Experts consider 4-strain influenza vaccine to fight B viruses

Selecting influenza strains to put in the flu vaccine each year is always difficult, given the ability of influenza viruses to change, but choosing the influenza B strain has become particularly vexing in recent years.  For a number of years, two distinct lineages of influenza B-Victoria and Yamagata-have been circulating and experts have found it almost impossible to predict which one would dominate in any winter.

Because a vaccine against one lineage offers little protection against the other, the US government's flu vaccine advisors have been discussing for several years the possibility of putting both lineages in the seasonal vaccine. That would turn the standard three-strain, or trivalent, vaccine into a quadrivalent vaccine. Trivalent vaccine contains two subtypes of influenza A-H3N2 and H1N1-and one of influenza B.

Dr. Anthony Fiore, a medical epidemiologist in the Influenza Division at the Center for Disease Control and Prevention (CDC) says that "putting both B families into the vaccine would offer protection against both B strains, which is regarded as particularly important for children. How much harm results when the vaccine misses the predominant B strain is not very clear, but experts say children are likely to get little protection against B viruses when this happens.  In general, B viruses cause less severe disease than influenza A,and they seem to be associated with smaller clusters or outbreaks, according to William Schaffner, MD, president-elect of the National Foundation for Infectious Diseases and chair of the Department of Preventive Medicine at the Vanderbilt University School of Medicine in Nashville.