H1N1 influenza pandemics: comparing the events of 2009 in Mexico with those of 1976 and 1918-1919.

Outbreaks of influenza A (H1N1) of avian- or swine-related origin have substantially impacted human populations. The most dramatic pandemic of influenza H1N1 occurred during 1918-1919 producing significant morbidity and mortality worldwide. In the 20th century, two other major pandemics took place but they were the H2N2 and H3N2 reassorted influenza strains. In 1976, a small outbreak of swine-related H1N1 in the U.S. led to a national scare but without any significant public health impact. More recently, in April 2009, in Mexico, and subsequently worldwide, an influenza (H1N1) triple reassortant strain produced >200,000 laboratory-confirmed cases and resulted in >2000 deaths. In August 2009, WHO declared this outbreak as the first influenza pandemic of the 21(st) century. It is critical to apply lessons learned during previous pandemics to mitigate the public health impact of the ongoing influenza pandemic in 2009. In particular, it is useful to compare the events in Mexico in 2009 to those during the Spanish influenza pandemic of 1918-1919.