Weekly influenza surveillance overview, March 23, 2012 - Week 11

The 2011–2012 influenza season started late and has been without any clear geographic progression. During week 11/2012, medium intensity was reported in 16 countries, high intensity in one country, and decreasing trends in 15 countries. Of those, 8 countries reported decreasing trends for two consecutive weeks, suggesting that their influenza seasons have peaked.

Virological surveillance for the period shows that out of 1 203 sentinel specimens tested, 502 (41.7%) were positive for influenza virus. The proportion of sentinel specimens testing positive for influenza virus has decreased over three consecutive weeks. Of the sentinel specimens that tested positive for influenza virus 82.9% were type A and 17.1% type B.  

There has been a degree of heterogeneity in the antigenicity of the A(H3) viruses this season and an imperfect fit with the A(H3) component in the seasonal vaccine.

As often observed late in the season, the proportion of influenza B viruses among the detected influenza viruses has been increasing and has doubled over the past two weeks.

Since week 40/2011, a total of 1 378 SARI cases, including 69 fatalities, have been reported by seven countries. Of these cases, most were influenza-related.

No resistance to neuraminidase inhibitors (oseltamivir and zanamivir) has been reported so far this season.