The duration and magnitude of influenza epidemics: A study of surveillancedata from sentinel general practices in England, Wales and the Netherlands

Weekly incidence data for influenza-like illness, routinely collected in se ntinel general practices in England and Wales and in the Netherlands over 1 0 winter periods (week 37 in one year to week 20 in the next, 1987/1988-199 6/1997) were examined in conjunction with matching virus isolate data to de fine epidemic periods of influenza in the two countries. We first defined t he background rates of recording influenza-like illness which occurred at t imes when only sporadic or no isolations of virus were reported. The backgr ound rates were similar in the two networks with mean weekly incidence in E ngland and Wales of 28.1 per 100,000 (all ages) and in the Netherlands 29.8 . Epidemic periods defined as lying above the upper 95% confidence level of the background rate lasted on an average of about 10 weeks. Once epidemics were recognised, peak incidence was generally achieved within 4 weeks. The excess population (all ages) consulting general practitioners during influ enza epidemic periods was calculated from the difference between the observ ed and background incidence rates, and expressed as a percentage of the tot al population. In the 10 periods surveyed, the percentage of the population consulting and diagnosed with influenza-like illness in England and Wales ranged from 0.4% in 1991/1992 to 1.7% in 1989/1990 and in the Netherlands f rom 0.5% in 1990/1991 to 2.1% in 1989/1990. The duration and epidemic perio ds were broadly similar in the two countries though the excess consulting p opulation during the 10 epidemics studied averaged 0.85% in England and Wal es compared with 1.39% in the Netherlands. There were substantial differenc es between the two countries in the impact of influenza in individual years , as measured in the consulting population even though the predominant viru s (sub)types were similar.