Tamiflu resistance in some H1N1 patients with badly weakened immune systems does not seem to reflect a major change in the virus' susceptibility to the frontline drug, the World Health Organization said on Thursday.
Nine people in Britain and the United States developed a Tamiflu-resistant form of swine flu while being treated in hospital mainly for blood cancers, said WHO flu expert Keiji Fukuda.
"We don't know the full answer. But it is more likely that we are not seeing a change, a major shift in the epidemiology or in the properties of these viruses with regard to oseltamivir resistance," he told a weekly news conference.
He said Tamiflu, known generically as oseltamivir and made by Switzerland's Roche, was effective when used correctly and early.
About 75 cases of oseltamivir-resistant viruses have been reported worldwide in recent months, mostly isolated cases which have arisen after preventive treatment with the drug, he said.
"Right now we do not see any evidence of a large impact in immuno-compromised people with milder forms and we do not see a large impact in HIV-infected populations," he said.
Flu marching on
Swine flu is expected to infect more people in the northern hemisphere in the next weeks before there is a downturn, but is less prevalent in the southern hemisphere, Fukuda said.
"It's still too early to say whether we are seeing peaking of activity in the northern hemisphere. Again, we see differences on a country by country basis," he said.
H1N1 has killed at least 6,770 people, according to the WHO. Most people suffer mild symptoms and recover without special treatment.
Some serious side effects have occurred after inoculation, but mostly "we have seen that the serious events are not related to the pandemic vaccine," Fukuda said.
In Canada, six people had severe allergic reactions after vaccination, but all recovered. Canadian provinces have stopped using a particular batch of GlaxoSmithKline Plc vaccine.
"As we understand, none of this vaccine was distributed outside Canada," Fukuda said. "All of the unused doses of vaccine have been put on hold, that is they are not being used at this point while investigations go on."