Management of Prevalent Infections in Children Following a Disaster

What is Influenza?

Influenza is a segmented, single-stranded enveloped RNA virus classified into influenza A, B and C based on antigenic differences. Influenza A is a potentially severe illness, causes epidemics and pandemics, is rapidly changing, and infects birds, swine, horses, seals, and humans. Influenza B is more uniform, causes epidemics and only infects humans. Influenza C is of minimal public health impact and infects humans and swine. Further subtyping of influenza A virus is based on the neuraminidase and hemagglutinin proteins on the viral surface. There are 16 different hemagglutinins and 9 different neuraminidase subtypes. Hemagglutinin proteins allow the virus to stick to cells by binding to a specific receptor. The neuraminidase protein helps newly formed viral particles get released from the cell surface so that they have the potential to infect other cells. Only H1N1, H2N2, H3N2 subtypes are associated with widespread epidemics in human. Since 1997, rare but severe infections in humans with influenza A subtype H5N1 viruses have been identified in Asia, Africa, Europe, and the Middle East where these viruses are present in domestic or wild birds.

Repeated seasonal influenza epidemics persist because the type A and type B viruses undergo constant and rapid change due to antigenic drift. Antigenic drift refers to a gradual change in the virus that occurs through a slow series of amino acids changes in the hemagglutinin or neuraminidase surface antigens. Occurring only after a particular viral strain has become established in humans, antigenic drift represents an adaptation to the development of host antibodies. Newly developed antigenic strains of influenza then prevail for a period of 2 to 5 years, only to be replaced by the next emerging strain. This new strain can then trigger a new epidemic, since it is now unfamiliar to the antibody repertoire of the population. The development of yet another set of host antibodies eventually protects the population–at the same time it puts pressure on the virus to drift yet again. Ongoing change caused by antigenic drift requires ongoing reformulation of influenza vaccines usually on an annual basis. The World Health Organization and the Centers for Disease Control and Prevention continually track these changes to better recommend strains to be contained in the next seasonal influenza vaccine.

In contrast to the gradual evolution of strains subject to antigenic drift, antigenic shift occurs as soon as a type A influenza virus with a completely novel hemagglutinin or neuraminidase moves into humans from another host species. The primary source is birds, certain species of which carry a reservoir of 15 influenza A subtypes. These subtypes either genetically reassort themselves with circulating human influenza virus or are transmitted directly into humans, typically via intermediate hosts such as swine. Antigenic shift of type A influenza viruses occurs less frequently than antigenic drift, but with more dramatic impact that can lead to a pandemic. A pandemic is defined by the emergence and global spread of a new influenza A virus subtype to which the population has little or no immunity and that spreads rapidly from human to human. Pandemics, therefore, can cause increased morbidity and mortality rates compared with seasonal influenza. During the 20th century, there have been four influenza pandemics, in 1918 (H1N1), 1957 (H2N2), 1968 (H3N2), and 2009-10 (H1N1). The recent influenza pandemics of 2009 H1N1 (“swine flu”) was caused by genetic reassortment between human, two avian and one swine influenza viruses. Avian influenza (H5N1) continues to cause outbreaks among poultry and wild birds worldwide but has caused relatively few cases of human H5N1 infection although case fatality rates are greater than 50 percent.