Antiviral drugs and pandemic (H1N1) 2009: Previous page | 1,2,3

6 October 2009 (originally posted on 21 May 2009)

Contents Antiviral drugs: definitions and treatment applications Antiviral drug resistance Antiviral drugs as prophylaxis

Antiviral drug resistance

Do antiviral drugs cause resistance, and why?

Development of resistance to antimicrobial agents (including antiviral drugs) is often seen when antiviral drugs are used in substantial quantities.

The speed with which such resistance develops, the percentage of viruses developing resistance and the ability of the new virus to spread will be influenced by several factors, including how the antiviral drug is used and whether the new (mutated) drug resistant virus can compete well enough to spread.

Almost half of the reported cases of oseltamivir resistant virus have occured in patients taking oseltamivir for prophylaxis. While the significance of this observation is not yet clear, prophylaxis should only be offered where there are strong reasons for doing so.

How many cases of antiviral drug resistance have been reported to WHO?

Up to 24 September 2009, 28 resistant viruses have been detected worldwide, characterized and reported to WHO.

Twelve of these drug-resistant viruses were from people using oseltamivir for post-exposure prophylaxis. Six were from patients with severe immunosuppression using oseltamivir for treatment. Four were from other patients receiving oseltamivir treatment. Two were from patients who were not taking oseltamivir for either treatment or prophylaxis. Characterization of the remaining viruses is under way.

Information about any cases of resistance will be regularly updated through the situation updates published on the WHO website on the Pandemic (H1N1) 2009 page.

Are more cases of antiviral drug resistance expected?

WHO expects more cases of oseltamivir-resistant virus to be reported as people continue to use oseltamivir over the coming months.

WHO receives notification of suspected cases of resistant virus through its laboratory network and will report on these as laboratory results are confirmed and the clinical and epidemiological data are available.

Is WHO concerned about resistance to antiviral drugs?

WHO will continue to monitor pandemic influenza (H1N1) 2009 viruses for signs that the number of antiviral drug resistant viruses is increasing enough to pose a major public health risk.

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