Global Alert and Response (GAR)

Pandemic (H1N1) 2009 - update 96

Weekly virological surveillance update

Overall influenza activity has remained at low level in most parts of the Northern Hemisphere. Little activity has been reported in the Southern Hemisphere in 2010 to date. In most countries where human infection of influenza virus was reported, the pandemic influenza A(H1N1) continues to be the predominating influenza A viruses subtyped (100% in Canada and the United States and 81.3% globally).

Seasonal A(H1N1) viruses were detected very sporadically in Russian Federation. Sporadic influenza A(H3N2) activity was reported from some countries including China, Japan, Russian Federation, Singapore and Thailand . Influenza B was reported as predominating influenza virus accounting for 68.6% of all influenza detections in Russian Federation, 72.7% in Cameroon, 88.7% in China, 95.2% in Republic of Korea, and 100% in Mongolia. Increased sporadic influenza B activity has also been observed in some other countries including Italy, Kazakhstan, Latvia, Singapore and Sweden.

Based on FluNet reporting for the week from 28 March to 3 April 2010, the total number of specimens reportedly positive for influenza viruses by National Influenza Centres (NIC) from 33 countries was 1,665. Of these, 355/1,665(21.3%) were typed as influenza A and 1,310(78.7%) as influenza B.

From the start of the pandemic in 19 April 2009 to 3 April 2010, based on FluNet reporting by 84 countries, the total number of specimens reported positive for influenza by NIC laboratories was 605,957 *. Of these, 467,539(77.2%) were pandemic A(H1N1), 8,700 (1.4%) were seasonal A(H1N1), 31,273(5.2%) were A(H3N2), 73,291 (12.1%) were A (not subtyped) and 25,059 (4.1%) were influenza B.

Since the beginning of the pandemic on 19 April 2009 to 10th April 2010, cumulatively 155 countries shared a total of 25174 (19333 clinical and 5841 isolates) with WHO CCs for further characterization. The majority of pandemic A(H1N1) 2009 influenza viruses analyzed to date are antigenically and genetically closely related to the recommended vaccine virus A/California/7/2009.

Antiviral susceptibility surveillance has been conducted by the WHO Global Influenza Surveillance Network (GISN) including WHO CCs. So far, pandemic A(H1N1) specimens and isolates from at least 92 countries have been tested. The data showed that oseltamivir resistant pandemic A(H1N1) viruses are sporadically detected with rare onward transmission. So far, 285 cases of oseltamivir resistance have been reported by GISN and other partners. All of these viruses showed the H275Y substitution and all remain sensitive to zanamivir. (See below to obtain more information on oseltamivir resistant viruses).

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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