Epidemiology of the 2009 Pandemic Influenza A (H1N1)

Publication Summary

The pandemic influenza A (H1N1) virus (pH1N1) spread quickly throughout the world following the April 2009 outbreak in Mexico, and it was the first major influenza pandemic since 1969. In order to effectively prepare for and respond to future pandemics, it is crucial to fully understand the characteristics of the 2009 outbreak. The objective of this review is to focus on the salient epidemiological features of the 2009 pandemic, the known risk factors for severe disease during infection, and specific vulnerable populations.

Published: 2011

Subject Area(s):


Project No. 29

Key Points

• Case studies and computer modelling data suggest that while transmission events in first class cabins are relatively rare, transmission is more likely to occur in economy class during long-haul flights.

• R0 of greater than 1 implies that the infection will continue to spread in the absence of control measures.

• Most studies agree that pH1N1 exhibited modest transmissibility, suggesting that swift implementation of antiviral treatment/prophylaxis and social distancing measures could have a high degree of success in limiting the spread of the pandemic.

• Case fatality rates (CFRs) are extremely difficult to accurately estimate due to the high levels of case underreporting, lack of complete pH1N1 laboratory confirmation and the occurence of subclinical cases.

• An estimate of the case-intensive care ratio (CIR) at 0.16% to 1.44% suggested that populations without access to sufficient intensive care services would experience a much higher CFR rate than the general populations of Canada and the U.S.

• There is no evidence to suggest a protective effect among contacts from the seasonal influenza vaccine, but antiviral prophylaxis did significantly reduce the risk of transmission in household contacts.

• The median age of patients who succumbed to pH1N1 was 51, reflecting the increased risk of dealth in older adults, despite their lower risk of infection.

• Between 25% and 50% of patients with severe pH1N1 disease also exhibited underlying health conditions.

• Severe obesity and morbid obesity (defined as BMI>35 and BMI>40, respectively) posed a 5- to 10-fold higher risk for severe or fatal infection.

• Pregnant women who took antivirals within 2 days of symptom onset were highly unlikely to develop severe illness.

• The increased fraction of the Aboriginal community presenting with severe pH1N1 disease was not unique for this pandemic and was also seen in the 1918 H1N1 Spanish influenza pandemic, to which mortality in Aboriginal communities in North America (3%-9%) was significantly higher than among non-Aboriginal communities.