Flu Friday (in August!)

The CDC issued some updates today on the influenza A (H3N2) variant virus. There are now 153 confirmed cases of flu A H3N2v, reported from Indiana, Ohio, Illinois and Hawaii. There is no evidence yet that the virus is particularly virulent (only two hospitalizations, and no deaths thus far). The median age of cases (7 years) is likely related to lower levels of cross protective antibodies in kids (kids also may be more likely to cuddle up to pigs at agricultural fairs). In every confirmed case where a contact history was obtained, there was contact with swine or attendance at a fair where swine were present (I’m still awaiting data on what percentage of the Midwestern population has not attended an event where swine were present). The interim case definition requires swine contact in the week prior to illness, or an epi link to a confirmed H3N2v case.

On the testing front, CDC reports a nice evaluation of the commercial rapid diagnostic tests, demonstrating great variability in performance—negative results from these tests don’t exclude the diagnosis. Molecular tests should perform well, but may give “unsubtypable” results or a falsely positive result for human H3 viruses. Any samples from suspected cases should be sent to state public health labs, where initial testing will be performed (followed by testing at CDC if results of the CDC RT-PCR panel demonstrate presence of the nucleoprotein, influenza A and seasonal H3 targets).


Cartoon image of influenza virus from the CDC Public Health Image Library