PHILADELPHIA, PA—Split-virion—versus subunit—inactivated influenza vaccines provided greater clinical effectiveness among adults ≥50 years of age who sought care for acute respiratory illness during three separate flu seasons, a study reported at IDWeek 2014 has found.

Although both split-virion and subunit inactivated influenza vaccines are assumed to have similar clinical effectiveness based on hemagglutination inhibition assay results, split-virion vaccines contain more internal protein and stimulate a greater cellular immune response, said H. Keipp Talbot, MD, MPH, of Vanderbilt University School of Medicine in Nashville, TN. However, “the clinical significance of this is unknown,” he added, in explaining the rationale for the study.

Influenza cases and controls were identified by positive and negative RT-PCR, respectively, for patients presenting for treatment for acute respiratory illness from November through April 2008–2009, 2010–2011, and 2011–2012.

The investigators determined whether the cases had been exposed to seasonal influenza split-virion vaccine (Afluria, Fluarix, FluLaval, & Fluzone Standard Dose) or subunit vaccine (Agriflu & Fluvirin) up to 2 weeks prior to the visit; excluded were those who received Fluzone High Dose.

Vaccine effectiveness was calculated for split-virion vaccine versus no vaccine and subunit vaccine versus no vaccine using logistic regression models. “Covariates included age, sex, race, smoking, oxygen use, insurance, medical conditions, immunosuppression, week of acute respiratory illness visit, influenza season, and site of enrollment,” Dr. Talbot reported.

Of the 840 participants enrolled, 539 had a known immunization status and complete clinical data. “Compared to influenza negative patients, the 68 (12.6%) influenza positive patients were younger (P<0.001), more likely to have been enrolled in the hospital than in either the emergency department or the acute care clinic (P=0.001), and more likely to have received subunit than split-virion vaccine (P<0.001),” the investigators found.

Except for the proportion who were influenza positive (5.4% vs. 12%; P=0.02), patients who received split-virion vaccines (n=204) were similar to those (n=150) who received subunit vaccines. Vaccine effectiveness for the split-virion influenza vaccine was 77.8% (58.5%, 90.3%) and, for the subunit vaccine, 44.3% (-11.8%, 70.9%). Relative vaccine effectiveness for split-virion compared to subunit vaccines was 33.5% (95% CI 6.9%, 86.7%).