Antiviral drugs: Definitions and Treatment Applications

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.

Antiviral Drug Resistance

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.

Almost half of the reported cases of oseltamivir resistant virus have occured in patients taking oseltamivir for prophylaxis. While the significance of this observation is not yet clear, prophylaxis should only be offered where there are strong reasons for doing so.

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

Up to 24 September 2009, 28 resistant viruses have been detected worldwide, characterized and reported to WHO.

Twelve of these drug-resistant viruses were from people using oseltamivir for post-exposure prophylaxis. Six were from patients with severe immunosuppression using oseltamivir for treatment. Four were from other patients receiving oseltamivir treatment. Two were from patients who were not taking oseltamivir for either treatment or prophylaxis. Characterization of the remaining viruses is under way.

Information about any cases of resistance will be regularly updated through the situation updates published on the WHO website on the Pandemic (H1N1) 2009 page.

Situation updates - Pandemic (H1N1) 2009

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 major public health risk.

Antiviral Drugs as Prophylaxis

Should individuals keep a personal stockpile of antiviral drugs?

WHO does not recommend that individuals stockpile antiviral drugs such as oseltamivir or zanamivir, unless recommended by a health care provider or other national health authority. In some instances, such authorities may provide antiviral drugs in advance to individuals at particular risk, or to those who may not have rapid access to medicines in the event of illness.

Should individuals take antiviral drugs for the prevention of pandemic (H1N1) 2009 infections?

In general, WHO does not recommend the use of antiviral drugs for prophylactic purposes. For people who have had exposure to an infected person and are at a higher risk of developing severe or complicated illness, an alternative option is close monitoring for symptoms, followed by prompt early antiviral treatment should symptoms develop.

Where can individuals purchase antiviral drugs?

Antiviral drugs, such as oseltamivir and zanamivir, are generally available by prescription only, though some countries have made special arrangements during the pandemic to ensure that antiviral drugs can be made available quickly.

Antiviral drugs should only be obtained through recognized medicines suppliers (e.g.: pharmacies) and in accordance with national prescription procedures. In particular, products sold outside the authorized national medicine supply chains should not be purchased.

In addition to the need for a prescription, caution should be exercised when buying antiviral drugs over the Internet.

Warning on purchase of antivirals without a prescription, including via the Internet [pdf 35kb]