UK findings hint at spread of resistant H1N1 strain

Feb 3, 2011 (CIDRAP News) – British researchers say 3 of 27 cases of oseltamivir (Tamiflu)-resistant 2009 H1N1 influenza detected in Britain so far this season were in outpatients who had no known exposure to the drug, suggesting that resistant strains may be spreading at a low level in the community.

Writing in Eurosurveillance today, the authors say the finding "suggests possible onward transmission of resistant strains and could be an indication of a possibility of changing epidemiology of oseltamivir-resistant influenza A(H1N1) virus." However, no transmission has been confirmed so far, they report.

Scattered cases of oseltamivir-resistant 2009 H1N1 have been detected since early in the 2009 pandemic, most of them in immunosuppressed patients or in hospital patients who had been treated with the drug. A Jan 14 report from the World Health Organization put the global total at 319 cases out of 20,000 samples tested, the British researchers note.

The researchers, from the United Kingdom Health Protection Agency and laboratories in Wales and Scotland, say the 27 resistant cases they found are the UK total for the season through the third week of January. The rate of resistance detection last season (2009-10) was similar, but all cases last year were in hospital patients, they report.

The 3 resistant cases in outpatients were among 364 outpatient samples tested, for a detection rate of 0.8%. The other 24 resistant cases were out of 2,500 hospital patient samples tested, a rate of 1.0%. The report offered no information on the patients' disease severity or outcomes.

"While the number of cases infected with a resistant strain who have been detected in the community is small, it is likely to have epidemiological significance," because no such cases were found among the 1,098 outpatient samples tested in the 2009-10 flu season, the report states.

Clinical and epidemiologic information was available for 24 of the 27 resistant cases from this season and 44 of 45 cases detected in 2009-10. Ten of 24 cases (42%) this season involved no known exposure to oseltamivir or contact with known cases of resistance, versus 5 of 44 cases (11%) last season, a significant difference, the authors say.

In addition, 7 of the 24 patients (29%) this season were immunosuppressed, compared with 34 of 44 patients (77%) last season, also a significant difference. The lower rate of immunosuppression as a risk factor this season may partly reflect increased alertness for resistance and increased use of zanamivir (Relenza), as advocated by UK national guidance, the authors write.

So far this season, there has been no documented onward transmission of resistant strains, whereas transmission was documented for 4 of 44 cases last season, the article says. That difference is not significant.

The researchers say they have not formally looked for possible clustering of resistant cases this season, but clustering is considered unlikely, "which therefore suggests the likelihood of low-level onward transmission of resistant strains."

They conclude that if the incidence of oseltamivir resistance in outpatients without evident risk factor rises above 10%, antiviral prescribing recommendations will need to be revised.

Lackenby A, Gilad JM, Pebody R, et al. Continued emergence and changing epidemiology of oseltamivir-resistant influenza A(H1N1) 2009 virus, United Kingdom, winter 2010/11. Euro Surveill 2011 Feb 3;16(5):pii=19784 [Full text]

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