The World Health Organization's recommendations guide vaccine manufacturers. (Charles Platau/Reuters)

Next season's flu shot for the Northern Hemisphere should include the H1N1 strain, the World Health Organization recommended Thursday.

Last year, the H1N1 virus emerged too late before the regular flu vaccine could be produced to include the strain.

For the coming flu season in the Northern Hemisphere, the two vaccines should be combined, said Keiji Fukuda, the WHO's top influenza expert. The announcement followed a closed-door meeting of influenza experts who advise vaccine makers and guide governments on vaccine stockpiles.

Flu vaccines normally include three strains of virus.

The WHO's recommended strains for the 2010-11 flu season in the Northern Hemisphere are:

• The pandemic strain, an A/California/7/2009 (H1N1)-like virus.
• Another influenza A strain known as A/Perth/16/2009 (H3N2)-like virus.
• An influenza B strain called B/Brisbane/60/2008-like virus.

"The inclusion of the H1N1 pandemic virus in the influenza vaccine does not signal that the pandemic is over," Fukuda told a news conference in Geneva.

"This virus is expected to be a significant threat to people as we go into the fall and winter period."

Defining pandemic end

Since it takes months for vaccine manufacturers to grow the virus in eggs and process it, experts meet in advance of flu season to determine which strains to focus on. In some years, the predicted and actual circulating strains are well-matched and other years the match is poorer.

On Tuesday, WHO's expert influenza committee will meet to consider whether the world is moving to a post-peak period, where the pandemic continues but the virus is declining and flu infection rates fall back to normal levels, Fukuda said.

Many countries have already scaled back their H1N1 response as cases dropped steadily. In January, the federal government announced it will donate five million doses of H1N1 vaccine to the World Health Organization for use in developing countries.

Defining when a pandemic ends remains unclear, said Michael Osterholm, a flu expert at the University of Minnesota.

"If H1N1 comes back in a couple of months, is it a wave or is it just normal flu?" Osterholm asked. "There's no scoreboard for us to check that."

With files from The Associated Press