2014-2015 flu vaccine reduces severe illness in hospitalized patients

SAN DIEGO — Despite widespread circulation of an antigenically drifted influenza virus, vaccination during the 2014-2015 season may have reduced illness severity among hospitalized adult patients, according to interim data reported here.

“The 2014-2015 influenza season was severe, with widespread circulation of influenza A(H3N2) viruses which were mismatched primarily to those that were included in vaccine,” Joshua G. Petrie, MPH, of the University of Michigan School of Public Health, said during a late breaker presentation. “[Vaccine effectiveness (VE)] against influenza A(H3N2) has generally been lower than for the other types and subtypes … [but] hospitalization and other severe outcomes of influenza have generally not been a major focus of the [U.S. Influenza Vaccine Effectiveness Network].”

Joshua G. Petrie

Petrie and colleagues examined 642 adult patients hospitalized for treatment of acute respiratory illness at two Michigan health care institutions during the 2014-2015 influenza season. Throat and nasal swab samples were collected and tested for influenza using real-time PCR. Clinical outcomes were compared among patients who did and did not receive vaccination using logistic regression models adjusted for factors such as age, sex, hospital and Charlson Comorbidity Index. Vaccination status was determined by medical records and patient self-reports.

Approximately two-thirds of the cohort were considered vaccinated, and 18% tested positive for influenza infection, according to Petrie. The majority of patients were aged 50 years and older, female and had one or more comorbidities. H3N2 was predominant in positive influenza samples, among which 88% belonged to the 3C.2a drifted genetic group.

The adjusted effectiveness of influenza vaccination among these patients was 47% against all influenza (95% CI, 13-68) and 46% against H3N2 (95% CI, 7-68), suggesting that the 2014-2015 vaccine provided some protection against severe illness.

“Despite the mismatch between the circulating and vaccine viruses, VE estimated in this study was … much higher than the 7% estimated by the [U.S. Influenza Vaccine Effectiveness Network] in the ambulatory care setting,” Petrie said. “One potential explanation for this difference could be that the influenza vaccine may have been more effective in preventing more severe illness outcomes, such as those requiring hospitalization. However, we want to be cautious with that interpretation.” – by Dave Muoio

Reference:

Petrie JG, et al. Abstract LB-9. Presented at: IDWeek; Oct. 7-11, 2015; San Diego.

Disclosure: Petrie reports no relevant financial disclosures. Please see the full study for a list of all other authors’ relevant financial disclosures.