Extremely obese people had a higher chance of dying than did others infected with the H1N1 swine flu virus that became a pandemic last winter.

Many critically ill patients with pandemic influenza A (H1N1) infection were noted to be obese, but whether obesity, rather than its associated co-morbidities, was an independent risk factor for severe infection was not known. Researchers analysed data from 534 adults hospitalised in California with H1N1 flu during the first four months of 2009 and found that extreme obesity was associated with a nearly three-fold higher risk for death.

Extreme obesity is defined as having a body-mass index (BMI) of 40 or greater. BMI is a statistical measure of whether a person is normal, overweight or obese based on whether weight and height are in proportion. The study did not include data on people who were pregnant or younger than 20 years.

However, further research is needed to study why extremely obese people were more likely to die from H1N1 infection.