Global Alert and Response (GAR)

Pandemic (H1N1) 2009 - update 90

Weekly virological surveillance update

Pandemic influenza A(H1N1) 2009 circulating viruses have continued to decrease in most countries of the Northern Hemisphere in recent weeks. Little activity has been reported in the Southern Hemisphere in 2010 to date. In most countries where detection of human influenza virus was reported, the pandemic influenza A(H1N1) continues to be predominating subtype among all influenza A viruses subtyped (87.3% in North America region, 86% in the European region and 75% globally). The number of pandemic influenza A(H1N1) viruses tested globally has decreased over recent weeks. Seasonal A(H1N1) viruses continue to be detected very sporadically in only a few countries. Sporadic influenza A(H3N2) activity has also been reported from some countries. As in recent weeks, influenza B activity has continued to increase in China and China, Hong Kong SAR. Other countries reporting an increase in influenza B activity included Iran, Mongolia and United Republic of Tanzania.

Based on FluNet reporting for the week from 14-20 February 2010, the total number of specimens reportedly positive for influenza viruses by National Influenza Centre (NIC) laboratories from 28 countries was 898. Of these, 365/898 (40.6%) were typed as influenza A and 533 (59.4%) as influenza B. Compared the previous reporting week, this represents an overall decrease in the proportion of influenza A positive specimens relative to the influenza B positives. Of all sub-typed influenza A viruses, 75% (243/365) were pandemic A(H1N1) 2009. China continued to report increased influenza B activity accounting for 88.4% of all influenza detections in the reporting week, while in China, Hong Kong SAR it represented 54.5% of circulating influenza.

From the start of the pandemic in 19 April 2009 to 20th February 2010, based on FluNet reporting by 82 countries, the total number of specimens reportedly positive for influenza viruses by NIC laboratories was 536,355 *. Of these, 419,289 (78.2%) were pandemic A(H1N1), 8,347 (1.6%) were seasonal A(H1N1), 29,938 (5.6%) were A(H3N2), 62,881(11.7%) were A (not subtyped) and 15,806 (2.9%) were influenza B. Cumulatively, the pandemic virus continued to be predominating subtype reported with detection of a very small proportion of seasonal strains.

Since the beginning of the pandemic in 19 April 2009 to 27thFebruary, 2010, cumulatively 153 countries shared a total of 24,304 specimens (18,748 clinical samples and 5,556 virus isolates) with WHO CCs for further characterization. The majority of pandemic A (H1N1) 2009 influenza viruses analysed to date are antigenically and genetically closely related to the 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 91 countries have been tested, showing that oseltamivir resistant pandemic A (H1N1) viruses are sporadic with rare onward transmission. So far, 264 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. Click here 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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