19 vaccines in the U.S. Researchers at the University of Miami Miller School of Medicine say they initiated the study after hearing concerns from some people that the vaccines could impact fertility. They noted the study has some limitations, given that the number of men enrolled in the study is a small sample size, and semen analysis is "an imperfect predictor of fertility potential." Back in April, concerns about fertility were cited as a top reason to avoid COVID-19 vaccination, according to the Kaiser Family Foundation , with 42% of adults between the ages of 18 and 29 saying they have heard the shots can cause infertility and 5% saying they believe this is true.