Mild and asymptomatic influenza B virus infection among unvaccinated pregnant persons: Implication for effectiveness of non-pharmaceutical intervention and vaccination to prevent influenza.

Abstract

We estimated symptomatic and asymptomatic influenza infection frequency in community-dwelling unvaccinated pregnant persons to inform risk communication. We collected residue sera from multiple antenatal-care blood draws during October 2016-April 2017. We determined influenza infection as seroconversion with≥4-fold rise in antibody titers between any two serum samples by improved hemagglutinin-inhibition assay including ether-treated B antigens. The serology data were linked to the results of nuclei acid testing (rRT-PCR) based on acute respiratory illness (ARI) surveillance. Among all participants, 43%(602/1384) demonstrated serology and/or rRT-PCR evidenced infection, and 44%(265/602) of all infections were asymptomatic. ARI-associated rRT-PCR testing identified only 10%(61/602) of total infections. Only 1%(5/420) of the B Victoria cases reported ARI and had a rRT-PCR positive result, compared with 33%(54/165) of the H3N2 cases. Among influenza ARI cases with multiple serum samples, 19%(11/58) had seroconversion to a different subtype prior to the illness. The incidence of influenza B infection in unvaccinated pregnant persons is under-estimated substantially. Non-pharmaceutical intervention may have suboptimal effectiveness in preventing influenza B transmission due to the less clinical manifestation compared to influenza A. The findings support maternal influenza vaccination to protect pregnant persons and reduce consequent household transmission.