New WHO Criteria May Qualify More Flu Viruses as Pandemic Strains
By Laurie Barclay, MD
Medscape Medical News
November 3, 2009 — Refining the pandemic criteria recently issued by the World Health Organization (WHO) might reduce the risk for a false alarm, according to an editorial reported online November 2 in the British Medical Journal: Clinical Evidence. Application of the new WHO criteria may result in more influenza viruses qualifying as pandemic strains.
"When changes occur in the influenza virus, it is important that the public health community use reliable, balanced criteria to evaluate the probability of an influenza pandemic," writes Peter A. Gross, MD, from Hackensack University Medical Center, University of Medicine and Dentistry of New Jersey–New Jersey Medical School. "Recent changes in the WHO's definition of a pandemic flu have caused significant confusion."
The 2005 WHO criteria for declaring a pandemic require an antigenic shift, or appearance of a new influenza subtype, such as a change from A (H2N2) to A (H3N2), whereas the new definition would result in almost any influenza virus demonstrating drift, or mutation from the original subtype, to qualify as a pandemic strain if it showed rapid transmission from human to human.
It is often difficult to predict how much morbidity and mortality will ensue from infection with a particular virus strain — case in point being the current influenza A (H1N1) pandemic, which thus far has not been as severe as one would expect from a true pandemic.
Nonetheless, the novel A (H1N1) influenza strain, which is only a drifted and not a shifted strain, is significantly divergent from seasonal A (H1N1) strains antigenically and genetically, as determined by advanced testing technology.
"In the future, it would be more appropriate for the WHO to clarify its current definition of an influenza pandemic virus to include any shifts in subtype, and only drifted strains which exhibit significant antigenic and genetic divergence from the seasonal strain of the same subtype," Dr. Gross writes.
The editorial describes additional evidence suggesting that the population is not completely immunologically naive to the novel A (H1N1) and that the younger population may have already been primed by previous exposure to an A (H1N1) virus, either naturally or through vaccination.
"Seasonal influenza will continue to occur, and annual immunisation with the trivalent vaccine containing three influenza strains — an A (H3N2) strain, an A (H1N1) strain, and a B strain — should continue for high-risk groups and people who want to avoid influenza infection," Dr. Gross concludes. "For prevention of infection with the novel A (H1N1), those at high risk (including pregnant women, those with chronic cardiac or respiratory diseases, diabetes, or immunosuppression) should receive the new monovalent vaccine for the 2009 novel A (H1N1) strain as recommended by national guidelines."
