Some reproductive-aged individuals remain unvaccinated against COVID-19 due to concerns about potential adverse effects on fertility. We examined the associations of COVID-19 vaccination and SARS-CoV-2 infection with fertility among couples trying to conceive spontaneously using data from an internet-based preconception cohort study. We enrolled 2,126 self-identified females residing in the U.S. or Canada during December 2020-September 2021 and followed them through November 2021. Participants completed questionnaires every 8 weeks on sociodemographics, lifestyle, medical factors, and partner information. We fit proportional probabilities regression models to estimate associations between self-reported COVID-19 vaccination and SARS-CoV-2 infection in both partners with fecundability, the per-cycle probability of conception, adjusting for potential confounders. COVID-19 vaccination was not appreciably associated with fecundability in either partner (female FR=1.08, 95% CI: 0.95, 1.23; male FR=0.95, 95% CI: 0.83, 1.10). Female SARS-CoV-2 infection was not strongly associated with fecundability (FR=1.07, 95% CI: 0.87, 1.31). Male infection was associated with a transient reduction in fecundability (FR=0.82, 95% CI: 0.47, 1.45 for infection within 60 days; FR=1.16, 95% CI: 0.92, 1.47 for infection >60 days). These findings indicate that male SARS-CoV-2 infection may be associated with a short-term decline in fertility and that COVID-19 vaccination does not impair fertility in either partner.

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Measurement error is pervasive in epidemiologic research. Epidemiologists often assume that mismeasurement of study variables is non-differential with respect to other analytic variables and then rely on the heuristic that “non-differential misclassification will bias estimates towards the null.” However, there are many exceptions to the heuristic for which bias towards the null cannot be assumed. In this paper, we compile and characterize seven exceptions to this rule and encourage analysts to take a more critical and nuanced approach to evaluating and discussing bias from non-differential mismeasurement.

Objective To evaluate the association between pregravid use of a variety of contraceptive methods and subsequent fecundability. Design Prospective cohort study. Setting Denmark and North America, 2007-19. Participants 17 954 women who had tried to conceive for up to six menstrual cycles at study entry. At baseline, participants reported their contraceptive histories, and personal, medical, and lifestyle characteristics. Main outcome measures Pregnancy, determined by bimonthly follow-up questionnaires for up to 12 months. Results Approximately 38% (n=6735) of participants had recently used oral contraceptives, 13% (n=2398) had used long acting reversible contraceptive methods, and 31% (n=5497) had used barrier methods. Women who had recently stopped using oral contraceptives, the contraceptive ring, and some long acting reversible contraceptive methods experienced short term delays in return of fertility compared with users of barrier methods. Use of injectable contraceptives was associated with decreased fecundability compared with use of barrier methods (fecundability ratio 0.65; 95% confidence interval 0.47 to 0.89). Users of injectable contraceptives had the longest delay in return of normal fertility (five to eight menstrual cycles), followed by users of patch contraceptives (four cycles), users of oral and ring contraceptives (three cycles), and users of hormonal and copper intrauterine devices and implant contraceptives (two cycles). Lifetime length of use of hormonal contraceptive methods was not associated with fecundability. Conclusions Use of some hormonal contraceptive methods was associated with delays in return of fertility, with injectable contraceptives showing the longest delay. The findings indicated little or no lasting effect of long term use of these methods on fecundability.

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