WHO working on a severity scale to accompany pandemic alert phases

Written by: Helen Branswell, Medical Reporter,
May. 11, 2009

TORONTO - The World Health Organization is devising an index to gauge the severity of the threat posed by a potential pandemic influenza virus, the agency's top flu expert said Monday.

The WHO hopes to introduce the new scale soon, said Dr. Keiji Fukuda, who said the tool could help national authorities make decisions about how aggressively they want to respond with measures like closing schools and discouraging public gatherings.

"These are hard decisions for country authorities to make. And so they really want as much guidance as possible," Fukuda, acting assistant director general for health security and environment, said in an interview.

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"And I think this provides some guidance, some help. But they're still difficult decisions to make."

Shortly after revealing a severity scale was in the works, the WHO posted an assessment of the situation to date with the new H1N1 swine flu virus. It noted that outside of the outbreak in Mexico - which is still not completely understood - the new virus typically causes "very mild illness in otherwise healthy people."

According to the WHO, there are about 4,800 confirmed cases in 30 countries, including 331 in Canada.

The WHO has faced criticism from some quarters that its pandemic alert scale has no mechanism to reflect the fact that a flu pandemic might cause mild, moderate or severe illness and trigger varying levels of societal disruption.

The agency has been stressing since the swine flu incident began that the term pandemic relates to the scope of transmission, not the severity of disease caused by a new spreading virus.

But after years of media focus on the dangerous H5N1 avian flu virus - which draws parallels to the 1918 Spanish Flu - there may be a mistaken public perception that high levels of severe disease and deaths must go hand-in-hand with a pandemic strain.

In reality, the two most recent pandemics - 1957 and 1968 - caused widespread illness and higher-than-normal numbers of deaths. But they were not catastrophic.

"Pandemics are not created equal," said Dr. Peter Palese, a leading influenza expert at Mt. Sinai Medical Center in Manhattan.

Fukuda said a group of experts that helped the WHO rewrite its pandemic preparedness guidelines has been working on a severity scale. While the aim is to have something simple - with categories such as mild, moderate and severe - the WHO is aware that outbreaks may play out differently in different places.

"On the one hand, it is important to have a very straightforward simple approach to describing it. On the other hand, one shoe doesn't necessarily fit all feet," Fukuda said.

And even in mild pandemics, such as the 1968 Hong Kong flu, lots of people will get infected. Of those, an as-yet unknown proportion will become severely ill and some will die.

"In a sense, it is almost illogical to be describing something like a pandemic as mild when you're spreading it over the number of people that will get infected," Fukuda said.

An infectious diseases expert welcomed the effort to try to make the situation clearer.

"The current pandemic phase model really needs one additional piece, namely that of disease severity," said Dr. Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota.

"By combining both transmission and severity characteristics it will provide for a much more accurate assessment of the status of any emerging pandemic."

Some countries already have a severity index in their pandemic plans. The U.S. Centers for Disease Control, for instance, use a scale similar to a five-point hurricane index, with the Spanish Flu (which killed upwards of 50 million people in 1918-19) at the low end of five.

The CDC hasn't yet determined where the H1N1 outbreak lies on that scale, said Dr. Francisco Averhoff, chief of quarantine and border health services in the division of global migration and quarantine.

But based on what has been seen to date, the CDC has dialled down some of its recommendations on so-called social distancing measures. Where initially it recommended closing schools when H1N1 cases were found, now it says infected kids should stay home but schools should stay open.

Meanwhile, disease investigators have been trying to get a clearer picture of just how bad an actor the new virus is.

An article published electronically on Monday by the journal Science estimated that 23,000 people in Mexico had been infected with the new virus by the end of April and that the case fatality rate - the proportion of cases that end in death - was about 0.4 per cent. In comparison, the case fatality rate for the Spanish flu was upwards of 2.5 per cent.

The authors, from a number of universities and the WHO, said the early signs were that the disease severity with the new virus was in the range of that seen during the 1957 and 1968 pandemics.

"The transmissibility looks like the less severe pandemics of '57-'58 and '68-'69, not a 1918 - at least currently," said Dr. Ira Longini, an expert in mathematical modelling at the University of Washington. Longini was not involved in the study.