Influenza B outbreak at an adult cystic fibrosis centre - Clinical impact and factors influencing spread

Introduction An outbreak of Influenza B occurred at a large United Kingdom (UK) regional adult cystic fibrosis (CF) centre in May 2016. This was late in the UK 2015–2016 influenza season and occurred on a specialist ward with strict infection control procedures. This study investigates the spread of influenza, clinical consequences and potential contributing factors. Methods Patient records, clinical status and pulmonary function data were reviewed for all inpatients during this period. Respiratory viral PCR results, influenza vaccination status of patients and staff, and environmental factors were also recorded. Affected patients were prospectively monitored for the following three months. Results 10 of 21 inpatients developed influenza B between 5th and 12th May 2016, an attack rate of 48%. All those characterised were confirmed as the same strain of influenza B/Victoria-lineage. Influenza infection resulted in a mean FEV1 reduction of 10.5% (SD 11.3, p = 0.012), which persisted at 3 months post infection (p = 0.003). Nine of the affected cases rooms were in close proximity on the ward while patients in the two isolation rooms with enhanced ventilation did not become infected. Ventilation measurements in affected rooms ranged from 1.75 to 2.10 air changes/hour, below national recommendations. Seventy percent of affected inpatients had received the 2015/16 trivalent seasonal influenza vaccine, which did not contain a B/Victoria-lineage influenza B virus. Conclusion This influenza B outbreak in CF adults had a high attack rate and a significant clinical impact. Room ventilation and a limited protection from the seasonal influenza vaccine were possible contributory factors.