The level of evidence is upgraded by consistent findings in better quality studies, although no randomized studies are included.
A systematic review included fifty-three studies (16 cohort studies including 2 563 519 participants, 33 case-control studies including 44 536 participants, and 4 cross-sectional studies including 12 098 221 participants). Of these, 7 cohort studies (44%), 26 case-control studies (79%), and all 4 cross-sectional studies were deemed to have a moderate to high risk of bias. Among studies deemed to have a low risk of bias, a weak association was found among cohort studies (7 studies; adjusted rate ratio, 1.05; 95% CI, 1.02 to 1.09; P < .001) and a similar but nonsignificant association was found among case-control studies (6 studies; adjusted odds ratio, 1.06; 95% CI, 0.88 to1.29; P = .54). Effect estimates were further from the null when studies with a moderate to high risk of bias were included. Associations between vasectomy and high-grade prostate cancer (6 studies; adjusted rate ratio, 1.03; 95% CI 0.89 to1.21; P = .67), advanced prostate cancer (6 studies; adjusted rate ratio, 1.08; 95% CI, 0.98 to 1.20; P = .11), and fatal prostate cancer (5 studies; adjusted rate ratio, 1.02; 95% CI, 0.92 to 1.14; P = .68;) were not significant (all cohort studies). Based on these data, a 0.6% (95% CI 0.3% to 1.2%) absolute increase in lifetime risk of prostate cancer associated with vasectomy and a population-attributable fraction of 0.5% (95% CI 0.2%-0.9%) were calculated.