BACKGROUND:

To define the pulmonary complications of influenza during the current interpandemic period, we reviewed clinical, laboratory, and radiographic data from 193 adults (> or =18 years old) (1999-2003) who were hospitalized with influenza A during 4 winters.

RESULTS:

The mean age was 75 years, 8% had documented bacterial infection, 15% required intensive care unit treatment, and 6% died. Chest radiograph (CXR) findings were classified as showing acute disease (AD; n=101) or no AD (NAD; n=92). Most CXR findings were subtle in nature. Subjects with AD were more likely to have > or =1 cardiac diagnosis (odds ratio [OR], 2.2 [95% confidence interval {CI}, 1.2-4.1]), to have rales on examination (OR, 1.9 [95% CI, 1.0-3.7]), to be symptomatic for >3 days (OR, 2.2 [95% CI, 1.2-4.1]), and to be less likely to wheeze (OR, 0.37 [95% CI, 0.20-0.70]). Total and neutralizing anti-influenza antibody titers were lower in patients with influenza than in respiratory syncytial virus-infected control subjects (P<.05), which suggests a protective effect of antibody. Interestingly, antibody titers did not differ between subjects with AD and those with NAD.

CONCLUSION:

In the absence of significant antigenic shifts, previous exposure to influenza, including vaccinations, may play a role in reducing the severity of influenza-associated lower respiratory tract disease.