Swiss pharmaceuticals specialist Roche reckons there is no new evidence that resistance to its influenza drug Tamiflu is on the rise, despite scientists' concerns.
The company made its claim a week after leading British academics warned that Roche's anti-viral treatment would not alone mitigate the effects of a bird flu pandemic.
"Over the past few months, there has been erroneous speculation that resistance to Tamiflu is increasing. This is an area that Roche and independent groups have been closely monitoring and there is no scientific evidence to suggest this is happening," said David Reddy, the company's head of pandemic flu, in a statement.
The Swiss firm said governments could be confident that Tamiflu remains a critical drug, as recommended by the United Nations' World Health Organization, for stockpiling to prepare for an influenza pandemic and for doctors and patients to treat and prevent flu when it hits.
Last week leading scientists called on the British authorities to stock more than one antiviral drug to tackle any bird flu pandemic.
A report from the Royal Society and the Academy of Medical Sciences in Britain said the H5N1 avian virus, which public health experts fear could develop into a pandemic strain, might develop resistance to Tamiflu.
To overcome any potential problems, the scientists recommended in a report the government should also stockpile GlaxoSmithKline's antiviral drug Relenza.
John Skehel, chairman of the working group that produced the report, said strains of the virus resistant to Tamiflu had appeared when the drug had been used against seasonal influenza and in a small number of patients infected with H5N1.
"It is known [...] that not all viruses that are resistant to Tamiflu are also resistant to Relenza. That is the specific basis for recommending a joint stockpile of the two," he said.
Britain has stockpiled about 14.6 million treatment courses of Tamiflu. In Switzerland, the figure is two million, part of a national influenza pandemic plan that has been set up and is to be discussed by Swiss experts next week in Geneva.
Earlier this year a Swiss study predicted that the deadly bird flu virus would develop resistance to new anti-viral drugs if a pandemic breaks out.
The mathematical model used by researchers at Zurich's Federal Institute of Technology to plot the likely course of an outbreak showed that resistance is more likely if drugs are used as prophylactics.
The study, commissioned by the journal Science, used data from previous influenza outbreaks combined with recent information about bird flu and the results of clinical trials on resistance to so-called neuraminidase inhibitor drugs such as Tamiflu.
Roche said that so far there had been little evidence of resistance to this type of medication, although this could occur if a patient failed to receive the prescribed dosage. According to the company, the last case of resistance was recorded in March 2005.
Tamiflu is one of Roche's best-selling products, with sales of SFr1.6 billion ($1.33 billion) in 2005.
swissinfo with agencies
Two drugs (in the neuraminidase inhibitors class), oseltamivir (Tamiflu) and zanamivir (Relenza), have been shown, in laboratory studies, to reduce the severity and duration of illness caused by seasonal influenza.
The efficacy of the neuraminidase inhibitors depends on their administration within 48 hours after symptom onset.
For cases of human infection with H5N1, the drugs may reduce the severity of disease and improve prospects of survival, if administered early, but clinical data are limited.
The H5N1 virus is expected to be susceptible to the neuraminidase inhibitors.
The number of confirmed cases of avian influenza worldwide is 258 with 153 deaths, according to the World Health Organization.
No cases of human bird flu have been reported in Switzerland.
According to the latest government plan, 1.85 million people could become infected with bird flu if the virus is able to pass from human to human.
In the event, 46,000 people would be admitted to hospital and around 7,400 people would die.
Such a pandemic would cost an estimated SFr2.3 billion.