Summary of influenza antiviral susceptibility surveillance findings, September 2010 - March 2011

6 June 2011

The WHO Global Influenza Surveillance and Response System (GISRS) (1) has been closely monitoring the evolution of influenza viruses infecting humans, including their susceptibility to antiviral drugs.

The vast majority of influenza A(H1N1) 2009 (hereafter A(H1N1)) viruses tested so far in the WHO GISRS laboratories were sensitive to oseltamivir. From September 2010 to March 2011, of 5814 A(H1N1) viruses tested in 5 WHO Collaborating Centres (CCs) of GISRS, 89 (1.5%) were resistant to oseltamivir. The overall monthly detection rate of resistance to oseltamivir ranged between 0% and 2.6% and increased slightly from October 2010, then declined in March 2011 (Figure 1). However, the detection rate and trend vary in individual WHO CCs. In Japanese viruses, the highest detection rate of 5.8% was reported in March 2011, with a consistent, though slight, increasing trend observed since December 2010 (Table 1). All these resistant viruses were associated with a histidine to tyrosine substitution at amino acid 275 (H275Y) in the neuraminidase.

All A(H1N1) viruses tested in WHO CCs to date were sensitive to zanamivir and all were resistant to the M2 inhibitors, amantadine and rimantadine.

Outside of the WHO CCs, based on TESSY/EuroFlu reporting, from September 2010 to March 2011, National Influenza Centres (NICs) in Europe tested 3080 A(H1N1) viruses, of which 91 (2.95%) were resistant to oseltamivir. Similarly, a slight increase in the detection rate was observed up to February 2011, but dropped significantly in March. Outside Europe, NIC Singapore reported one detection in February of oseltamivir resistant virus to FluNet.

Associated clinical history information has been scarce. Based on available data, among Japanese oseltamivir resistant viruses, the vast majority were from cases treated or prophylaxed with oseltamivir. However, from NIC UK, the percentage of resistant viruses from cases with no known exposure to oseltamivir increased from 11% in the 2009-2010 season to at least 28% in the 2010-2011 season.

Global influenza virological surveillance has been conducted in the past 59 years by the WHO GISRS, currently comprising 136 National Influenza Centres, 6 WHO Collaborating Centres, 4 Essential Regulatory Laboratories and 12 WHO H5 Reference Laboratories.

(1) Former GISN. The WHO Global Influenza Surveillance Network (GISN) has been renamed as the WHO Global Influenza Surveillance and Response System (GISRS) following the adoption of WHA 64.5 on 24 May 2011.