WHAT ARE THE COMPLICATIONS OF INFLUENZA AND CAN THEY BE PREVENTED - EXPERIENCE FROM THE 1989 EPIDEMIC OF H3N2 INFLUENZA-A IN GENERAL-PRACTICE

Objectives-In an epidemic: to measure the incidence and risk of compli cations of influenza; to determine the effect of pre-existing disease on complications; to estimate vaccine uptake and efficacy. Design-Case -control study. Setting-Primary care: two group practices. Subjects-34 2 of the 395 cases of clinically diagnosed influenza reported to the g eneral practice surveillance of infectious diseases scheme of the Publ ic Health Laboratory Service during the 1989 epidemic, and 342 age and sex matched controls. Interventions-Examination of records. Main outc ome measures-Documented recognised complications; hospital admission; previous vaccination. Results-Of 15 recognised complications, bronchit is was the commonest (rate 190.1/1000 cases) and significantly commone r in cases (summary odds ratio 9.7) after adjusting for higher consult ation rates (mean 6.1 per annum v 4.2 among controls; p<0.0001). No de aths were recorded. The risk of bronchitis complicating influenza was higher in patients with pre-existing illnesses regarded as an indicati on for vaccination (odds ratio 3.3; p<0.0001). Observed vaccination ef ficacy in those with pre-existing illnesses and in elderly subjects wa s high (63% and 77% respectively) but uptake was low (4.5% and 6.1% re spectively). Conclusions-Bronchitis complicates about one fifth of all cases of influenza presenting to general practitioners. Patients with pre-existing illnesses regarded as an indication for vaccination are particularly at risk. Vaccine uptake is extremely low, precluding an u nequivocal demonstration of a protective effect.