The 2009 H1N1 Changed the Definition of Pandemy

The H1N1 seems to come back in the newspaper headlines, so researchers from Philipps University of Marburg in Germany, published a paper that explains in what way the 'swine flu' contributed to a better understanding of pandemic outbreaks.

The authors reveal important lessons about the origins of 2009 H1N1, the main one being that an 'antigenic shift' can occur even if the odds are minute, and the virus can emerge from an existing influenza type, like in this case.

Professor Hans Dieter Klenk from Philipps University of Marburg, made a review of the pandemia that focused on antigens – substances that, when present in the body, trigger the immune system.

The influenza viruses have two antigens – hemagglutinin (HA) and neuraminidase (NA).

Klenk said that “it was widely believed that a pandemic occurs when a virus with a new HA, or a new HA and a new NA that are not recognized by the human immune system emerges and spreads throughout the population,” and this is called an antigenic shift.

Even though this process was believed to always involve new NA or HA subtype, the 2009 'swine flu' proved that a pandemic can be the outcome of a shift within the lineages of the existing subtypes.

“There are 16 HA and 9 NA subtypes, which differ significantly, but contain multiple lineages that were always believed to be too similar to allow antigenic shift,” explained Klenk.

“However, this is exactly what occurred in 2009.”

And even though the H1N1 had already circulated through the human population, a strain of H1N1 containing new HA and NA lineages caused a pandemic, revealing an antigenic shift from within the same subtype.

“H1N1 emerged in February 2009 in Mexico and swept around the globe within 6 months,” said Professor Klenk.

“The conventional ideal is that pandemics are fueled by new strands which emerge in the human population, yet it was because H1N1 did not conform to this ideal that its spread was so unexpected.

“From studying the influenza outbreaks of 1918, 1957 or 1977 it looks as if pandemics only occur when a new HA or NA subtype enters the population.

“However, the 2009 outbreak overturns this rule, revealing that a pandemic may not depend on the introduction of a virus with a new HA subtype.

“This meant that vaccination against the previous viruses offered little protection against infection by the new strain,” Klenk concluded.

In other words, the Professor said that “future research should not simply monitor one or a few viruses and that plans to deal with pandemics must be flexible enough to handle the unexpected.”

The paper was published today in BioEssays.

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