Risk of spontaneous abortion after mRNA COVID-19 vaccination received just prior to or during pregnancy: Complete data from CDC COVID-19 vaccine pregnancy registry.

Data on spontaneous abortion after COVID-19 vaccination during pregnancy are limited. We used data from the Centers for Disease Control and Prevention's COVID-19 Vaccine Pregnancy Registry (C19VPR) to assess risk of spontaneous abortion (SAB) among women receiving ≥1 mRNA COVID-19 vaccine up to 30 days before last menstrual period or during pregnancy. We calculated risk of SAB from 6 to 20 weeks' gestation using a fetuses-at-risk approach (n = 12,097). Pregnancies entered the risk pool at gestational age at vaccination. Risk of SAB in C19VPR was compared with pre-pandemic estimates. Cox proportional hazard models were used to compare risk by vaccine manufacturer. Cumulative risk of SAB was 10.79% (95%CI 10.01, 11.56). Risk of SAB among Moderna recipients was similar to Pfizer-BioNTech (adjusted hazard ratio 0.92; 95%CI 0.79, 1.07). Risk estimates of SAB were below or within pre-pandemic estimates. Generalizability may be limited due to homogeneity of the C19VPR cohort.