The United Nations World Health Organization says the H1N1 virus - commonly known as swine flu - has become the dominant flu strain in the world.

U.S. World Health Organization Assistant Director Keiji Fukuda attends a conference on swine flu in Cancun, Mexico (File)In a briefing with reporters Thursday, WHO Special Pandemic Influenza Adviser, Dr. Keiji Fukuda, said the H1N1 virus has been particularly active in the Northern Hemisphere in recent months.

He said he expects that trend to continue through the winter months, with more severe cases and even deaths.

Fukuda said the vaccine currently in use in some 20 countries around the world remains effective and safe. He said no rare or dangerous side effects have been reported.

The H1N1 virus has differed from normal, seasonal flu in that it has persisted during the summer months and affected relatively healthy people under the age of 65. The WHO says most people recover without hospitalization.

The World Health Organization has declared the swine flu outbreak a pandemic. As of October 25, 2009, the WHO says there have been more than 440,000 laboratory confirmed cases of H1N1 influenza and more than 5,700 deaths reported to the U.N. agency.