Atlanta—While most of the influenza viruses collected in the U.S. since October 1, 2017, were characterized antigenically or genetically as being similar to the cell-grown reference viruses included in the 2017–2018 Northern Hemisphere influenza vaccine viruses, suggesting little significant antigenic drift, some currently circulating A(H3N2) viruses are less similar to egg-adapted viruses used for production of the majority of influenza vaccines in this country, according to the national CDC. 

From October 1 to November 25 this year, A(H3N2) viruses were most commonly detected, but A(H1N1)pdm09 and influenza B viruses also were reported. 

That news comes as influenza appears to be on the march. Public health officials report that influenza activity in the U. S. for the 2017–2018 season was low during October but has been increasing since early November. Exact timing of influenza activity is difficult, however, the article in the Morbidity & Mortality Weekly Report notes; while influenza activity in the U.S. usually peaks from December to February, substantial influenza activity can be observed through May. 

The bottom line: The CDC emphasizes that the work of pharmacists and other vaccine providers is not nearly done this season.

“Although influenza vaccine effectiveness can range widely from season to season, influenza vaccination is the most effective currently available method to prevent influenza and its complications. However, less than half of the U.S. population has been vaccinated in recent influenza seasons,” study authors write. “Even with influenza vaccine effectiveness in the range of 30% to 60%, influenza vaccination prevents millions of infections and medical visits and tens of thousands of influenza-associated hospitalizations each year in the U.S. Healthcare providers should recommend influenza vaccine now and throughout the influenza season to all unvaccinated persons aged ≥6 months who do not have contraindications.”

The report also discusses studies finding reduced vaccine effectiveness against A(H3N2) viruses (30%–40%), even when no significant antigenic drift occurs, as compared with A(H1N1) and influenza B viruses. 

“This reduction in effectiveness might result, in part, from the egg propagation of influenza A(H3N2) vaccine virus components required for most influenza vaccine products licensed in the United States,” the report explains. “For example, egg adaptation of current A(H3N2) viruses typically results in a loss of N-linked glycosylation motif at residues 158-160 of the HA protein, which is within an important antibody epitope (site B). Other factors that might also contribute to the reduced effectiveness against A(H3N2) viruses include the naturally occurring, high level of genetic diversity and rapid evolutionary rate of this particular subtype and modification of the immune response to vaccine because of prior infection or vaccination. Vaccine effectiveness studies are needed to ascertain the level of protection that influenza vaccination provides to the population, but these data will not be available until later in the season.”
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