The first pandemic of the 21st century: a review of the 2009 pandemic variant influenza A (H1N1) virus.

Swine influenza was first described in the 1918 pandemic and made a resurgence in April 2009 in the form of a triple-reassortant influenza A virus, which is composed of a combination of human, swine, and Eurasian avian strains. As evidenced with previous influenza pandemics, young adults and children aged < 24 years are the population most affected. Definitive diagnosis has largely been limited by the inability of conventional influenza testing to distinguish among influenza A subtypes; however, the surge in pandemic cases clearly emerged at the end of the annual influenza season in the northern hemisphere. The pandemic variant influenza A (H1N1) strain is typically susceptible to oseltamivir and resistant to adamantanes, unlike the 2008 to 2009 seasonal influenza A (H1N1). However, 2 cases of oseltamivir-resistant pandemic-variant influenza A (H1N1) were reported in late August 2009. The full impact of the current pandemic is not yet clear, and further reassortment with the circulating seasonal influenza strains in the upcoming 2009 fall season could potentially lead to acquisition of widespread oseltamivir resistance. Vaccination will become paramount in importance for prevention and public health safety.