Patients hospitalized with laboratory-confirmed, influenza-associated pneumonia had a 57% lower odds of having received the influenza vaccine than controls whose pneumonia was due to other causes, investigators reported Oct. 5 in JAMA.
The findings could be used in future studies to estimate the number of hospitalizations prevented by influenza vaccination, according to Dr. Carlos Grijalva of Vanderbilt University, Nashville, Tenn., and his associates.
Seasonal influenza causes about 226,000 hospitalizations and 3,000 to 49,000 deaths every year in the United States. Observational studies show that influenza vaccination helps prevent hospitalizations for acute respiratory illness, but whether it also cuts the odds of hospitalization for community-acquired pneumonia is unknown, the investigators wrote.
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To explore this question, they conducted an observational, multicenter study of 2,767 patients who had been hospitalized with community-acquired pneumonia over 3 consecutive influenza seasons at four sites in the United States. Patients were at least 6 months old, were not severely immunosuppressed, and had not been recently hospitalized or resided in a long-term care facility (JAMA. 2015 Oct 5, doi:10.1001/jama.2015.12160.).
In all, 162 (6%) patients had laboratory-confirmed influenza, including 17% who had received the influenza vaccine, the researchers wrote. In contrast, 29% of controls had received the vaccine, for an adjusted odds ratio of 0.43 (95% confidence interval, 0.28 to 0.68) after controlling for demographic characteristics, comorbidities, influenza season, study site, and time of disease onset. The estimated vaccine effectiveness was 57%.
The test-positive case, test-negative control design is widely used to study vaccine effectiveness and is better than comparing hospitalized cases with population controls, because it “implicitly” accounts for the risk of hospitalization, the researchers wrote. But “despite enrollment over 3 consecutive seasons, a relatively small number of influenza-associated pneumonia cases met eligibility criteria, resulting in limited precision for some subgroup analyses,” they added. “Thus, the association between influenza vaccines and pneumonia among older adults remains controversial, and additional studies in this group are needed.”
Dr. Grijalva reported having served as a consultant to Pfizer. Several coauthors reported having received grant and other support from the National Institutes of Health, the Agency for Healthcare Research and Quality, Medscape, MedImmune, Roche, Abbvie, and a number of pharmaceutical companies.