Seniors still make up the bulk of flu cases, hospitalizations

Last year’s flu vaccine was about as effective as in previous years, but the elderly remained the most impacted by illness, according to an updated report on flu surveillance from the Centers for Disease Control and Prevention (CDC).

Influenza activity during the 2016-17 was moderate, according to the CDC, with low activity in November that increased through December and peaked in February.

The predominant virus for the season was influenza A (H3N2), but influenza B viruses were most commonly reported in later season cases from March through May, the CDC notes.

In terms of who was impacted most last flu season, 8% of the influenza-positive samples tested in public health laboratories were from children from birth to 4 years of age, 30.4% were from individuals aged 5 years to 24 years, and 31.8% were from older adults aged 65 years and up. Influenza A viruses were identified in 70% of the cases in children from birth to 4 years, and in 80% of the cases in adults over age 65. Influenza B viruses were most often reported in individuals aged 5 years to 24 years, and accounted for 28% of those flu cases, according to CDC.
“Last season’s flu vaccine likely prevented thousands of hospitalization and millions of illnesses despite the fact that the vaccine was less effective than we'd like,” Kristen Nordlund, a health communications specialist with CDC told Medical Economics. “While it is clear we need better flu vaccines, it's important that we not lose sight of the important benefits of vaccination with currently available vaccines.”

Hospitalization rates for the 2016-17 flu season were highest among the elderly, with 60% of flu-related hospitalizations occurring in individuals aged 65 and older. Among other age groups, 16.7 per 100,000 of children aged 5 years to 17 were hospitalized, 19.8 per 100,000 adults aged 18 to 49 were hospitalized, 65.1 per 100,000 adults aged 50 to 64 were hospitalized, and 290.5 per 100,000 adults aged 65 and older were hospitalized, according to CDC. Influenza A was diagnosed in 78% of hospitalizations, and influenza B was diagnosed in 21.2% of hospitalizations, according to CDC.

Among adults hospitalized for influenza, 94.1% had at least one other medical condition that placed them at a higher risk of infection or flu-related complications, and 56.4% of children hospitalized for flu had at least one underlying medical condition. Additionally, CDC reported that 26.6% of women aged 15 to 44 that were hospitalized with the flu were pregnant.

Next: Effect on this year's flu season

Effect on this year’s flu season

There were some new viruses that popped up during the last few season, with three novel influenza A viruses identified. One was an influenza A (H1N2) variant reported in Iowa, the second was an influenza A (H7N2) strain reported in a patient in New York City who had prolonged exposure to sick cats in an animal shelter and the third was an influenza A(H3N2) variant/(H3N2v) virus reported in Texas in a patient who had close contact with pigs in the week before falling ill. None of the three patients required hospitalization and all recovered fully from their illness, according to CDC.

In terms of how the circulating viruses responded to antiviral medications including oseltamivir, zanamivir and peramivir, CDC found that all the viruses tested were susceptible to these antiviral medications.

Looking at vaccine efficacy for the 2016-17 season, CDC said that data indicates an overall reduction in influenza-associated medical visits of 42%. The vaccine was 56% effective against influenza B cases and was 34% effective against the predominant influenza A (H3N2) virus.

Based on this data, the U.S. Food and Drug Administration’s Vaccines and Related Biologic Products Advisory Committee (VRBPAC) has recommended that the 2017-18 trivalent vaccine contain protection against the H1N1 and H3N2 influenza A viruses, and the influenza B virus, according to CDC, representing an update to H1N1-type coverage from the 2016-17 season. The World Health Organization has already created a list of which strains will be covered in the 2017-18 vaccine for the Northern Hemisphere.

CDC concluded in its report that vaccine efficacy for the 2016-17 flu season was similar to previous seasons despite evidence that protection against influenza A (H3N2) has been waning since the 2011-12 season. Inactivated vaccines generally offered better protection against influenza A (H1N1) and influenza B viruses, CDC said.

Although CDC won’t release a final report on the prior flu season efficacy until September 28, summer flu activity is typically low and vaccine coverage updates are released throughout the season. CDC will also release updated data on flu vaccine projections and distribution in late September.