by Jon Cohen
Next winter in the Southern Hemisphere, influenza vaccines should no longer be designed to protect against the seasonal H1N1 strain as the pandemic H1N1 strain has replaced it, according to World Health Organization (WHO) recommendations issued today.
As the recommendations explain, the seasonal strain has caused few outbreaks since the discovery of the swine flu strain last April.
WHO thus is recommending that vaccine makers drop the seasonal H1N1 virus from formulations that also contain an H3N2 strain and a type B, Brisbane strain. One possibility is that vaccine makers will simply replace the dropped virus with the pandemic H1N1 strain in a new trivalent product. Or countries may decide to use a monovalent vaccine that protects against the pandemic strain and a bivalent product designed to work against H3N2 and the Brisbane strain. WHO’s Strategic Advisory Group of Experts will debate these issues when it meets in late October. WHO will not make recommendations for the next Northern Hemisphere flu vaccines until February of next year.
(This item was originally titled "WHO Simplifies Flu Vaccine for Sourthern Hemiphere" and has been corrected.)
Jon, you left out a second significant aspect of the WHO announcement.
WHO recommended changing the current H3N2 strain (which is in the 2009-2010 seasonal vaccine) to a different H3N2 strain, because of evidence that circulating H3N2 is drifting away from the current vaccine strain.
That underscores the possibility that this year's northern hemisphere seasonal vaccine (2009-2010) may not be a good match for the H3N2 strain that ends up predominating.
H3N2 usually causes the most severe disease of the seasonal strains, and in people over 65, it causes far more severe disease than the pandemic strain causes in people under 65.
So if seasonal H3N2 isn't "crowded out" by the pandemic virus, the old folks may have a bad H3N2 year because of poor vaccine protection against it, while the younger folks are coping with pandemic flu H1N1.
Fortunately, there is some evidence that H3N2 may be partially crowded out. It is circulating in very reduced percentages in most southern hemisphere countries going through their pandemic seasons, dominated by pandemic flu H1N1.