New mothers and obese people have a higher risk of death and other severe outcomes from influenza, according to a new study out of McMaster University.

The research, published in BMJ, the journal of the British Medical Association, suggests that people with a body mass index (BMI) of 30 or higher and women who have recently given birth are more susceptible to flu complications such as the development of pneumonia.

American Aboriginal people and pregnant women are, contrary to current assumptions, not higher risk.

This is an important finding, according to the study's lead author, Dr. Dominik Mertz, because of vaccination shortages in past flu seasons.

Dr. Mertz, an assistant professor of medicine at McMaster, said that women late in their pregnancy should still have priority for vaccination, but risk factors are at their highest not during the pregnancy but in the "first four weeks post partum."

He also told CBC News that the study "did not find any evidence" that Aboriginal people are at a higher risk of severe flu outcomes.

"The data suggest that Aboriginal people in North America and Australia and Hispanic people in the United States are more likely to be admitted to hospital," but not more likely to have severe outcomes that require admittance to intensive care.

Dr. Mertz said that the medical assumptions that vaccination priority lists are based upon rely on a "striking lack of evidence."

The researchers reviewed 239 studies conducted between 1918 and 2011 to reach their results.