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NASHVILLE – Hospitals might want to prepare for a busier than usual flu season this year.
One reason, according to the CDC, is about half the H3N2 flu viruses analyzed so far this influenza season are drift variants – viruses with antigenic or genetic changes that make them different from the viruses for which the season’s vaccine offers maximum protection.
Another is that, so far this year, the most common seasonal influenza is H3N2 viruses, which generally cause more severe flu illnesses, hospitalizations, and deaths than some other strains.
H3N2 viruses predominated during the 2012-13, 2007-08, and 2003-04 seasons, the three “moderately severe” seasons with the highest mortality levels in the past decade, according to the CDC.
“It’s too early to say for sure that this will be a severe flu season, but Americans should be prepared,” said CDC director Tom Frieden, MD, MPH. “We can save lives with a three-pronged effort to fight the flu: vaccination, prompt treatment for people at high risk of complications, and preventive health measures, such as staying home when you’re sick, to reduce flu spread.”
A recent study found that hospital encounters, including ED visits and admissions, increased for young children when H3N2 circulated.
“Influenza-related encounters were greater when influenza A(H3N2) circulated than during other years with median rates of 8.2 vs 3.2 hospitalizations and 307 vs 143 ED visits per 10 000 children, respectively,” according to the report published recently in the journal Pediatrics.
For the study, Vanderbilt University researchers examined the annual incidence of influenza-related health care encounters and vaccine uptake among children 6- to 59-months-old from 2000-01 through 2010-11 in Davidson County, TN.
They found that the proportion of fully vaccinated children increased from 6% in 2000-01 to 38% in 2010-11. During that period, influenza-related hospitalizations ranged from 1.9 to 16.0 per 10 000 children – a median of 4.5 – per year. Influenza-related ED visits, meanwhile, ranged from 89 to 620 per 10 000 children, median 143, per year.
“Significant decreases in hospitalizations and increases in ED visits over time were not clearly related to vaccination trends,” the authors write, although they noted the increase when H3N2 circulated.
Public health officials emphasized that even if the vaccine isn’t an exact match, it still is better than doing without. During the 2007-08 flu season, for example, the predominant H3N2 virus was a drift variant yet the vaccine had an overall efficacy of 37% and 42% against those types of viruses.
Depending on the formulation, flu vaccines protect against three or four different flu viruses. Even during a season when the vaccine is only partially protective against one flu virus, it can protect against the others, public health officials said.