Vaccination halved the risk for influenza-associated hospitalization among adults during the 2015-2016 influenza season in the United States, most of whom were at an increased risk for serious influenza complications due to comorbidity or age, researchers reported.
In the Journal of Infectious Diseases, Jill Ferdinands, PhD, an epidemiologist in the CDC’s Influenza Division, and colleagues noted that there is limited available evidence regarding influenza vaccine effectiveness for the prevention of severe illness.
“It is widely speculated that, by attenuating disease severity following infection, vaccination may provide better protection against severe influenza outcomes, and several studies in the U.S. have indeed suggested better protection against influenza hospitalization than against outpatient influenza illness,” Ferdinands and colleagues wrote. “However, this trend has not been observed consistently, and much remains unknown about the protective effect of vaccination against severe influenza, most of which occurs among older adults with a wide range of comorbidities that may be associated with poorer vaccine response.”
During the 2015-2016 influenza season, the CDC initiated the U.S. Hospitalized Adult Influenza Vaccine Effectiveness Network (HAIVEN), a multistate, multiyear, test-negative, case-control prospective study that included adults aged 18 years or older who were admitted with an acute respiratory illness to eight hospitals in Texas, Pennsylvania, Tennessee and Michigan and tested positive for influenza by PCR, Ferdinands and colleagues explained. Patients who tested negative were included as controls.
For the study, Ferdinands and colleagues estimated vaccine effectiveness, adjusting for age, comorbidities and confounding factors, and stratified the data by frailty, 2-year vaccination history and clinical presentation. They analyzed data from 236 cases and 1,231 controls. The mean age was 58 years and more than 90% of patients had one or more comorbidity. Half of case patients and 70% of control patients were vaccinated.
According to Ferdinands and colleagues, the seasonal vaccine was 51% (95% CI, 29-65) effective in preventing hospitalization due to influenza A(H1N1)pdm09 — the predominant strain that season — and 53% (95% CI, 11-76) effective in preventing hospitalization due to influenza B virus infection. The findings showed that the vaccine was 81% (95% CI, 49-93) effective in preventing influenza-associated ICU admission.
Ferdinands and colleagues said the vaccine was protective for all age groups and underscored the importance of an annual evaluation of vaccine performance due in part to the yearly changes in influenza vaccine formulation.
“While results from a single season, such as the 2015-16 season reported here, may not generalize to other seasons in which influenza A(H3N2) is the predominant circulating strain” — such as last season — “the annual reassessment of [vaccine effectiveness] allows accumulation of evidence needed to critically evaluate performance of the U.S. influenza vaccine program over time,” they wrote.
“As additional vaccines become available for older adults and other groups at increased risk for serious influenza complications, evaluation of these vaccines will be essential to determine their ability to improve protection against serious outcomes in these populations.” – by Marley Ghizzone
Disclosures: Ferdinands reports no relevant financial disclosures. Please see the study for all other authors relevant financial disclosures.