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Background. Few studies have examined respiratory syncytial virus (RSV) infections in adults. We assessed the characteristics and outcomes of RSV relative to other viral infections.Methods. Patients ≥50 years old with acute respiratory illness were recruited for studies of influenza vaccine effectiveness from 2004 through 2010. Nasopharyngeal swabs from enrollees were analyzed for the presence of RSV and other respiratory viruses by multiplex reverse transcription polymerase chain reaction. Clinical data were obtained from interview and medical records.Results. A total of 2225 samples were tested across all seasons. The mean age was 64.2 (SD, 10.7) years; the mean interval from illness onset to sample collection was 4 (SD, 2.2) days. One or more viruses were detected in 1202 (54%) participants. In a multivariable logistic regression model, RSV was associated with ages 65–79 years (vs 50–64 years), symptoms of cough, nasal congestion and wheezing, and longer interval from illness onset to clinical encounter. RSV was not associated with the presence of chronic obstructive pulmonary disease or congestive heart failure in univariate analyses. Hospital admission within 30 days after illness onset was less common among patients with RSV compared to those with influenza (unadjusted odds ratio = 0.54 [95% confidence interval, .29–1.01], P = .06).Conclusions. RSV is a common cause of acute respiratory illness in adults aged ≥50 years; the risk of infection increases with age. Delays in healthcare seeking and reduced risk of hospital admission in patients with RSV suggest a milder course of illness relative to influenza.