Timely Prediction of Peak Seasonal Influenza Activity Estimation Using Sentinel Surveillance Data

Influenza sentinel surveillance provides valuable information to help plan resources and preparedness. Study aim was to determine which age group could give timely prediction on onset of seasonal epidemic activity. Methods: Study period comprised influenza epidemic seasons 2006-2009. Incidence rates (IR) of influenza like illness (ILI), for 2 and 3 weeks prior to onset of epidemic activity by age group and peak epidemic activity weeks for general population were calculated. Rate Ratios (RR) between influenza virus positive samples (IV+) and ILI, between 2 and 3 weeks prior to epidemic ILI activity by age group and for peak activity period (PA) were obtained. Differences in rate proportions and their 95% confidence intervals were assessed by statistic z. Results: The highest RR between IV+ and ILI rates corresponded to 0-4y (0.34; 95% CI: 0.28-0.41), and lowest (0.26; 95%CI: 0.23- 0.30) to 15-64y. Considering two and three weeks prior to epidemic activity onset related to overall population IRs during PA period, the only RR above 1 corresponded to 0-4y (1.38; 95%CI: 1.08,1.75)for two weeks and (1.29; 95%CI:1.04,1.6) for three weeks. Conclusion: High correspondence between ILI and IV+ was observed in 0-4y prior to other age groups; in this age group rates of ILI during two and three consecutive weeks prior to increase above threshold activity can be used to predict the onset of peak epidemic activity in a timely manner.