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H1N1 (Swine Flu):Tamiflu Resistance in H1N1

July 6, 2009

With a vaccine still months away, currently our only medical bullet in the arsenal to fight H1N1 remains antivirals.  So the news of cases of Tamiflu resistance is met with concern.  However all cases of Tamiflu resistance are not created equal. So while the first three instances of H1N1 infection with Tamiflu-resistant viruses were reported in the past week, it was Number three, not Number one (Denmark) that put influenza experts on edge. Public health authorities in Hong Kong announced Friday [3 Jul 2009] they have found a case of Tamiflu resistance in a woman who hadn’t taken the drug. That means she was infected with H1N1 viruses that were already resistant to Tamiflu, the main weapon in most countries’ and companies’ pandemic drug arsenals.

Tamiflu-Reuters

The two earlier cases (Denmark and Japan) involved people who had been taking the medication. While always unwelcome, that type of resistance is known to occur with seasonal [influenza virus] strains and may be less of a threat to the long-term viability of this key flu drug. “It was not at all surprising to see resistance in patients on treatment, but seeing it in someone who was not treated, it certainly is more concerning,” says Dr. Malik Peiris, a flu expert at the University of Hong Kong.

There is currently no evidence Tamiflu-resistant viruses are spreading widely. Still, some experts see the Hong Kong case as a warning that Tamiflu’s role in this pandemic may not be as long-lived as pandemic planners would like. “I think it’s too early to judge,” says Dr. Frederick Hayden, an expert on influenza antivirals who teaches at the University of Virginia. “But I think that possibility has existed from the beginning, and it’s something that needs to be certainly considered in making determinations about things like antiviral stockpiling, management of patients with more serious illness in hospital and how the available drugs will be used.”

Some experts say this early sign of resistance should prompt a rethink of how often and in which circumstances Tamiflu is used to battle the novel H1N1 virus. “It … probably highlights the importance of not using these antiviral drugs indiscriminately, given that the disease is relatively mild,” says Peiris, whose hospital monitored the woman who was found to be carrying the resistant virus. “In people who don’t have underlying risk factors, they probably should not be treated with Tamiflu, basically.”

Others suggest countries should limit how often they use the drug to prevent infection, a regimen known as prophylaxis. In prophylaxis, people who’ve been exposed to the virus are given one pill a day for 10 days, compared to the treatment regime of 2 pills a day for 5 days. Some countries, including Canada, have been reserving prophylaxis for people at high risk from this flu, such as pregnant women. But others have taken a different approach, using Tamiflu to try to curb spread of the virus. For instance, Britain has made the drug widely available to contacts of confirmed cases, though it announced this past week it was changing that policy.

The World Health Organization is drafting guidance for countries on the use of antivirals. While the WHO advises rather than instructs, it has been stressing that saving these drugs for treatment makes the most sense, says Dr. Keiji Fukuda, the agency’s top flu expert. “In general we have been pushing the advice that using these drugs for treatment is definitely the priority use of them,” says Fukuda, the acting assistant director general for health security and environment.

“And I think this is not just from a theoretical resistance perspective but also from the fact that if you have limited amounts of antiviral drugs, then you need to make some choices about how you use them.”

There is a risk inherent in using the drug to prevent illness. If people who are already infected but aren’t yet experiencing symptoms are put on prophylaxis, there won’t be enough drug in their systems to kill all the viruses they house. Those that survive develop resistance to the drug. And that, it appears, may be what happened in the resistance cases in Denmark and Japan. In both instances the women involved had been given Tamiflu prophylaxis after a contact developed swine flu.

But the Hong Kong case was different. A 16-year-old girl travelling  from San Francisco was stopped in Hong Kong’s airport in mid-June [2009] after setting off a fever detection device. She was taken to hospital where she tested positive for H1N1. She had not been taking antivirals and declined to be treated with the drug. She was kept in isolation until she recovered.

Stay tuned…things are changing rapidly.

ProMED Digest V2009 #324 – www.promedmail.org

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