Since week 34/08, several sporadic, laboratory-confirmed influenza infections have been detected in the United Kingdom (UK): isolates have included influenza A(H3N2), A(H1N1) and influenza B. The first oseltamivir resistant influenza A(H1N1) for the 2008/09 season has also been identified in the UK through the HPA sentinel GP virological surveillance scheme. The virus contains the H274Y mutation but remains sensitive to zanamivir and amantadine, and is antigenically similar to the H1N1 reference strain A/Brisbane/59/2007, which is included in this season´s influenza vaccine. Antiviral susceptibility tests on A(H3) isolates showed that they are sensitive to oseltamivir and zanamivir.

With laboratory-confirmed sporadic influenza infections of various strains in circulation at the start of this autumn season, it is important to emphasise that people in the defined influenza risk groups should take up the recommendation of influenza vaccination. It is too early in the season to predict the course of the 2008/09 influenza season, and whether it will be dominated by the circulation of H1N1, H3N2, or influenza B. The Agency will be closely monitoring the characteristics of circulating isolates in order to determine the overall prevalence of drug resistant influenza A and B isolates.

Influenza virus detections across Europe have been low so far in the 2008/09 influenza season (13 to date) and the above-mentioned H1N1 oseltamivir-resistant influenza isolate is, to the Agency´s knowledge, the first detected in Europe. Influenza A oseltamivir resistance first emerged last season with a number of circulating influenza A(H1N1) isolates with the H274Y mutation, which confers resistance to oseltamivir, but not to zanamivir. By the end of the 2007/08 season, 26 out of 33 reporting European countries reported H1N1 oseltamivir resistance ranging from 4% in Spain to 67% in Norway, with 11% (38/347) in the UK [1,2]. The epidemiological evidence from the 2007/08 season suggested no reported increase in morbidity associated with these confirmed oseltamivir-resistant cases.

Reports between the second quarter 2008 and September 2008 from WHO showed high prevalence of resistance in the southern hemisphere with 100% (129/129) of H1N1 strains oseltamivir resistant in South Africa, and 96% (25/26) strains in Australia [2].

References
1. Centre for Disease Prevention and Control. Antivirals and antiviral-resistant influenza – resistance to oseltamivir (Tamiflu) in some influenza A(H1N1) virus samples. ECDC website [online] September 2008 [cited 24 October 2008]. http://ecdc.europa.eu/Health_topics/influenza/antivirals.html
2. WHO. Epidemic and Pandemic Alert Response: influenza A(H1N1) virus resistance to oseltamivir. WHO website [online] 13 October 2008 [cited 24 October 2008].http://www.who.int/csr/disease/influenza/h1n1_table/en/index.html.