2009 novel H1N1 influenza: the impact of viral genomic reassortment on immune evasion and vaccine strategy.

281 The outbreak of the novel H1N1 influenza A, which began in Mexico, has attracted global attention due to the initial high mortality rate and rapid spread since April 2009. Zoonotic infection from pig to human was initially considered as most of the infected cases in Mexico had a history of close contact with pigs. By genotyping and sequencing the causative virus, it was found that the candidate virus is a novel reas-sortant virus derived from a triple-reassortant swine influenza A (H1) virus and a Eurasian swine influenza virus. 1,2 The triple-reassortant swine virus, composed of genetic material from pigs, humans, and birds, has been known of in pigs for a decade. Sporadic cases of swine-to-human transmission of the triple-reassortant swine virus have been reported, 3 but human-to-human transmission of the triple-reassortant swine virus is rare. Influenza virus is a negative-strand RNA virus, belonging to the Orthomyxoviridae. 4 The error-prone feature of the RNA polymerase causes the virus to evolve constantly. The novel H1N1 influenza A virus is the consequence of an additional viral genomic segmental reassortment gaining the ability to be transmitted among humans. 2 So far, the virulence of the novel virus remains unknown, but its transmissibility may be substantially higher than that of seasonal flu based on a World Health Organization (WHO) report. 5 The history of influenza pandemics includes outbreaks recorded in 1918 (Spanish flu), 1957 (Asian flu), and 1968 (Hong Kong flu). It is estimated that influenza pandemics occur every 30–50 years. 6 In addition to the pandemics, several milder influenza epidemics , such as swine flu in 1976, Russian flu in 1977, and avian flu beginning in 1999, occur periodically around the world and continue to threaten human beings. The rapid global spread of the novel H1N1 influenza A virus implies its pandemic potential. 5 The clinical manifestations of the novel H1N1 influenza are similar to those of seasonal influenza, and include fever, cough and sore throat; gastrointestinal symptoms with vomiting or diarrhea are also common. 1 An early finding suggests that the mortality rate caused by the novel H1N1 influenza virus is less than that of the 1918 pandemic, but comparable to that of the 1957 pandemic. 5 Antiviral drugs, including oseltamivir (Tamiflu) and zanamavir (Relenza), have so far been effective against the novel virus; however, oseltamivir resistance in the H1N1 influenza strain has already been reported. Therefore, the best way to counteract …