A large, population-based study of 2009 pandemic influenza a virus subtype H1N1 infection diagnosis during pregnancy and outcomes for mothers and neonates

Background. Pregnant women were at increased risk for serious outcomes of 2009 pandemic influenza A virus subtype H1N1 (influenza A[H1N1]pdm09) infection, but little is known about the overall impact of the pandemic on neonatal and maternal outcomes.Methods.We identified live births that occurred from 1 July 2008 through 31 May 2010 in 5 Kaiser Permanente regions. Pregnant women were considered to have influenza if they had a positive result of a laboratory test for influenza virus or if they received a diagnosis of influenza during a period in which seasonal influenza virus or A(H1N1)pdm09 was the predominant circulating virus.Results.There were 111 158 births from 109 015 pregnancies involving 107 889 mothers; 368 pregnant women (0.3) received a diagnosis of influenza due to seasonal virus, and 959 (0.9) received a diagnosis of influenza due to A(H1N1)pdm09; 107 688 did not receive an influenza diagnosis. Pregnant women with influenza due to A(H1N1)pdm09 were more likely than women with seasonal influenza infection to be hospitalized within 30 days of the diagnosis (27 vs 12; odds ratio [OR], 2.84 [95 confidence interval CI, 2.01-4.02]). Pregnant women with A(H1N1)pdm09 who started antiviral treatment ≥2 days after the diagnosis were significantly more likely to be hospitalized than those who started antiviral treatment <2 days after diagnosis (OR, 3.43 [95 CI, 1.55-7.56]). Mothers with seasonal influenza virus infection had an increased risk for having a small-for-gestational-age infant (OR, 1.59 [95 CI, 1.15-2.20]).Conclusions.In this large, geographically diverse population, A(H1N1)pdm09 infection increased the risk for hospitalization during pregnancy. Late initiation of antiviral treatment was also associated with an increased risk for hospitalization.