A “striking” number of women are becoming seriously ill after contracting H1N1, according to a new study, and researchers admit they have no explanation.

Although it’s been known that pregnant women — along with the young and elderly — are at increased risk, the authors of the report, Critically Ill Patients With 2009 Influenza A (H1N1) Infection in Canada, write “the tendency of females to develop severe 2009 influenza A (H1N1) infection in this series is striking,” especially since “female susceptibility has not been observed in other influenza outbreaks.”

And they can’t say why.

“The explanation for increased risk of severe disease and death among females in this report is unclear,” the authors admit, “but the role of pregnancy as a risk factor has been noted in previous influenza pandemics.”

About 67 per cent of the cases under study in the Canadian report were women. The study also found that, in one survey of 168 critically ill patients, 29 died, the majority in two weeks. The majority of the deaths, 72 per cent, were women.

Similar results were found last month after the Montreal public health department tracked cases from April to September, and found about 56 per cent of the 1,280 confirmed cases of H1N1 were women.