Appropriateness of thresholds currently used to describe influenza activity in England.

The timing and magnitude of influenza virus activity in England each winter remain unpredictable. Nevertheless, it is important to describe levels of activity in a timely manner, using defined 'threshold values'. This informs public health practitioners and the general public of the level of influenza virus circulation, and provides an indication of when sufficient activity is occurring in the community to warrant the use of antiviral drugs. Data presented here suggest that the current numerical thresholds, and their corresponding descriptions, are no longer appropriate for the levels of activity recently observed in England. Based on integrated clinical and virological data, we suggest alternative threshold values of 'baseline' activity (0-30/100,000 population), 'normal seasonal' activity (30-200/100,000) and 'epidemic' activity (> 200/100,000).