New WHO Criteria To Qualify Flu Viruses As Pandemic Strains
A change from, for example, A [H2N2] influenza to A [H3N2] is a process known as a "shift". Under the 2005 criteria, the appearance of a new influenza subtype was required before a pandemic was declared. However, according to the new definition, almost any influenza virus that mutates from the original subtype (a process known as "drift") would qualify as a pandemic strain if it spread readily from human to human.
While this seems to be a subtle change, it is really a significant change. Dr Peter Gross from Hackensack University Medical Centre in New Jersey warns that there is a potential problem with the new WHO pandemic definition.
To avoid confusion and limit any future false alarms, he says WHO needs to refine its current definition of an influenza pandemic.
Dr Gross writes. "Predicting how much harm a virus strain will cause is notoriously difficult and often controversial. The effects of the swine flu A [H1N1] pandemic have, so far, been less severe than many had feared, raising the question as to whether this infection has in fact reached pandemic proportions."
However, advanced testing technology reveals that the new A [H1N1] swine flu strain does exhibit significant antigenic and genetic divergence from seasonal A [H1N1] strains even though, in theory, it is only a drifted strain.
The author explains it is unclear whether deaths from the swine flu virus will approach the same level as other influenza A pandemics and epidemics over the last century. In the meantime, seasonal influenza will continue to occur. Annual immunization should be maintained for high-risk groups and people who want to avoid influenza infection.
In order to prevent swine flu infection, high risk-groups (including pregnant women, individuals with chronic cardiac or respiratory diseases, diabetes, or immunosupression) should also receive the new vaccine as recommended by national guidelines.
He says in closing that it would be more appropriate in the future for the WHO to clarify its current definition of an influenza pandemic to include any shifts in subtype, and only drifted strains which exhibit significant antigenic and genetic differences from the seasonal strain of the same subtype.
"Does every new influenza reassortant virus qualify as a pandemic virus?"
Peter A Gross, MD, Hackensack University Medical Center
BMJ Clinical Evidence
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