Model for containment of pandemic

IOM says community measures may help in a pandemic

Robert Roos * News Editor

Dec 14, 2006 (CIDRAP New) – The Institute of Medicine (IOM) weighed in with a clear "maybe" this week on whether community interventions such as school closures, quarantine, and respiratory etiquette could help blunt the impact of an influenza pandemic.

After looking at mathematical models and historical evidence, an IOM committee said that a wide range of community interventions may be helpful, but there is no conclusive evidence for their effectiveness.

"It is almost impossible to say that any of the community interventions have been proven ineffective," says the committee's report. "However, it is also almost impossible to say that the interventions, either individually or in combination, will be effective in mitigating an influenza pandemic."

"There is simply a dearth of strong evidence concerning the efficacy of community containment strategies, which is particularly troublesome given the fact that many of the interventions will carry significant economic, social, ethical, and logistical consequences," adds the report, titled "Modeling Community Containment for Pandemic Influenza: A Letter Report."

Containment measures endorsed by the panel include home isolation of patients plus social support, voluntary sheltering at home, quarantine, hand hygiene, respiratory etiquette, preventive antiviral treatment, and community restrictions such as school closures. The group also supported standard public health measures such as disease surveillance and contact tracing.

But the panel warned that public health officials, in recommending such steps, should take care not to overstate the evidence for their effectiveness. The group also said that any plans to use such measures should be linked with plans for mitigating their side effects.

The report, released Dec 12, was prepared by a 13-member committee chaired by Adel A.F. Mahmoud, MD, PhD, former president of Merck Vaccines. It is based on a workshop held Oct 25 in Washington, DC.

The committee reviewed 6 mathematical simulations of community containment strategies and found none of them entirely convincing. Many key assumptions used in the models, such as those regarding virus transmissibility and compliance with interventions, were based on little evidence, the report says. Accordingly, the panel calls for prospective epidemiologic studies of seasonal flu to bolster the assumptions used in the models.

The report also critiques existing models for focusing too narrowly on flu-related outcome measures and ignoring other effects of interventions. For example, extended school closings could expose children to increased violence or result in malnutrition by depriving children of free or subsidized school lunches.

The committee also reviewed several analyses of data from the 1918 flu pandemic. These included preliminary results from Dr. Howard Markel of a study of nonpharmaceutical interventions (NPIs) in 45 US cities. He concluded that NPIs may have lowered peak death rates and flattened the epidemic curves in those cities, though some cities had severe epidemics despite using NPIs.

Similarly, Dr. Marc Lipsitch found in a study of 17 cities that early interventions were significantly associated with lower peak death rates and that early school closures were most closely linked with lower peaks. Further, an analysis presented by Dr. Neil Ferguson, combining 1918 data with mathematical modeling, suggested that community interventions could significantly reduce overall illness rates if they were imposed for the full duration of the pandemic.

Summarizing the lessons of the simulation models and historical analyses, the report says, "The models generally suggest that a combination of targeted antivirals and NPIs can delay and flatten the epidemic peak, but the evidence is less convincing that they can reduce the overall size of the epidemic. Delay of the epidemic peak is critically important because it allows additional time for vaccine development and antiviral production. Lowering the peak of the epidemic is crucial also because it can reduce the burden on healthcare infrastructure by avoiding an extremely large influx of patients."

Participants in the workshop said differences between the world of 1918 and today may limit the usefulness of historical data, the report notes. For two examples, population density is different today, and antibiotics now available to treat secondary infections could increase survival.

The committee's conclusion on the key question of community restrictions, such as closing schools and limiting public gatherings, is that they have a role, but the evidence does not permit any predictions about the effects of specific types of restrictions or the comparative effects of voluntary versus mandatory restrictions.

As with any infectious disease, the evidence indicates that early restrictions are better than later ones," the report says. "The main effect might be to slow the time to peak of the outbreak in a community, which could be important for hospital-based management of ill patients and to allow for delivery of vaccine if available."

In other conclusions, the report says there is probably a role for isolating sick people at home while providing social support for them, though this is based mainly on common sense and evidence from other illnesses.

Other measures the committee affirmed as potentially beneficial, based on varying kinds of evidence, include:

* Surveillance and case reporting, rapid diagnosis, hand hygiene, and respiratory etiquette
* Antiviral prophylaxis and treatment in households and healthcare settings
* Contact tracing to allow contacts to take actions such as voluntary sheltering and quarantine
* Risk communication, meaning the identification of "key and trusted spokespersons" to promote public acceptance of community containment measures

The committee offers 11 recommendations for improving the understanding and use of community interventions. One calls for the development of "decision-aid models that can be readily linked to surveillance data to provide real-time feedback during a pandemic."

The IOM report's release came a day after health officials meeting in Atlanta heard about Markel's research suggesting that cities which had implemented early "social distancing" measures in the pandemic of 1918 had lower death rates than other cities.

According to an Associated Press (AP) report on the meeting, researchers found that cities such as St. Louis, which instituted social distancing at least 2 weeks before the peak of the local epidemic, had flu-related death rates less than half that of Philadelphia, which was slower to act.

Markel, of the University of Michigan, is conducting the research with the Centers for Disease Control and Prevention, the AP reported. Markel called the effort "a Manhattan project of history."

So far, evidence shows that the more restrictions were used and the longer they were in place, the milder the pandemic, the story said. Wearing masks in public, restricting door-to-door sales, canceling church, and quarantining sick people were among the measures that seemed helpful. But the researchers said they hadn't determined which measures were most effective, and they couldn't prove those steps were the reason some cities did better than others.

One factor that may confuse the interpretation of the 1918 data on community interventions is that the two waves of the pandemic that year might have affected cities differently, according to Michael T. Osterholm, PhD, MPH, director of the University of Minnesota Center for Infectious Disease Research and Policy, publisher of the CIDRAP Web site.

The pandemic began with a relatively mild wave in the spring of 1918, and was followed by a second, much more severe and widespread wave in the fall. Osterholm told CIDRAP News there is strong evidence that military camps that were hard hit in the spring had lower illness and death rates in the fall wave, presumbably because many people developed immunity. He suggested that the same might have been true of cities.

"All of us really want to demonstrate protection from these interventions—it's our greatest hope for a future pandemic—but we also want to be certain that the information we give people is based on science and not wishful thinking," Osterholm commented.

IOM. Modeling community containment for pandemic influenza: a letter report. Released Dec 11, 2006 [Full text]