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By Stephanie Nebehay
GENEVA (Reuters) - The H1N1 swine flu virus has picked up steam in the northern hemisphere and is expected to cause more serious infections and deaths as cold weather sets in, the World Health Organisation (WHO) said on Thursday.
But the virus is not known to have mutated, including in people infected in a large outbreak in Ukraine, meaning that current pandemic vaccines continue to provide protection, it said.
Mexico is reporting more H1N1 cases than early in the pandemic, which began in April, and the United States shows higher levels of flu-like illness than in past years, top WHO flu expert Keiji Fukuda said. Swine flu is also on the rise in Europe and Central Asia.
"We anticipate seeing continued or increased activity during the winter period in the northern hemisphere. This also means that we expect to see continued reports of serious cases and deaths," Fukuda told a news conference. "At WHO we remain quite concerned about the pattern that we are seeing."
Most people recover without specialised medical care for symptoms such as fever, cough and sore throat, but pregnant women and people with underlying chronic conditions like asthma are at higher risk of potentially fatal complications, he said.
At least 5,712 people worldwide have died from swine flu, which is now present in virtually every country, according to the United Nations agency. Most serious illness and fatalities occur in patients younger than 65, a different pattern to seasonal influenza, which traditionally strikes the elderly.
The European Centre for Disease Prevention and Control said that as of Wednesday, some 500,000 cases of acute respiratory illness and 86 related deaths had been reported in Ukraine.
Pandemic vaccines given to millions of people in some 20 countries in recent weeks have shown them to be "very safe", providing protection with no unusual side effects, Fukuda said.
The WHO has yet to receive some 200 million vaccine doses donated by 11 countries, which are intended for distribution in 95 poor countries lacking supplies, he said.
"Vaccine companies out there are producing as much vaccine as quickly as possible. Much of the vaccine has been allocated to different countries on the basis of contracts," he added, referring to deals between drugmakers and governments.
GlaxoSmithKline and Sanofi-Aventis are among some 25 companies producing pandemic vaccine.
The known cases of resistance to antiviral drugs have been "isolated and infrequent", according to Fukuda.
"We see no evidence at all that there is widespread occurrence of antiviral resistance," he said.
Antiviral drugs such as oseltamivir, marketed by Swiss drugmaker Roche Holding as Tamiflu, are considered the frontline drug against the H1N1 virus.
Norway's decision to allow over-the-counter sales of the drug so as to relieve stress on primary health-care systems appears to be "innovative and prudent", Fukuda said. (Additional reporting by Jonathan Lynn in Geneva and Kate Kelland in London; editing by Philippa Fletcher)