Early use of antivirals is effective in treating H1N1 flu and health authorities must be vigilant for signs of drug resistance, the World Health Organisation said on Friday.

Drug-resistant pandemic flu viruses have appeared infrequently so far and there is no evidence they are spreading, but further cases are likely, the WHO said on its website.

Growing international experience showed the importance of the early use of oseltamivir, manufactured as Tamiflu by Roche Holding and Gilead Sciences, or zanamivir, an inhaled medicine produced as Relenza by GlaxoSmithKline in swine flu cases, the WHO said.

The experience of clinicians, including those who have treated severe cases of pandemic influenza, and national authorities, suggests that prompt administration of these drugs following symptom onset reduces the risk of complications and can also improve clinical outcome in patients with severe disease, the Geneva-based U.N. agency said.

This experience further underscores the need to protect the effectiveness of these drugs by minimizing the occurrence and impact of drug resistance, it said.

Most people who contract pandemic influenza suffer only mild symptoms before recovery without treatment, but children, pregnant women and those with some existing health conditions are vulnerable to a more severe attack or even death.

The WHO, which declared H1N1 a global pandemic in June, says one third of the world's population of nearly seven billion people could catch it.

The risk of resistance is higher in patients who suffer from weak immune systems and have already been treated with oseltamivir, it said.

It is also high in people who are treated with the antivirals as a prophylactic -- as a precaution after exposure to someone with influenza, but nevertheless develop the disease.

In such cases medical staff should investigate whether resistance has developed and take precautions to prevent the spread of a resistant virus, it warned.

Health authorities should also investigate whether a drug-resistant virus is spreading through communities from person to person.

The WHO repeated guidance that it did not recommend taking antivirals as a prophylactic. People exposed to pandemic flu victims should be monitored closely and treated promptly with antivirals if symptoms develop, it said.

Global monitoring has detected 28 resistant viruses so far. In each case the viruses were resistant to treatment with oseltamivir but not zanamivir, which should be used to help patients suffering severe illness from an oseltamivir-resistant virus, it said.

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