Fortnightly influenza surveillance overview, 18 June – 1 July 2012, weeks 25 – 26

The 2011–2012 influenza season has ended but the surveillance of influenza will continue during the off-season period and the bulletin appears on a fortnightly basis until week 40/2012.
During weeks 23–24/2012, all sixteen countries reporting experienced low-intensity influenza activity.
Of 68 sentinel specimens, one was positive for influenza type B virus. Since week 40/2011, 89.3% of the detected influenza viruses have been type A and 10.7% type B. The A(H3) subtype represented 98.5% of influenza A type viruses detected during the 2011–2012 season.
Many of the A(H3) viruses reacted poorly with post-infection ferret antisera raised against the A/Perth/16/2009 H3N2 vaccine component, prompting the WHO’s decision to recommend a change to the A(H3N2) component for the northern hemisphere 2012–13 influenza season. The need for change is consistent with an observed low vaccine effectiveness for the A(H3N2) component.