RENO, NEV. — The influenza vaccine is both safe and effective for women in the second half of pregnancy, results of a large prospective study suggest.
When given at least 2 weeks before exposure, the vaccine reduced the rate of influenza 19-fold, with no evidence of worsening in several obstetric and neonatal outcomes, including premature rupture of membranes, stillbirth, low birth weight, neonatal pneumonia, neonatal death, and major malformations.
The study, which was conducted at the University of Texas Southwestern Medical Center at Dallas, included 2,889 women who received the inactivated influenza vaccine during the 2003-2004 flu season, which began earlier than usual and was moderately severe. They were compared with 1,988 gestational age-matched pregnant women who did not get vaccinated during the same time period, Jeanne S. Sheffield, M.D., and her associates wrote in a poster presented at the annual meeting of the Society for Maternal-Fetal Medicine.
Six women in the vaccinated group (2.4 per 1,000 women) and 13 women in the nonvaccinated group (6.5 per 1,000 women) developed laboratory-confirmed influenza, a statistically significant difference. The overall efficacy of the vaccine was 68%.
But when the analysis was restricted to women who developed influenza more than 2 weeks after vaccination (the time required to develop immunity), there was only one case of influenza (0.4 per 1,000 women) in the group, yielding an efficacy of 94%. The relative risk of developing influenza decreased almost 19-fold when the vaccine was given 2 weeks before exposure.
There were some statistically significant differences between the groups, study co-investigator Scott W. Roberts, M.D., told this newspaper.
Women in the vaccination group were seen more frequently than the controls, they had more repeat cesarean deliveries and cases of dystocia, and larger body-mass indexes.
The estimated gestational age (EGA) at delivery was slightly, but significantly, higher in the vaccination group (39.6 weeks), compared with the unvaccinated group (39.4 weeks).
The vaccinated group had significantly fewer births with EGAs of 36 weeks or less, compared with the unvaccinated group. Infants whose mothers were vaccinated were significantly less likely to be admitted to the intensive care nursery.
Women in the vaccinated group were significantly more likely to undergo a cesarean delivery (27% vs. 23%). Dr. Roberts said that most or all of these differences may reflect the fact that women who were seen in the clinic more often were more likely to choose vaccination.
When given at least 2 weeks before exposure to influenza, the vaccine dramatically reduced the rate of infection. Lynda Banzi