The 2009 pandemic influenza A/H1N1 virus has a predilection for the lower respiratory tract. In some cases, infection results in a pneumonitis and severe acute respiratory distress syndrome, which can be difficult to manage despite advanced ventilatory techniques. The effects of H1N1 infection on pregnant women became cause for concern early in the course of the current pandemic because of initial experience1 and data from previous pandemics.2 3 4 In the linked study (doi:10.1136/bmj.c1279), the ANZIC influenza investigators provide a detailed analysis of a cohort of pregnant women who developed critical illness as a result of pandemic H1N1 infection in 2009.5

During pregnancy changes occur to a woman’s immune system to facilitate tolerance of paternally derived fetal antigens. While the pregnancy is recognised by the maternal immune system, the immune response is characterised by an altered TH1-TH2 (type 1-type 2 T helper cell) balance. The maternal immune response favours humoral immunity (TH2 response), and suppresses cell mediated immunity …