Influenza virus is highly contagious, affecting people of all ages and all socioeconomic backgrounds, and has a particularly profound effect on children. Community studies indicate that school-aged children have had the highest rates of influenza infection, with annual rates as high as 42% in prospective surveillance studies.1
Furthermore, children who do contract influenza are particularly susceptible to nonrespiratory complications. The most common of these complications is acute otitis media, which annually affects 3% to 5% of children. There has also been a report of a substantial increase (ie, 10% to 30%) in the number of antimicrobial courses prescribed to children during the influenza season, and influenza infection is sometimes associated with development of pneumococcal and staphylococcal pneumonia in children.1
There have been reports of febrile convulsions, sinusitis, myositis, myocarditis, pericarditis, and encephalopathy. Approximately 1% of all infected children require hospitalization as a result of these primary and secondary sequelae of influenza.2
Seasonal influenza, which arises each year, is due to small adaptive mutations (termed antigenic drift
) that occur as a result of immunologic pressures on strains of influenza already circulating within the human population.3
In the spring of 2009, an entirely new strain of influenza A (H1N1) arose owing to a chance recombination of swine, avian, and human influenza.4
As a result of the genetic uniqueness of the 2009 H1N1 virus (ie, 27% and 18% change in the amino acid sequences of hemagglutinin and neuraminidase, respectively), there is a complete lack of herd immunity, allowing this strain to spread quickly and efficiently across the globe.4
The emergence of any novel influenza strain, H1N1 included, poses the risk of pandemic levels of infection.