End-of-season influenza vaccine efficacy for the 2017-2018 flu season was a mediocre 38% (95% CI 31%-43%), but flu shots were still estimated to have prevented 7 million illnesses and 8,000 deaths, researchers found.
With respect to individual flu strains, vaccine efficacy last season ranged from 62% (95% CI 50%-71%) against influenza A(H1N1)pdm09 to 22% (95% CI 12-31%) against influenza A(H3N2), reported Melissa A. Rolfes, PhD, of the CDC, and colleagues.
Influenza vaccinations are estimated to have prevented 10% of expected hospitalizations overall, and 41% among children ages 6 months to 4 years, the authors wrote in Clinical Infectious Diseases.
Last year's widely publicized "high severity" flu season featured "the highest morbidity and mortality since the 2009 pandemic," the authors noted, including a record number of flu-related hospitalizations and the largest number of pediatric flu-related deaths since the 2009 pandemic year. However, only 42% of the U.S. population received the flu shot during the 2017-2018 season, the researchers added.
Among persons age ≥6 months who were eligible for the flu shot, the researchers estimated that there were 47.9 million illnesses, about 22 million medical visits, 953,000 hospitalizations, and 79,400 influenza-associated deaths in 2017-2018. Adults ages ≥65 comprised 15% of the illnesses, but 70% of flu-related hospitalizations and 90% of flu-related deaths.
Influenza A(H3N2) was linked with the highest rates of illness, and the authors found this strain affected 15% of adults ages 50-64 and 9% of children ages 6 months to 4 years. When applying these rates to the total U.S. population, the researchers estimated that influenza A(H3N2) was linked with 28.4 million illnesses, 13.0 million medical visits, 587,000 hospitalizations, and 49,000 deaths overall.
Examining data from the U.S. Flu VE Network, the authors found that influenza A(H3N2) viruses accounted for 84% of influenza A virus infections. Moreover, the authors found that among those enrolled in this network, 42% of influenza-positive case-patients and 53% of influenza-negative controls were vaccinated against influenza.
Further estimates found that influenza vaccination prevented 3.7 million medical visits and 109,000 hospitalizations. Not only were an estimated 8,000 deaths prevented by influenza vaccination, but 9% of expected deaths overall, the authors said. In addition, influenza vaccines prevented an estimated 39% of influenza-related mortality in children ages 6 months to 4 years.
An accompanying editorial by Kathleen Neuzil, MD, and Meagan C. Fitzpatrick, PhD, both of the University of Maryland School of Medicine in Baltimore, said that while the data "strongly support use of the influenza vaccine," it also illustrates the large burden of illness and "a lot of disease we are not preventing." While the wait is on for better vaccines, the editorialists advised clinicians not to forget about using antiviral medications to prevent and treat influenza, but also emphasized the importance of "strongly recommending" the flu shot to patients.
"Unfortunately, last year, influenza vaccine coverage dropped in the U.S. across all age groups, [which] ... may be partially driven by public perceptions that a moderately effective vaccine is not worth the effort," Neuzil and Fitzpatrick wrote. "However, improving coverage is likely to avert more disease than a similar improvement in efficacy – and furthermore, we can emphasize that personal risk is heightened when vaccine efficacy is low."
Limitations to the data, the researchers said, include that because "multipliers are used to scale surveillance data to national burden estimates" and the data to calculate these multipliers often lag by 2 years, the team used multipliers measured during previous influenza seasons, which would not take into account 2017-2018 changes in testing practices, disease severity, or care-seeking behavior. Rolfes and colleagues also noted that this model does not "currently account for waning effectiveness of influenza vaccination over the season."
The research was supported by the CDC through Emerging Infections Programs, the Influenza Hospital Surveillance Project, the University of Michigan, Kaiser Permanente Washington Research Institute, Marshfield Clinic Research Institute, the University of Pittsburgh, Baylor Scott and White Healthcare, and the National Institutes of Health.
Rolfes disclosed no conflicts of interest; other co-authors disclosed support from Sanofi, Seqirus, Merck, Pfizer, Micron, MedImmune, PaxVax, AbbVie, and the CDC; one co-author received payment to co-edit a book on infectious disease surveillance, with royalties donated to the Minnesota Department of Health.
Neuzil and Fitzpatrick disclosed no conflicts of interest.
Flu vaccine was 38 effective last season, 22 effective
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