Development of a swine flu vaccine was partly triggered by evidence that seasonal influenza strains are developing resistance to antiviral drug Tamiflu, a World Health Organisation expert said Tuesday.
Nikki Shindo said relatively high rates of hospitalisation for swine flu in the United States and Mexico "rightly" prompted vaccine development "because we are also facing the risk of having resistant viruses."
"Last year we have seen widespread evidence of oseltamivir resistance in seasonal influenza," said the pandemic flu expert at a daily WHO briefing on the outbreak of influenza A(H1N1).
"So given that we have winter in Southern American countries and also the other parts of the southern hemisphere, there will be a risk of having viruses that will be highly resistant to antivirals."
Flu outbreaks are at their most active during winter.
Oseltamivir is the active ingredient in Tamiflu, an antiviral made by Swiss pharmaceutical manufacturer Roche which is at the centre of global precautions, led by the WHO, against influenza and a possible pandemic.
Roche said Tuesday it was donating 5.65 million treatment courses of Tamiflu to help the WHO fight the swine flu outbreak and boosted its production capacity for the next five months.
In the absence of an efficient vaccine against influenza A(H1N1) for now, only Tamiflu and Relenza, made by GlaxoSmithKline, are considered efficient drugs to keep the virus in check.
Data from the United States and Mexico suggested that nine to 10 percent of confirmed cases of swine flu require hospitalisation, Shindo said, adding: "This clearly is different from what we see from seasonal influenza, that's for sure."
However, the WHO is preparing to publish clinical guidance to doctors and nurses on the new A(H1N1) virus this week highlighting the fact that most patients would not require hospitalisation or antiviral therapy, she said.
Shindo also highlighted different approaches to the use of antivirals to treat swine flu on either side of the Atlantic.
While the United States and Mexico tended to confine such treatment for the most vulnerable, such as those with chronic illness or pregnant women, European countries resorted to the drugs more readily.
"The European countries which are mainly importing cases have been using anti-virals very aggressively," Shindo remarked.
Mexico and the United States, where the outbreak was first uncovered last month, together account for nearly 90 percent of the 5,251 cases of the A(H1N1) virus confirmed by the WHO in 30 countries.
However, there were hints this week that the virus might have already been circulating more widely there when it was first uncovered.
Anne Schuchat, the interim deputy director for science and public health at the Centres for Disease Control said the 2,600 confirmed cases in the United States represented just the tip of the iceberg of actual infections.
The vast majority of people who became ill with the virus tended to recover in the same way as with seasonal flu and probably did not seek laboratory testing, she explained.
Science magazine also released online a study by the WHO Rapid Pandemic Assessment Collaboration, which includes CDC scientists, which estimated that 23,000 people had been infected by the A(H1N1) virus in Mexico.