Vaccination provides similar protection against hospital-associated influenza, milder outpatient illness
Influenza vaccination demonstrated similar protection against hospital-associated influenza and milder, outpatient instances of the illness over three influenza seasons, according to findings published in Clinical Infectious Diseases.
“The CDC has large U.S. vaccine effectiveness networks in outpatient and hospital settings that assess how well each season’s influenza vaccine protects against influenza. Many of the network sites are in the same cities,” Mark Tenforde, MD, PhD, MPH, EIS officer in the CDC’s National Center for Immunization and Respiratory Disease, Influenza Division, told Healio. “The goals of our study were to see how similar patients were who sought care for influenza or other acute respiratory illness in outpatient vs. hospital settings and to compare estimates for vaccine protection against milder outpatient illness vs. more severe hospitalizations from influenza.”
Tenforde and colleagues tested adults aged 18 years and older with acute respiratory illness for influenza within two vaccine effectiveness networks, one based in the outpatient setting and the other one based in the hospital setting, from 2015 to 2018. According to the study, a test-negative design was used to compare vaccination odds in influenza-positive cases vs. influenza-negative controls. Vaccine effectiveness was calculated using logistic regression, adjusting for site, age, sex, race/ethnicity, peak influenza activity, time to testing from symptom onset, season (in regard to overall vaccine effectiveness) and underlying conditions.
In total, 14,573 outpatients including 4,144 who tested positive for influenza and 6,769 inpatients including 1,452 who tested positive for influenza were enrolled in the vaccine effectiveness networks. Inpatients were older (median age, 62 years vs. 49 years) and had more high-risk conditions (median, 4 vs. 1).
The study showed that overall influenza vaccine efficacy across seasons was 31% (95% CI, 26%-37%) among outpatients and 36% (95% CI, 27%-44%) among inpatients. Strain-specific vaccine efficacy among outpatients vs. inpatients was 37% (95% CI, 25%-47%) vs. 53% (95% CI, 37%-64%) against H1N1pdm09, 19% (95% CI, 9%-27%) vs. 23% (95% CI, 8%-35%) against H3N2 and 46% (95% CI, 38%-53%) vs. 46% (95% CI, 31%-58%) against B viruses.
Patients in need of care for acute respiratory illnesses who were hospitalized were “much different” than those seeking care in outpatient settings, even in similar parts of the U.S., according to Tenforde. Hospitalized patients were older by more than a decade, on average and had more chronic conditions that increased their risk for influenza-related complications, including ICU admission or death. Despite those differences, however, vaccine effectiveness was similar.
“Reassuringly, our study found that, over three seasons, vaccination offered similar protection against hospital-associated influenza as against milder outpatient illness. This is important because being hospitalized for the illness is associated with poor outcomes and other personal and health system costs,” Tenforde said. “The study provides additional support for the importance of influenza vaccination, especially for high-risk populations like the elderly and those with chronic medical issues.”