Pandemic (H1N1) 2009 - update 94
Weekly virological surveillance update
1 April 2010 - Overall influenza activity has remained at low level in most part 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 (99.4% in North America and 93.8% globally).
Seasonal A(H1N1) viruses continue to be detected very sporadically in a couple of countries. Sporadic influenza A(H3N2) activity has been reported from some countries in recent weeks including China, Ghana and Thailand. Influenza B, as in recent weeks, has been predominating subtype accounting for 66.7% of all influenza detections in Republic of Korea, 71.6% in Russian Federation, 84.2% in China and 100% in Mongolia and Iran. Increased sporadic influenza B activity has also been observed in some European countries.
Based on FluNet reporting for the week from 14 to 20 March 2010, the total number of specimens reportedly positive for influenza viruses by National Influenza Centres (NIC) from 28 countries was 1,658. Of these, 414/1,658 (25%) were typed as influenza A and 1,244 (75.0%) as influenza B.
From the start of the pandemic in 19 April 2009 to 20 March 2010, based on FluNet reporting by 83 countries, the total number of specimens reportedly positive for influenza viruses by NIC laboratories was 599,659 *. Of these, 466,055 (77.7%) were pandemic A(H1N1), 8,618 (1.4%) were seasonal A(H1N1), 30,703 (5.1%) were A(H3N2), 72,702 (12.1%) were A (not subtyped) and 21,489 (3.6%) were influenza B.
Since the beginning of the pandemic on 19 April 2009 to 27th March 2010, cumulatively 155 countries shared a total of 24,703 specimens (18,877 clinical samples and 5,826 virus 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 91 countries have been tested. The data showed that oseltamivir resistant pandemic A(H1N1) viruses are sporadically detected with rare onward transmission. So far, 268 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.
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.