People who are considering carrying or using an antiviral drug should consult their doctor for an individual recommendation.
To date, influenza A/H1N1 is sensitive to the antiviral prescription medications oseltamivir (Tamiflu) and zanamivir (Relenza). This means these drugs may be useful in treating H1N1. They are used to reduce the duration of illness in uncomplicated infections, and reducing severe illness and death. They are sometimes used in non-infected people to prevent them from contracting the virus. Pandemic H1N1 is resistant to the older antiviral medications amantadine and rimantidine.
The World Health Organization advises that antiviral medications should be used in accordance with national pandemic preparedness plans. Doctors should decide on use based on an individual's risk and benefit.
In a November 2009 press conference, WHO emphasized the importance of treating people at risk for serious infections immediately after they fall ill. Risk groups include pregnant women and people with some underlying health conditions. However, they said, otherwise healthy people with non-severe symptoms do not require treatment routinely.
See the WHO Recommended use of antivirals from February 2010.
US CDC guidance
Treatment: Early antiviral treatment is recommended for people hospitalized with suspected or confirmed H1N1 AND for anyone who is at higher risk of severe illness (including children under 2 years, people over 65 years, pregnant women, people with underlying chronic health problems).
Prophylaxis: Antiviral medication can be considered for prophylaxis after exposure for some people. These groups include people who are at high risk for severe influenza AND:
- are close contact of a confirmed, probable or suspected case
- are healthcare workers or first responders who had unprotected, close contact with a sick person while the person was infectious
US CDC advises pre-exposure prophylaxis should only be used in limited circumstances under the guidance of local health authorities. The guidance also contains interim recommendations on treatment and chemoprophylaxis for pregnant women, children and infants under the age of 1 year.
A study published in The New England Journal of Medicine showed that prompt antiviral treatment is important to reduce complications and fatalities in pregnant women and postpartum women who are vulnerable to severe pandemic flu infections. The US Centers for Disease Control and Prevention (CDC), which was involved in the study, altered their antiviral recommendations slightly in late October 2009 to reflect this data.
UK Health Protection Agency guidance: Advises offering treatment with antiviral medication for all suspected and confirmed cases. However, "it will remain a matter of clinical discretion as to whether antivirals should be prescribed in individual cases". Emphasis is put on giving treatment promptly to people in high risk groups. Close contacts are recommended preventive antiviral medication only if in high risk groups for severe illness.
Australian Department of Health Guidelines: Protect Phase recommends the use of antiviral medication for people showing symptoms of infection, without waiting for confirmation of diagnosis, if they are in high risk groups, have moderate or severe disease, or are deteriorating. The medication should be started as soon as possible, within 48 hours of onset of symptoms. Preventive medication is not generally recommended.
US FDA approves emergency use of experimental flu drug: The US Food and Drug Administration granted temporary approval for the emergency use of an intravenous drug, peramivir, for flu. The medication is still in the clinical studies stage. Between April 2009 and June 2010, the drug was approved for emergency use in hospitalized adults and children who are not responding to other antivirals, or who cannot use oral or inhaled antivirals. Peramivir can also be used in adults if doctors think it is appropriate. The approval was lifted in June 2010.
In early 2010, the drug gained a manufacture license in Japan and import license in Mexico. Later in 2010, the South Korean Food and Drugs Administration approved the marketing and manufacturing approval for peramivir to treat patients with influenza A and B viruses, including H1N1 2009 and avian influenza.
WHO has highlighted two particular instances in which the virus has increased opportunities to become drug-resistant.
- When immunocompromised people are treated with the drugs. They tend to stay infected longer, and require longer periods of treatment. Every moment that the virus and the drug are both present in the body provides an opportunity for the virus to develop drug resistance.
- When people take antivirals preventatively (prophylaxis) to try and prevent infection following a possible exposure to the flu. WHO advises against using drugs in this way. Instead, people who may have been exposed to pandemic flu should monitor their health and, if they are at-risk for severe infection, seek immediate care (and drug treatment) if they fall sick.
Officials have tested many thousands of samples of the pandemic H1N1 flu. Of them, fewer than 500 were resistant to oseltamvir (Tamiflu). This is within the expected parameters of virus behavior. Importantly:
- All virus samples that showed Tamiflu resistance were still responsive to the antiviral drug zanamivir (Relenza).
- Most of the people who had resistant strains had been taking Tamiflu preventatively before getting sick, which means the resistance developed within their own bodies. (They did not catch a resistant strain from someone else.)
- Almost every case was an isolated incident in which the patient who had the resistant strain did not pass it on to anyone else. (There may have been person-to-person spread of a resistant strain between two bunkmates at a summer camp in the US in July 2009.)
- Tamiflu-resistant pandemic flu probably spread among immunocompromised patients in a hospital in Wales, UK, in November 2009. The Health Protection Agency considers the risk to the general public low.
- It is possible, but not confirmed, that six students in Vietnam passed Tamiflu-resistant pandemic flu amongst themselves in June 2009.
- Additional information about antiviral resistance is available at http://www.cdc.gov/h1n1flu/recommendations.htm