Global Alert and Response (GAR)

Antiviral drugs for pandemic (H1N1) 2009: definitions and use

22 December 2009

What are antiviral drugs?

Antiviral drugs are medicines that act directly on viruses to stop them from multiplying.

Are antiviral drugs used for treatment of pandemic (H1N1) 2009 infection?

Yes, two antiviral drugs are being used to treat pandemic influenza infections. These are oseltamivir and zanamivir, which both block the action of an influenza virus protein called neuraminidase. In clinical trials with seasonal influenza, these antiviral drugs have been shown to reduce the symptoms and duration of illness. Direct reports and observations from clinicians around the world also suggest that these treatments contribute to preventing severe disease and death related to pandemic influenza (H1N1) 2009 infections.

For the treatment of pandemic (H1N1) 2009, how many antiviral drugs are there?

There are two approved antiviral drugs for influenza that are available for treatment of pandemic influenza. These are the neuraminidase inhibitors oseltamivir and zanamivir, more commonly known by their trade names Tamiflu and Relenza.

Another class of approved antiviral drugs known as M2 inhibitors (amantadine and rimantadine) can be effective for treating seasonal influenza. However, the pandemic (H1N1) 2009 virus has been shown to be resistant to these particular antiviral drugs.

What is WHO’s guidance on the use of antiviral drugs?

For patients with symptoms of severe illness that are probably due to pandemic influenza, WHO recommends that treatment with oseltamivir should start immediately, no matter when the illness started and without waiting for laboratory results to confirm infection.

For patients at higher risk for serious disease from pandemic influenza, including pregnant women, children under age 5 and those with certain underlying medical conditions, WHO recommends treatment with either oseltamivir or zanamivir as soon as possible after the onset of symptoms, and without waiting for the results of laboratory tests.

People who are not from a higher risk group but who have persistent or rapidly worsening symptoms should be treated with antivirals. These symptoms include difficulty breathing or a high fever that lasts beyond three days.

In all cases, where oseltamivir is unavailable or cannot be used for any reason, zanamivir may be given.

What is meant by a higher risk group?

Individuals who have been identified as higher risk of more complicated or severe illness associated with infection by the pandemic influenza virus include:

What is meant by at-risk groups?

Individuals that have been identified as “at-risk” of more complicated or severe illness associated with infection by influenza virus include:

  • pregnant women (particularly in the later stages of pregnancy);
  • infants and children (<age 5);
  • patients with chronic health conditions such as cardiovascular, respiratory or liver disease, or diabetes;
  • patients with immunosuppression related to treatment for transplant surgery, cancer or due to other diseases.

The elderly (>65) appear less susceptible to infection by pandemic H1N1 influenza virus, but are assumed to be at higher risk of more severe or complicated illness if infected.

Are antiviral drugs suitable for everyone?

Antivirals should only be used when prescribed by a qualified health care provider, as they will be able to assess each situation and make the appropriate decisions on care.

People who are not from a higher risk group and experience mild symptoms need not take antivirals. Most people recover from the pandemic influenza without the need for medical care.

Is it necessary to wait for a laboratory result before starting antiviral drug treatment?

No, if antiviral drug treatment is indicated by the clinical presentation, then treatment should start as soon as possible. If there is a delay, treatment may be less effective.

What is the standard treatment regimen for antiviral drugs?

For oseltamivir, the standard adult treatment course is one 75 mg capsule twice a day for five days. For severe or prolonged illness, physicians may decide to use a higher dose or continue the treatment for longer.

Zanamivir is taken as a powder by inhalation. The recommended dose for treatment of adults and children from the age of 5 years is two inhalations (2 x 5mg) twice daily for five days.

(updated from 6 October 2009 and 21 May 2009)