Following an outbreak of variola in New York City early in 1947, more than five million people were vaccinated. Among these there were observed 4,172 women in the first trimester of pregnancy. They gave birth to 68 children with malformations, an incidence of 1.63%. A control, nonvaccinated group of 2,186 women in the same period of pregnancy, gave birth to 30 malformed infants, an incidence of 1.37%.
The vaccinated group gave birth to 343 premature infants, a rate of 8.2%, while the nonvaccinated group gave birth to 185 prematures, a rate of 8.5%.
Deaths from congenital malformations in New York City during the four months representing the first trimester of pregnancy of the observed women numbered 259, a rate of 48.8 per 10,000 live births. In the corresponding months of the preceding year the number of deaths was 290, a rate of 47.2 per 10,000 live births.
There was no apparent increase in the ratios of stillbirths to total births in 1947, during the months when the stillbirths may have been conceived, over the ratios of the corresponding months of the previous year.
There is no evidence from these studies that smallpox vaccination of women in the first trimester of pregnancy has any deleterious effect on the developing embryo.
Mass Smallpox Vaccination and Cardiac Deaths, New York City, 1947