The latest study suggests that patients with flu-associated pneumonia were less likely to have received a flu shot.
The study is published in the online issue of JAMA.
Most of us are familiar with symptoms of the flu, a contagious disease of the respiratory system that affects the nose, throat and lungs. Serious complications, including pneumonia and bronchitis, are likewise associated with the disease.
A serious condition in itself, pneumonia - swelling of the lung tissue - is the leading infectious cause of hospitalization and death in the US.
Every year, new flu vaccines are made to protect patients against the flu viruses that research suggests will be the most common. After a year, immunity from vaccinations declines, which is why health officials recommend getting a new vaccine each flu season.
Still, seasonal flu epidemics account for around 226,000 hospitalizations and between 3,000-49,000 deaths in the US each year.
In this latest study, researchers set out to determine whether flu vaccines can lower the risk of flu-associated hospitalizations for community-acquired pneumonia, a question that - until now - has remained unclear.
Vaccine effectiveness was 57%
To further investigate, Dr. Carlos G. Grijalva - of the Vanderbilt University School of Medicine in Nashville, TN - and colleagues conducted an observational study of hospitalizations for community-acquired pneumonia from January 2010-June 2012.
The multi-center study included data from four different sites in the US and included patients 6 months or older with lab-confirmed flu infections and verified vaccination status. In total, 2,767 patients hospitalized for pneumonia were eligible for the study, nearly 162 of whom had lab-confirmed flu.
Of these flu cases, 17% of those with flu-associated pneumonia had been vaccinated. Meanwhile, 29% of the controls (those without flu) who had flu-negative pneumonia had been vaccinated.
The researchers say the estimated effectiveness of the vaccine was 57%, which means the odds of flu vaccination among patients who were hospitalized for flu-associated pneumonia was 57% lower than for patients who did not have the flu but who did have non-flu pneumonia.
Commenting on their findings, the researchers write:
"The estimated odds ratio of vaccination between cases and controls, and derived vaccine effectiveness from this study, could be used to inform subsequent estimations of the national number of hospitalizations for pneumonia averted by influenza vaccination."
Though the study benefits from some strengths, including a large sample size, there are some limitations. The observational design, for example, leaves room for some misclassification and "residual confounding."
The authors also note that there was a relatively small number of flu-associated pneumonia cases that met eligibility criteria, resulting in limited data for some subgroups. As a result, the link between flu vaccines and pneumonia among older adults "remains controversial."
Additionally, although the study included a diverse population, it only included four geographical areas in the US, which could limit its generalizability to other settings. The authors suggest conducting further studies in the ambulatory setting to complement their findings.
In August, Medical News Today reported on new vaccine candidates that could result in a universal flu vaccine.