Global Alert and Response (GAR)

Pandemic (H1N1) 2009 - update 82

Weekly virological surveillance update

For this reporting week (20 to 27 December 2009), pandemic A(H1N1) accounted for more than 99% of 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 almost 10% of the influenza A viruses were positive for seasonal influenza A(H1N1) and A(H3N2). Sporadic detections of seasonal A(H1N1), A(H3N2) and influenza B viruses were reported from a few countries including Canada, United States of America and 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 4,710. Of these, 4,048 (85.9%) were pandemic (H1N1) 2009, 17 (0.4 %) were seasonal A (H1), 81 (1.7%) were A (H3), 274 (5.8%) were A (not subtyped), 290 (6.2%) were influenza B.

The majority of all influenza detections reported to FluNet were from China and the Russian Federation followed by Greece. Of the 23 countries that reported to FluNet, on average, the pandemic A (H1N1) accounted for 85.9% of all influenza detections (85.9%; 4,026/4,688) for the northern hemisphere from 21 reporting countries and (100%; 22/22) for southern hemisphere from 2 reporting countries. The above numbers represent only the specimens and results reported to FluNet. As some laboratories (NICs), under pressure of the pandemic surge, are not routinely testing for seasonal subtypes, this data should be interpreted with caution. Data from countries with NICs which have good virological surveillance nevertheless indicates pandemic (H1N1) 2009 is the predominant circulating virus with low levels of seasonal influenza activity.

Since the beginning of the pandemic in 19 April 2009 to 2nd January 2010 cumulatively 153 countries/areas/territories shared a total of 23,335 specimens (18,171 clinical samples and 5,164 virus isolates) with WHO CCs. Of these, 15,944 specimens (clinical samples and isolates) were tested; 12,876 (80.8%) were positive for influenza viruses. Of these positives, 8,628(67 %) were pandemic A(H1N1), 3,385(26.3%) were seasonal influenza A, and 848 (6.6%) were influenza B.

Systematic surveillance conducted by the Global Influenza Surveillance Network (GISN) including WHOCCs, continues to detect sporadically pandemic A (H1N1) viruses that are resistant to oseltamivir. So far, antiviral susceptibility testing was conducted by WHO CCs and other GISN labs on pandemic A (H1N1) specimens and isolates from at least 86 countries, and 190 cases of oseltamivir resistance have been reported by GISN so far. All of these viruses showed H275Y mutation but all these viruses remain sensitive to zanamivir. Worldwide, more than 15,000 clinical specimens (samples and isolates) of the pandemic A (H1N1) virus have been tested and found to be sensitive to oseltamivir.

All pandemic A(H1N1) 2009 influenza viruses analysed to date were antigenically and genetically closely related to the vaccine virus A/California/7/2009.

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.

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