A previous study in California provided evidence that pregnant women were much more likely to die of pandemic H1N1 than other people. Now, a new study based on information from the Australian and New Zealand Intensive Care (ANZIC) Influenza Investigators registry, provides further information documenting and quantifying this risk.
From the study:
The overall risk of admission to an intensive care unit for non-pregnant women of childbearing age was 1 in 35 300 compared with 1 in 14 600 for gestations less than 20 weeks, 1 in 2700 for gestations of 20 weeks or more, and 1 in 5500 for the postpartum period. Compared with non-pregnant women of childbearing age, pregnant women with a gestation of 20 weeks or more had a 13-fold greater risk of admission to an intensive care unit as a result of 2009 H1N1 infection (relative risk 13.2, 95% confidence interval 9.6 to 18.3). The corresponding risk for postpartum women was 6.4 (2.6 to 15.7) and for women with a gestation of less than 20 weeks was 2.4 (1.3 to 4.6). Overall, compared with non-pregnant women of childbearing age the relative risk of women who were pregnant or post partum being admitted to an intensive care unit with 2009 H1N1 infection was 7.4 (5.5 to 10.0).
Overall, seven women (11%) died. Generally, influenza has been an extremely rare cause of maternal death. Between 1997 and 2005 in Australia, 24 maternal deaths occurred as a result of any infection, or 1.1 deaths per 100 000 births. Only one of these deaths was due to influenza. No maternal death were attributed to influenza in the United Kingdom between 1997 and 2005….a maternal mortality of 11% is high when compared with any other obstetric condition.
Pregnant women, particularly in the second half of pregnancy, are more likely than non-pregnant women to develop critical illness associated with 2009 H1N1 influenza. Among women who developed critical illness, the outcomes were poor, including death of the mother or baby.
This study indicates that pandemic H1N1 significantly increased the odds of critical illness and death among pregnant women in Australia and New Zealand in 2009.
The number of pregnant women who were infected with pandemic H1N1 in Australia and New Zealand in 2009 is not known. However, preliminary reports suggest that the number of adults infected was a relatively small percent of the total population. If pandemic H1N1 gains the ability to spread more efficiently among adults, we can therefore expect a much higher amount of critical illness and death among pregnant women unless they have been effectively vaccinated or receive prompt treatment with antivirals.
The Australian and New Zealand Intensive Care (ANZIC) Influenza Investigators (2010) Critical illness due to 2009 A/H1N1 influenza in pregnant and postpartum women: population based cohort study. BMJ, 340:c1279