Novel swine-origin influenza virus A (H1N1): the first pandemic of the 21st century.

An influenza epidemic was detected in April 2009 at the border between the United States and Mexico. The virus was identified soon after to be a swine-origin influenza virus A (S-OIV A) (H1N1). This virus has an HA gene that is derived from the 1918 swine influenza virus and other genes from human, avian, and Eurasian swine influenza viruses. Clinically, it behaves similarly to seasonal influenza. The only differentiating characteristics are vomiting and diarrhea in a quarter of infected patients, which are rare in seasonal influenza. On June 11, 2009, the World Health Organization declared the first pandemic of the 21st century, caused by S-OIV A (H1N1). Vaccination is the only way to dampen this pandemic. Many questions await answers, including the clinical impact of the pandemic, optimal doses of vaccine, and the future destiny of the virus. A breakthrough in vaccinology against influenza is needed to address the recurring influenza pandemic.