Global Alert and Response (GAR)

Pandemic (H1N1) 2009: antiviral drug resistance

22 December 2009

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.

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

Up to 8 December 2009, 109 oseltamivir-resistant H1N1 viruses have been detected worldwide, characterized and reported to WHO.

Although the majority of reported cases are associated with oseltamivir treatment, cases of resistance have occurred in patients taking oseltamivir to prevent against pandemic influenza infection.

Around one third of the cases of oseltamivir-resistant H1N1 viruses have occurred in patients whose immune systems were severely immunocompromised.

Severely immunocompromised hospital patients need to be regarded as an especially vulnerable group, according to clinical and other experts who recently investigated two clusters of oseltamivir resistance in the United Kingdom and the United States. These patients are highly susceptible to infection, particularly difficult to treat, and especially likely to develop resistance. The experts recommended modified treatment approaches with antivirals, as well as vaccination against the pandemic influenza for health workers, care givers, family members and others in contact with these patients.

Although all incidents of oseltamivir resistance merit investigation, and are fully investigated, no evidence suggests that events to date constitute a public health threat.

Information about any cases of resistance will be regularly updated through the pandemic influenza situation updates.

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 public health risk.

(updated from 6 October 2009 and 21 May 2009)

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