Anthrax vaccine appears to have no effect on pregnancy rate or outcome

Mar 29, 2002 (CIDRAP News) – Anthrax vaccination in more than 3,000 nonpregnant US Army women had no effect on pregnancy or birth rates and no discernible impact on the risk of adverse birth outcomes, according to a study published in this week's Journal of the American Medical Association.

"These results do not support the hypothesis of a decrease in pregnancy rates nor an increase in fetal loss rates or adverse fetal outcome among those receiving anthrax vaccination prior to pregnancy," states the report by Andrew R. Wiesen, MD, MPH, and Christopher T. Littell, DO. However, the study did not assess the effect of anthrax vaccination during pregnancy, and there were too few adverse birth outcomes to rule out a small effect of the vaccine on the risk of such outcomes.

The authors examined vaccination and pregnancy data on all Army women between the ages of 17 and 44 years who were stationed at two Georgia installations (Fort Stewart and Hunter Army Airfield) any time between January 1999 and March 2000. The Department of Defense launched a program in 1998 to vaccinate all military personnel, but the program was cut back in 2000 because of a vaccine shortage.

Of 4,092 women who met the study criteria, 3,136 received at least one dose of anthrax vaccine, and most received two or three doses. There were 385 pregnancies in 28, 815 person-months of follow-up among women who had had at least one dose of vaccine, for a pregnancy rate of 159.5 per 1,0000 person-years. Unvaccinated women had 130 pregnancies in 9,734 person-months of follow-up, for a rate of 160.0 per 1,000 person-years. This yielded a pregnancy-rate ratio between vaccinated and unvaccinated women of 0.94 (95% confidence interval [CI], 0.8-1.2; P=.60).

In comparing pregnancy outcomes for the two groups, the authors found that vaccinated women were 1.2 times as likely to give birth as unvaccinated women (95% CI, 0.8-1.8; P=.52). This odds ratio remained the same after adjust for age, marital status, and race.

Among 327 births for which complete ICD-9 coding data were available, 11 infants were of low birth weight. Vaccinated women were 1.3 times as likely as unvaccinated women to have a low-birth-weight infant, but this finding was nonsignificant (95% CI, 0.2-6.4; P=.72). The odds ratio for structural abnormalities in babies born to vaccinated women was 0.7 (95% CI, 0.2-2.3; P=.71), and the overall odds ratio for any adverse birth outcome in this group, after adjustment for age, was 0.9 (95% CI, 0.4-2.4; P=.88). The number of low-birth-weight infants in the vaccinated group was about half the 7.5% rate seen nationwide, but this may have been due to the young age of the women, the article states. The rate of structural abnormalities in the babies was in line with national rates.

Given the small number of adverse birth outcomes, the study lacked the statistical power to rule out a small effect of vaccination on adverse birth outcome, the authors state. "A post hoc power analysis showed the study only had a 12% power to detect a 20% increase in adverse birth outcomes, based on potential effects on likelihood of pregnancy," the report says.

Wiesen AR, Littell CT. Relationship between prepregnancy anthrax vaccination and pregnancy and birth outcomes among US Army women. JAMA 2002;287(12):1556-60
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