Last Updated: 2003-06-06 12:24:19 -0400 (Reuters Health)

LONDON (Reuters) - Influenza drugs are effective in treating and preventing the illness but they are expensive and there is not enough evidence showing they work in high risk groups such as children and the elderly, doctors said on Friday.

Oseltamivir, which is produced by Swiss healthcare group Roche Holding AB under the brand name Tamiflu, and GlaxoSmithKline Plc's zanamivir or Relenza, are drugs known as neuraminidase inhibitors.

They work by blocking the action of viral enzymes.

In an analysis of clinical trials of the drugs, researchers at the University of Leicester in England found they reduced the likelihood of getting flu by about 70-90 percent and cut the duration of the illness by about a day.

But annual vaccination is still regarded as the best way to prevent flu epidemics which affect between three million and five million people each year and kill as many as 500,000.

"The drugs are both effective at reducing the length of symptoms and incidence if properly administered. They have to be given within 48 hours of onset," Alexander Sutton, a researcher at the university, said in an interview.

But he added that more research is needed into the effect of the drugs in the very young and old and in other high-risk people with heart problems and asthma to see if they can prevent deaths or other serious outcomes.

In a commentary in the research reported in the British Medical Journal, Klaus Stohr, the project leader of the World Health Organisation Global Influenza Programme, said the drugs cannot replace annual influenza vaccination.

"Neuraminidase inhibitors are clinically effective complements to the current influenza intervention tools," he said.

"However, costs and lack of data on their effectiveness in the groups most severely affected by influenza limit their use in many industrialised countries and make them largely unaffordable in developing countries," he added.

The drugs, which cost about 24 pounds ($39.23) in Britain for a course of treatment, are used mostly in Japan and the United States.

Only about 50 countries, most in the industrialised world, have influenza immunisation policies. The WHO is urging countries to increase their vaccination coverage to all people at high risk to at least 50 percent by 2006 and 75 percent by 2010.