Introduction Viral respiratory infection (VRI) is associated with an increased rate of decline in lung function in children with cystic fibrosis (CF) but the long-term clinical impact of VRI in adults is poorly described. We performed a prospective observational study to determine the effect of VRI on lung function in adults with CF.

Methods 100 adults with CF were followed for 12 months. Patients were seen every two months routinely and also at onset of new respiratory symptoms. Sputum, nose- and throat-swabs were collected at each visit for virological analysis. Polymerase chain reaction assays for adenovirus, influenza A&B, metapneumovirus, parainfluenza 1–3, respiratory syncytial virus and rhinovirus were performed on each sample. Spirometry was recorded at each visit. Treatment failure was defined as a failure of the FEV1 to return to ≥90% of baseline after intravenous antibiotics. Statistical analysis utilised generalized linear models and multiple linear regression as appropriate, taking into account multiple observations from participants.

Results 191/626 (30.5%) study visits were positive for ≥1 virus with rhinovirus accounting for 72.5%. The incidence of VRI and pulmonary exacerbation (PEx) was 1.6 and 2.5 cases/patient-year respectively. VRI was associated with increased risk of PEx (OR 2.2; 95% CI 1.6 – 3.1; p<0.001).

There was no significant difference in relative fall from baseline FEV₁ at virus-positive compared with virus-negative visits (8.7 vs 9.4%, p=0.4). Acute fall in FEV₁ was lower in virus-positive PEx compared with virus-negative PEx (12.7 vs 15.6%; p=0.04). Rate of PEx, but not of VRI, was associated with a statistically significant decline in FEV₁ over one year, adjusted for age, sex and baseline lung function (β coefficient –1.79; 95% CI –3.4 to –0.2; p=0.02).

Intravenous antibiotics were given for 122 PEx of which 90 had pre- and post-antibiotic FEV₁ data available. 26/90 (29%) were classified as treatment failures. There was a trend towards lower likelihood of treatment failure in virus-positive PEx (OR 0.55; 95% CI 0.1 to 2.7; p=0.46).

Conclusions Incidence of PEx, but not VRI, is associated with accelerated decline in FEV1 in adults with CF. Virus-positive PEx are associated with a lower acute fall in FEV1 than virus-negative PEx.