There are three types of influenza viruses that infect humans. These are designated A, B, and C. According to the Centers for Disease Control and Prevention, types A and B can produce serious illness and cause epidemics, while type C causes only mild respiratory infections and are not thought to cause epidemics.
The annual flu shots currently contain three different strains of influenza virus, two for type A and one for type B.
Influenza B viruses are the predominant circulating strain approximately one out of three years, according to Dr. David Jackson and colleagues in the UK. These get less publicity than the type A viruses that also infect birds and swine. The natural reservoir of animal infection gives influenza A the potential for pandemic infection, when new strains emerge from their animal hosts and jump to humans.
Type B has no known animal host, and therefore less potential for pandemics. However, Jackson noted, “Influenza B viruses are no less important than their influenza A virus counterparts as etiological agents of seasonal influenza epidemics, and their infections result in hospitalization of at-risk patient such as the young and elderly.”
Molecular studies show that influenza B has recently evolved into two separate lineages, and vaccine strains produced from one lineage may have little effect against the other lineage. “If this situation continues, decisions have to be made to include both lineages into vaccine formulations, or choose a single strain,” Jackson reported.
Newly drifted influenza B strains can cause outbreaks of the flu even in a population vaccinated against the expected influenza B virus. Dr. B. Camilloni and colleagues at the University of Perugia, Italy, reported circulation of a B strain in elderly people living in a nursing home during the 2007-2008 winter. All had been vaccinated with an earlier B strain.
Dr. Haripriya Sridharan and colleagues at the University of Texas investigated why influenza B does not infect birds or swine, as influenza A does. They found the virus protein that binds to the host cell is specific for humans and other primates.