Global Alert and Response (GAR)

Pandemic (H1N1) 2009 - update 83

Weekly virological surveillance update

For this reporting week (27th December 2009 to 2nd January 2010), pandemic A (H1N1) accounted for 98.1% of all subtyped influenza A viruses detected in many of the countries reporting influenza activity in both northern and southern hemispheres. One of the exceptions was China where the pandemic (H1N1) 2009 virus accounted for 90.2% of all specimens tested positive for influenza A viruses. Furthermore, in China, influenza B accounted for 12.6% among the specimens tested positive for influenza viruses. Sporadic detections of seasonal A(H1N1), A(H3N2) and influenza B viruses were reported from a few countries including some European countries.

During this week a total of 23 countries reported to FluNet. The total number of specimens reportedly positive for influenza viruses by NIC laboratories was 2,878. Of these, 2,354 (81.8%) were pandemic (H1N1) 2009, 4 (0.1 %) were seasonal A (H1), 41 (1.4%%) were A (H3), 165 (5.7%) were A (not subtyped), 314 (10.9%) were influenza B. The percentage of pandemic (H1N1) 2009 was 98.1% of all subtyped influenza A viruses.

The majority of all influenza detections were reported by China and France followed by Greece. Among the 23 countries reported to FluNet, on average, the pandemic A (H1N1) accounted for 81.8% of all influenza detections (81.6%; 2,348/2,878) for the northern hemisphere from 21 reporting countries and (100%; 6/6) for southern hemisphere from 2 reporting countries. The above numbers represent only the specimens and results reported to FluNet*.

Since the beginning of the pandemic in 19 April 2009 to 9th January 2010 cumulatively 153 countries/areas/territories shared a total of 23,376 specimens (18,203 clinical samples and 5,173 virus isolates) with WHO CCs for further analysis. All pandemic A(H1N1) 2009 influenza viruses analysed to date appear to be antigenically and genetically closely related to the vaccine virus A/California/7/2009.

The WHO Global Influenza Surveillance Network (GISN) including WHOCCs conducts systematic surveillance for antiviral susceptibility. So far, antiviral susceptibility testing conducted by WHO CCs and other GISN laboratories on pandemic A (H1N1) specimens and isolates from at least 87 countries indicates that oseltamivir resistant pandemic H1N1 viruses are still detected sporadically and there is no wide community circulation of oseltamivir resistant viruses except for the recently reported transmission of resistant viruses in a few local settings**. So far, 199 cases of oseltamivir resistance have been reported by GISN and other WHO partners. All of these viruses showed H275Y mutation but all these viruses remain sensitive to zanamivir.

Since the first appearance of pandemic influenza A (H1N1) 2009 viruses, certain mutations including those leading to the D222G substitution in the haemagglutinin (HA) protein and the K340N substitution in the polymerase basic protein 2 (PB2) have been detected sporadically. These substitutions in HA and/or PB2 have been reported in viruses obtained from mild, severe or fatal illness cases but such viruses have neither formed distinct phylogenetic clustering nor been associated with consistent changes in virus antigenicity. Based on currently available virological, epidemiological, and clinical information, the D222G substitution does not appear to pose a major public health issue. A preliminary review has been published*** , and the WHO GISN and its partners will continue to closely monitor pandemic viruses for the D222G and other amino acid substitutions and assess associated risks.

WHO, through the GISN, continues monitoring the evolution and global circulation of influenza viruses, including pandemic, seasonal and other influenza viruses infecting, or with the potential to infect humans.

*Some NICs report data to FluNet retrospectively and updates of previous data with new results are frequent.

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