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Antiviral drugs and pandemic (H1N1)

6 October 2009 (originally posted on 21 May 2009)


Antiviral drugs: definitions
and treatment


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 (H1N1) 2009 infection. 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 and may also contribute to preventing severe disease and death. Since these antivirals have been effective in treating seasonal influenza, they are also expected to be effective for pandemic (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?

Studies show that early treatment, preferably within 48 hours after the first sign of symptoms, appear associated with better clinical outcome.

For patients who initially present with severe illness or whose condition begins to deteriorate, WHO recommends that treatment with oseltamivir should start immediately, no matter when illness started and without waiting for laboratory results.

For patients at risk for serious disease, including 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.

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

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. WHO recommends that all patients (including pregnant women) and all age groups (including young children and infants) should be treated with oseltamivir in the event of severe or deteriorating illness. Treatment with either oseltamivir or zanamivir should also be offered to all patients in at-risk groups in the event of illness, even if mild or uncomplicated.

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 (<5 years);
  • 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.

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.

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