Pandemic H1N1 influenza is now in virtually every country in the world, and health officials are bracing for an upsurge in cases as winter sets in, World Health Organization officials said Thursday.

"At WHO, we remain concerned about the pattern we are seeing, particularly because a sizable number of people do develop serious complications and death," Dr. Keiji Fukuda, a special adviser to the WHO director-general on pandemic influenza, said at a news conference. "We anticipate seeing continued or increased activity during the winter period in the Northern Hemisphere. This also means we expect to see continued reports of serious cases and deaths."

The most recent figures available show at least 5,700 people worldwide have died from swine flu, with 4,175 of those in the Americas. Fukuda said that vaccinations against swine flu have started in 20 countries and that millions of doses have been delivered.

He complained, however, that the agency had yet to receive most of the 200 million doses of vaccine that were to be donated by 11 countries. Delays in production of the vaccine have led to shortages, and most countries, like the United States, have chosen to vaccinate their high-priority groups before making good on their pledges.

In other flu news:

• Swine flu has struck the remote Yanomami tribe in Venezuela, showing that no one is safe from the virus. Seven people have died from the flu out of a population of 28,000. Several of the victims were babies, and one was a pregnant woman. Fukuda said the virus has struck aboriginal populations in Australia severely, and it is not clear yet whether the high rate of infection is related to a genetic susceptibility or poor health care.

• Cold weather has brought an outbreak of swine flu in Mongolia, and the country has been requesting additional doses of the antiviral drug Tamiflu to combat its spread. The country has confirmed 859 cases of swine flu and six deaths, but health care facilities have been struggling to cope with a surge in unconfirmed cases.

• In an effort to ease the burden on its health care system, Norway has decided to allow sale of Tamiflu without prescription. Some health officials have feared that such sales would lead to indiscriminate use of the drug, increasing the risk of the virus developing resistance to what is currently the most valuable tool to fight infection. Fukuda, however, called the decision to allow over-the-counter sales "innovative and prudent."

• An outbreak is occurring in Ukraine, with 500,000 cases of respiratory distress and 85 deaths, according to the European Centre for Disease Control. Both WHO and European officials have been sending in teams to help out.