Risk factors linked to severe H1N1 pandemic flu infection are similar to those for seasonal flu, with some distrinct differences, such as younger age groups and overweight/obesity, scientists reported in the journal PLoS Medicine.

This research study assessed the frequency and distribution of already established and newly found potential risk factors of severe influenza pandemics - a total of 70,000 H1N1 hospitalized patients were enrolled across 19 countries from April 2009 to January 2010.

The scientists analyzed and interpreted data collected in surveillance programs conducted by the Ministries of Health or National Public Health Institutes in the following 19 countries: Argentina, Australia, Canada, Chile, China, France, Germany, Hong Kong (administrative region) Japan, Madagascar, Mexico, The Netherlands, New Zealand, Singapore, South Africa, Spain, Thailand, United States, and United Kingdom.

They concluded that risk factors of H1N1 infection and seasonal flu are similar and that many individuals with severe and fatal episodes usually have chronic co-morbid conditions like cardiac disease, chronic respiratory disease, and diabetes. Contrary to this, nearly two-thirds of the hospitalized patients and 40% of those who died due to H1N1 did not have any pre-existing chronic condition.

The role of obesity and pregnancy as potential risk factors needs further study, although there is evidence supporting vaccination and proactive treatment in pregnant women. Pregnant women in their third trimester consistently made up the majority of the total of infected pregnant women.

The authors concluded:

"Our results demonstrate that risk factors for severe H1N1pdm infection are similar to those for seasonal influenza, with some notable differences, such as younger age groups and obesity, and reinforce the need to identify and protect groups at highest risk of severe outcomes."

Funding: MDVK, CAD, and AK acknowledge funding from the Medical Research Council UK and the Bill and Melinda Gates Foundation (MDVK) for funding. No funding bodies had any role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

"Risk Factors for Severe Outcomes following 2009 Influenza A (H1N1) Infection: A Global Pooled Analysis"
Van Kerkhove MD, Vandemaele KAH, Shinde V, Jaramillo-Gutierrez G, Koukounari A, et al. (2011) Risk Factors for Severe Outcomes following 2009 Influenza A (H1N1) Infection: A Global Pooled Analysis.
PLoS Med 8(7): e1001053. doi:10.1371/journal.pmed.1001053

Written by Brian Windsor