WHO picks pandemic strain for next seasonal flu vaccine

Feb 18, 2010 (CIDRAP News) – Given signs that the pandemic H1N1 virus will continue its dominance over other flu strains, the World Health Organization (WHO) today recommended adding the pandemic strain as the H1N1 component of the seasonal flu vaccine for the Northern Hemisphere's next flu season.

The WHO also changed the other influenza A strain, replacing the Brisbane H3N2 component with a Perth H3N2 strain. The influenza B component remains the same Brisbane strain included in this year's seasonal flu vaccine.

At a media briefing today to announce the WHO's new seasonal flu vaccine recommendation, Keiji Fukuda, MD, special adviser to the WHO director-general on pandemic flu, said that including the pandemic strain in next season's flu vaccine isn't a signal that the pandemic is over. Instead, adding the strain to the vaccine is a separate issue that simply addresses which strains the WHO's experts believe the seasonal flu vaccine should cover. "The virus is expected to be a significant threat as we go into next fall and winter," he said.

In view of a continuing downturn in pandemic flu activity in the Northern Hemisphere, with sporadic activity elsewhere, Fukuda said the WHO's emergency committee will meet on Feb 23 to discuss the possibility of a move to the postpeak pandemic phase. "As we begin to transition, we can still expect to see significant outbreaks in different countries," he said, pointing to recent reports of community spread in the West African nations of Senegal and Mauritania.

Today's recommendation came at the end of a 4-day meeting of advisors from the WHO influenza collaborating centers and national regulatory authorities. Fukuda said a number of observers, mainly representatives from national labs and technical groups, attend the proceedings but have no formal input. He said pharmaceutical representatives are not part of the strain selection process. Some elected officials in Europe have accused the WHO of bending to vaccine maker pressure, a charge that the WHO and others familiar with WHO proceedings have denied.

Each February the WHO assesses the flu strains circulating throughout the globe before recommending the strains for the next Northern Hemisphere flu season. It takes about 6 months for vaccine manufacturers to grow the viruses in chicken eggs and formulate them into trivalent vaccines.

Changing one or two strains is not unusual. This year's recommendation, a change of two strains, closely matches the recommendation made in September for the Southern Hemisphere's flu season, which spans May through October. For the Southern Hemisphere's H3N2 component the WHO recommended A/Wisconsin/15/2009, which is a Perth-like virus. Fukuda said that though the pandemic H1N1 virus has pushed out other flu strains in many parts of the world, some countries have reported circulation of H3N2 and B strains, particularly China, where influenza B has recently become the dominant strain.

Of the few H3N2 isolates the US Centers for Disease Control and Prevention (CDC) has analyzed over the past few months, most matched the Perth strain, not the Brisbane strain included in this season's vaccine.

In a technical report released with its recommendation today, the WHO said circulation of the seasonal H1N1 strain this season has been markedly lower than previous years, but the few isolates received were related to the Brisbane strain covered by this season's vaccine.

Fukuda said even if the seasonal H1N1 continues to circulate, WHO experts don’t think it will pose a major public health threat. The seasonal H1N1 virus had become widely resistant to oseltamivir (Tamiflu). The pandemic H1N1 virus has been generally susceptible to the drug, though a few resistant cases have been detected, mostly in those who received prophylaxis or treatment.

Influenza B viruses from two major lineages, Victoria and Yamagata, have been circulating, but the WHO said the Victoria lineage has been predominant since September. The B/Brisbane/60/2008 strain recommended for both hemispheres' seasonal flu vaccines is a member of the Victoria lineage.

The WHO recommends the following for next season's vaccine:

  • For the H1N1 component, a strain similar to A/California/7/2009, replacing A/Brisbane/59/2007
  • For the H3N2 component, a strain similar to A/Perth/16/2009, replacing A/Brisbane/10/2007
  • For the B component, a strain similar to B/Brisbane/60/2008-like virus

Decisions about whether to continue using the monovalent pandemic vaccine or move to a multivalent seasonal vaccine that contains the pandemic virus will be made by national regulatory authorities, Fukuda said, adding that countries that don't routinely conduct seasonal flu vaccination may still need the monovalent vaccine. The WHO's Strategic Advisory Group of Experts on Immunization (SAGE) group will also meet in April to discuss this and other issues and make a recommendation, he said.

See also:

WHO recommended strains

Full WHO report on strain selection

Sep 23, 2009, CIDRAP News Story "WHO picks novel H1N1 for 2010 southern hemisphere flu vaccine"

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