Although up to 500,000 people world-wide die of severe influenza each year, there has been no clear evidence about who is susceptible for influenza complications and it may not be who people think, says a study from McMaster University.

This is important because issues during past influenza seasons and pandemics have included vaccine shortage; the time needed to develop vaccines for specific influenza strains and which groups are first in line for vaccination.

New mothers and obese people, two groups not typically regarded as risk groups, were found to have a higher risk of death and other severe outcomes from influenza, according to the global study sponsored by the World Health Organization.

But, in contrast to current assumption, ethnic minorities such as American Aboriginal People and pregnant women were not found to have more complicated influenza and would not need priority vaccination.

The report is published online in the BMJ, the journal of the British Medical Association.

“Policy makers and public health organizations need to recognize the poor quality of evidence that has previously supported decisions on who receives vaccines during an epidemic,” said Dr. Dominik Mertz, lead author of the study and an assistant professor of medicine of McMaster’s Michael G. DeGroote School of Medicine.

“If we can define the risk groups we can optimally allocate vaccines, and that is particularly important when and if there is vaccine shortage, say during a new pandemic.”

The researchers reviewed 239 observational studies between 1918 and 2011, looking at risk factors for complications of influenza including developing pneumonia or needing ventilator support, admission to hospital or its intensive care unit or dying.

“These data reinforce the need to carefully define those conditions that lead to complications following infection with influenza,” said Dr. Mark Loeb, senior author on the paper. He is also a microbiologist and professor of medicine of the Michael G. DeGroote School of Medicine.

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