Three years of low influenza activity--no reason for complacency.

Influenza activity in England and Wales has been unusually low over the last three years. Despite this, substantial morbidity and mortality has occurred in subgroups of the population. Furthermore, the influenza season 2001/02 was characterised by the emergence of a new subtype of the influenza A virus (H1N2), a timely reminder that it is not possible to assess the impact of a change in the virus strains circulating without having a comprehensive surveillance infrastructure in place.