Novel Influenza A (H1N1): Where Are We?

an outbreak of H1N1 influenza A virus infection was detected in Mexico, with subsequent cases observed in several other countries including the United States (1, 2). Though limited information was available on the early pre-pandemic situation in Mexico, subsequent cases in the U.S. and Canada had a common travel history of returning from Mexico, the epicenter of the current outbreak. After that, however, secondary cases without a travel history developed in the U.S. and Canada, which spread to the United Kingdom and Spain over several weeks with increasing local community-wide transmission. As the days go by, the number of infected patients and countries with laboratory-confirmed cases is increasing. As of May 19, 2009, there have been over 9,830 laboratory-confirmed cases in 40 countries (3). Current novel A/H1N1 virus has been found to contain a unique combination of genes from pig, bird, and human flu viruses. This peculiar recombinant influenza virus is entirely new, and has not been seen before elsewhere (3). This creates an almost universal vulnerability to infection in nearly all people. Therefore, the emergence of novel A/H1N1 infection among humans presents the greatest pandemic threat since the 1968 pandemic caused by A/H3N2 (4). The current pandemic alert level still remains at phase 5-just one level short of a full pandemic. However, considering the situation in Japan and other countries these days, raising the pandemic level to 6 is imminent. Given that novel A/H1N1 influenza viruses nearly met the three prerequisites for pandemic; emergence of a novel virus with little or no pre-existing immunity in the world's population, the potential to infect humans causing clinically apparent illness, and efficient transmissibility from one human to another. Therefore, it may be quite difficult to predict how the pandemic situation will evolve as time goes on. That 60% of patients were 18 yr of age or younger suggests that children and young adults have a higher susceptibilty to novel A/H1N1 infection than elderly groups. Likewise, in the early periods of the previous pandemics, young adults were more likely to acquire A/H1N1 infection through frequent social activities. In turn, there is a concern that those infected young adults transmit the influenza virus to elderly of local communities. On the other hand, it is also possible that elderly may show partial protectivity against novel A/H1N1 influenza with preexisting antibodies, as proposed by 1976 swine influenza vaccine studies (5, 6). There is a possibility of …