Hurricane H5N1? New pandemic 'storm' rankings

Community strategies depends on severity

The Department of Health and Human Services (HHS) and the Centers for Disease Control and Prevention (CDC), have released new guidance that calls for community response to pandemic flu to be triggered by a severity index much like the ranking system used for hurricanes.

Should avian influenza A (H5N1) mutate into a pandemic strain, community strategies that delay or reduce the impact of a pandemic — also called nonpharmaceutical interventions — will be critical to reduce the spread of disease until a vaccine that is well matched to the virus is available.

"Pandemic influenza is not necessarily imminent, but we believe it is inevitable," Julie Gerberding, MD, MPH, said a recent press conference. ". . . And this isn't just about H5N1 avian flu. This is about any novel influenza virus to which people have not been exposed and to which we might all be susceptible. Planning requires that a whole network is engaged. It means individuals and families. It means communities and it means the whole system of business, education, health care and government really work together so that we have a strong linkage throughout the entire network. We are only as strong as our weakest link."

The new guidelines focus primarily on community-level measures that could be used during an influenza pandemic in an effort to reduce the spread of infection. In order to help authorities determine the most appropriate actions to take, the guidelines incorporate a new pandemic influenza planning tool for use by states, communities, businesses, schools and others. The tool, a Pandemic Severity Index (PSI), takes into account the fact that the amount of harm caused by pandemics can vary greatly, with that variability having an impact on recommended public health, school, and business actions. (See table.)

The PSI, which is modeled after the approach used to characterize hurricanes, has five different categories of pandemics, with a Category 1 representing moderate severity and a Category 5 representing the most severe. The severity of pandemic primarily is determined by its mortality rate. A Category 1 pandemic is as harmful as a severe seasonal influenza season, while a pandemic with the same intensity of the 1918 flu pandemic, or worse, would be classified as Category 5.

"Now we all know that if a pandemic virus emerged, the first thing we would try to do is completely extinguish it or quench it," Gerberding said. "But that might not be realistic given the speed with which virus can move around the world. So, the next best thing we can do is to try and slow down the spread and buy some time. The best way to protect people is of course a vaccine. But we are not likely to have an effective vaccine in the first six months of a pandemic. So we have to put our heads together and figure out what can we do in the first six months before the pandemic virus vaccine is available."

To develop a plan, public health officials analyzed the last three pandemics, which occurred in 1918, 1957, and 1968. The later two pandemics were classified as only Category 2 in the new system, primarily because they did not result in a high case fatality rates, explained Martin Cetron, MD, director of CDC's division of global migration and quarantine.

"We've had few pandemics — fortunately — to draw experience on, but each of them threw us curve balls, whether it was high attack rates and low case fatality ratios or very rapid spread and high case fatality ratios like 1918," he said.

The tool used to construct the severity index uses the major parameters, such as illness rates and case fatality ratios to forecast or categorize a pandemic threat as it begins to emerge. "We have also modeled looking at what we know about seasonal flu transmissions," Gerberding said. "And we have been able to draw some important conclusions. One is that the earlier you initiate an intervention, the more likely it is to make a big impact."

Key points of the plan include:

  • Based on the projected severity of the pandemic, government and health officials may recommend different actions communities can take in order to try to limit the spread of disease. These actions, which are designed primarily to reduce contact between people, may include:
  • Asking ill persons to remain at home or not go to work until they are no longer contagious (seven to 10 days). Ill persons will be treated with antiviral medication if drugs are available and effective against the pandemic strain.
  • Asking household members of ill persons to stay at home for seven days.
  • Dismissing students from schools and closing child care programs for up to three months for the most severe pandemics, and reducing contact among kids and teens in the community
  • Recommending social distancing of adults in the community and at work, which may include closing large public gatherings, changing workplace environments, and shifting work schedules without disrupting essential services.

(Editor's note: The latest CDC guidance on pandemic flu is available at: