Pandemic (H1N1) 2009 - update 91
Weekly virological surveillance update
12 March 2010 - 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 (99.5% in North America region, 89.3% in the European region and 93% 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 been predominant China and China, Hong Kong SAR. Other countries reporting an increase in influenza B activity included Iran, Mongolia and the Russian Federation.
Based on FluNet reporting for the week from 21-27 February 2010, the total number of specimens reportedly positive for influenza viruses by National Influenza Centre (NIC) laboratories from 28 countries was 813. Of these, 563/813 (69.2%) were typed as influenza A and 250 (30.8%) as influenza B. Of all sub-typed influenza A viruses, 92.8% (426/459) were pandemic A(H1N1) 2009. China, Hong Kong SAR continued to report high influenza B activity accounting for 63,5% of all influenza detections in the reporting week.
From the start of the pandemic in 19 April 2009 to 27th February 2010, based on FluNet reporting by 82 countries, the total number of specimens reportedly positive for influenza viruses by NIC laboratories was 538,192 *. Of these, 420,761 (78.2%) were pandemic A(H1N1), 8,352 (1.6%) were seasonal A(H1N1), 29,942 (5.6%) were A(H3N2), 63,128 (11.7%) were A (not subtyped) and 15,925(3.0%) 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 6 March, 2010, cumulatively 153 countries shared a total of 24,332 specimens (18,756 clinical samples and 5,576 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.
A reassortant influenza A(H1N1) virus of swine origin distinct from the pandemic A(H1N1) 2009 virus isolated from 3 patients in Saskatchewan, Canada has recently been reported (1). All patients worked at the same large hog operation. The virus is from genetic reassortment between seasonal A(H1N1) and triple-reassortant influenza virus that emerged in North American swine during the late 1990s. The neuraminidase and haemagglutinin genes of these viruses were derived from human H1N1 virus and were closely related to those of the A/Brisbane/59/2007 vaccine virus. So far no there have been no further detections or transmission of this virus reported. This study highlights the need for increased virological surveillance to enable early detection of reassortant viruses with the potential for human-to-human transmission.
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.
For more information
Weekly update on oseltamivir resistance to pandemic influenza A (H1N1) 2009 viruses
- National Influenza Centres
- EuroFlu Weekly Electronic Bulletin
- PAHO Weekly Regional Update
- FluNet web site
(1) Bastien N et al, 2010. Human Infection with a Triple-Reassortant Swine Influenza A(H1N1) Virus Containing the Hemagglutinin and Neuraminidase Genes of Seasonal Influenza Virus. JID 201:000 - 000