Influenza surveillance in the US has at least five component parts (depending on how you count it is as many as seven). We discussed the virologic surveillance system in another post. CDC has two surveillance sub-systems that look at hospitalized cases with laboratory confirmed influenza, the New Vaccine Surveillance Network (NVSN) and the Emerging Infections Program (EIP). The NVSN is confined to cases in children less than five years old, while the EIP covers all ages. Let’s take a look at the most recent EIP data.
EIP doesn’t cover the entire country. Instead it collects data from 60 counties and 12 metropolitan areas in 10 states (San Francisco CA, Denver CO, New Haven CT, Atlanta GA, Baltimore MD, Minneapolis/St. Paul MN, Albuquerque NM, Las Cruces, NM, Albany NY, Rochester NY, Portland OR, and Nashville TN). It is an active surveillance system that examines routine hospital laboratory and admissions data for cases that have a record of a positive test for influenza (viral culture, direct/indirect fluorescent antibody assay (DFA/IFA), reverse transcription-polymerase chain reaction (RT-PCR), or a commercial rapid antigen test). The counties and metropolitan areas are chosen to allow an estimate of hospitalization rates for the populations of the hospitals’ catchment areas. Using these data, EIP estimates influenza hospitalization rates every two weeks of the flu season (weeks 40 of the previous calendar year to 20 of the new calendar year).
Here are the most recent data through week week ending May 16 (week 19), graphed separately for different age categories:
Source: CDC Fluview
What’s striking is that this year’s flu season looks pretty much like the mild seasons of 2005-2006 and 2007-2008 and quite unlike last year’s moderately severe season — except for the 5 – 17 year group, where it begins to diverge around week 9 (week ending March 9).
We wonder if this was the first sign of this pandemic virus, whose age distribution is markedly left shifted.