H3N2: 3 notes on the dominant flu strain

The majority of U.S. influenza cases detected this season have been H3N2, an influenza A variant associated with more severe illness. 

Of more than 4,400 flu specimens tested by public health laboratories since October, about 77 percent have been positive for H3N2, according to the CDC's latest FluView report, which includes data through Nov. 12. 

Three notes on the strain:

1. During seasons in which H3N2 is circulating, there have been higher rates of hospitalization and death, according to health officials. The strain was dominant during the 2017-19 flu season, when the U.S. saw 710,000 flu-related hospitalizations and 52,000 flu-related deaths — the worst since the 2009 H1N1 flu pandemic, health officials told NBC News. 

2. The strain may be particularly risky for children and seniors. "There are also early signs of influenza causing severe illness in precisely these two groups of individuals this season," Jose Romero, MD, director of the CDC's national center for immunization and respiratory disease, said during a call earlier this month, CNBC reported. 

The nation's overall cumulative flu hospitalization rate is 8.1 per 100,000 — the highest it has been this early in the season since the 2010-11 flu season. The rate is more than double among adults ages 65 and older at 18.6 per 100,000. Kids 4 and younger are the second-hardest-hit group, with a hospitalizations rate of 13.6 per 100,000. 

3. Influenza A (H3N2) viruses tend to change more more quickly in terms of their genetic and antigenic properties than influenza B viruses, which makes it more difficult to formulate vaccines against the strain. 



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