February 2005

Information

Antivirals

Antivirals are drugs used for the prevention and early treatment of influenza. If taken shortly after (within 48 hours) getting sick, they can reduce influenza symptoms, shorten the length of illness and potentially reduce the serious complications of influenza. Antivirals work by reducing the ability of the virus to reproduce but do not provide immunity against the virus.

Vaccines, the most effective public health tool to fight an influenza pandemic, provide immunity against the influenza virus but cannot be produced until the pandemic strain of influenza has emerged. Antivirals, however, can be stockpiled in advance.

Two classes of antivirals are currently available in Canada and have a role in the prevention and treatment of influenza: M2 ion channel inhibitors and neuraminidase inhibitors.

M2 ion channel inhibitors stop the virus from reproducing and are effective against influenza A viruses but not influenza B. Amantadine and rimantadine are examples of M2 ion channel inhibitors. Currently, only amantadine is licensed for use in Canada. Research, however, shows that influenza viruses rapidly develop resistance to amantadine when it is used for treatment.

Zanamivir and oseltamivir are examples of neuraminidase inhibitors. These drugs interfere with replication of both influenza A and B viruses in three ways:

• They interfere with the release of virus from infected cells
• They cause the aggregation of virus
• They may help respiratory secretions make the virus inactive

Based on research, neuraminidase inhibitors are the preferred drugs for use during a pandemic because they have a lower risk of adverse events and decreased evidence of drug resistance.

The World Health Organization (WHO) has recommended oseltamivir specifically for treatment of avian influenza and has recommended that countries consider stockpiling it for use against a pandemic strain of influenza. Studies done through the WHO Global Influenza Surveillance Network have shown that the H5N1 strain of avian influenza, considered a pandemic-like virus, is susceptible to oseltamivir. The strain has demonstrated resistance to the M2 inhibitors.

• Frequently Asked Questions - Avian Influenza
• Frequently Asked Questions - Pandemic