Global Alert and Response (GAR)

Pandemic (H1N1) 2009 - update 89

Weekly virological surveillance update

Pandemic influenza A(H1N1) 2009 infections 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 nearly all countries where human influenza infection is reported, the pandemic influenza A (H1N1) continues to be the predominant subtype among all influenza A viruses subtyped (87.3% in North America region, AMR, 94% in EUR and 90% global). Seasonal A(H1N1) viruses continue to be detected very sporadically. Sporadic influenza A(H3N2) activity has been reported from some countries in recent weeks. Influenza B activity continued to increase in China and Hong Kong SAR China.

Based on FluNet reporting for the week from 6-13 February 2010*, the total number of specimens reportedly positive for influenza viruses by NIC laboratories was 2,076. Of these, 1,003/2,076 (48.3%) were typed as influenza A and 1,073 (51.7%) as influenza B. Of all sub-typed influenza A viruses, 90% (781/866) were pandemic A(H1N1) 2009. Hong Kong SAR China has reported increased influenza B activity in recent weeks accounting for 56.1% of all influenza detections in the reporting week, while in China it accounted for 83.5%.

From the start of the pandemic in 19 April 2009 to 13th February 2010, based on FluNet reporting, the total number of specimens reportedly positive for influenza viruses by NIC laboratories was 535,731. Of these, 418,795 (78.2%) were pandemic A(H1N1), 8,345 (1.6%) were seasonal A(H1N1), 29,936 (5.6%) were A(H3N2), 62,817(11.7%) were A (not subtyped) and 15,754 (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 20thFebruary, 2010, cumulatively 153 countries shared a total of 24186 specimens (18663 clinical samples and 5523 virus isolates) with WHO CCs for further characterization. The majority of pandemic A (H1N1) 2009 influenza viruses analysed to date were antigenically and genetically closely related to the vaccine virus A/California/7/2009.

Following WHO Consultation on the Composition of Influenza Vaccines for the Northern Hemisphere 2010-2011 and on Vaccine Viruses for Pandemic Preparedness from 14 - 18 February 2010, WHO recommended the viruses to be used for influenza vaccines in the 2010-2011 northern hemisphere influenza season.

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, 253 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 cases.

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.

*A total of 32 countries reported to FluNet for the week 6 - 13 February 2010.

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