Influenza vaccine: a safe bet for mother and baby.

Viral influenza is a common and potentially life-threatening infection. In every influenza epidemic in the 20th and 21st centuries, older patients, immunocompromised patients, and pregnant women have suffered disproportionate morbidity and mortality. In 2009, a novel strain of the influenza virus, H1N1, emerged. As with regular seasonal flu, it also caused major complications in obstetric patients and, surprisingly, in children under the age of 18. In a recent report from Louie et al, the number of maternal deaths attributed to H1N1 in the initial phase of the epidemic was 4.3/100,000 live births. Moreover, as noted by Jamieson et al, the rate of hospitalization for treatment of H1N1 infection has been significantly higher in pregnant women compared with nonpregnant women. In recognition of the dangers associated with influenza in pregnancy, the American College of Obstetricians and Gynecologists and the Centers for Disease Control and Prevention have strongly recommended that all women who will be pregnant during influenza season, regardless of the trimester of pregnancy, receive the seasonal and H1N1 influenza vaccine at the appropriate time. Despite these recommendations, however, influenza vaccination rates in pregnant women remain disappointingly low. One of the major explanations for low vaccine-acceptance rates is apprehension or misconception on the part of physicians and patients alike about the safety of the vaccine—for both the mother and the fetus or neonate. Two articles in this issue of Obstetrics & Gynecology should go a long way toward alleviating this concern. Nordin et al (see p. 519) conducted a retrospective observational cohort study of 75,906 pregnant women who received the regular trivalent seasonal flu vaccine during the 2002–2003 through the 2008–2009 influenza seasons. These women were matched in a ratio of 1:2 with unvaccinated women. One major endpoint of the investigation was the frequency of medically attended adverse events within 3 days of vaccination: allergic reaction, cellulitis, fever, malaise, limb soreness or swelling, rash, seizure, and altered mental status. The second key endpoint was the frequency of medically attended adverse reactions within the first 42 days of vaccination, including various neurologic disorders, psychosis, myocarditis, pericarditis, and thrombocytopenia. Twenty-eight percent of women were vaccinated in the first trimester, 44% in the second, and 27% in the third. Overall, there were no significant differences in the frequency of adverse events in vaccinated compared with unvaccinated women. When only those women vaccinated in the first trimester were considered, again, there were no significant differences in the frequency of adverse outcomes. This study did not address the risk of vaccine-associated fetal injury. Because of the time period in which it was conducted, it could not address the safety of the H1N1 vaccine. By design, it did not address the safety of the live flu vaccine. In addition, despite the very large sample size, it was underpowered to detect significant differences in extremely rare events See related articles on pages 511 and 519.