This month, the World Health Organization finally declared that the new H1N1 virus has become pandemic. This week it reported a big jump in cases and fatalities.
How many people this virus will sicken and kill depends, ultimately, on three things:
The virus itself.
The impact of what are known as "non-pharmaceutical interventions," or NPIs.
The availability and effectiveness of a vaccine.
The virus will be the most important factor. Influenza is one of the fastest-mutating organisms in existence, which makes it unpredictable, and a virus newly infecting the human population is likely to be even more unpredictable as it adapts to a new environment.
There have been four pandemics that we know about in some detail: 1889-92, 1918-20, 1957-60 and 1968-70. All four followed similar patterns: initial sporadic activity with local instances of high attack rates -- just as H1N1 has behaved so far -- followed four to eight months later by waves of widespread illness with 20 to 40 percent of the population sickened. (In a normal influenza season about 10 percent of the population gets sick.) Subsequent waves followed as well.
In all four pandemics, lethality changed from wave to wave -- sometimes increasing, sometimes decreasing. It's impossible to know what will happen this time, but in 1999 the Centers for Disease Control and Prevention modeled a moderate pandemic in the United States, including a vaccine in its calculations, and concluded that the death toll would probably be 89,000 to 207,000. If the virulence of this virus does not significantly increase -- and right now there is no reason to think it will -- something close to the lower number looks probable.
What could help bring about such a best case?
Again, the virus is the most important factor, and we have no control over it. But we do have non-pharmaceutical interventions and the possibility of a vaccine. Such interventions would come into play primarily in a moderate or severe pandemic.
For a mild one, we may not need to take steps beyond washing hands, exercising "cough etiquette" and keeping the sick at home. But if the virus increases its virulence, other measures, such as closing schools, urging people to telecommute and even banning public meetings, could mitigate the impact.
However, the usefulness of non-pharmaceutical interventions is limited, and even if they work, their chief impact will be to flatten the pandemic's peak and stretch out the duration of a wave of illness to make it easier to handle.
Consider: Those telecommuting are likely to run into Internet capacity problems, while the impact of closing schools -- aside from the burden that creates on working parents and their employers, or on children who get good meals only at school -- depends on how much kids congregate while out of school. And sustaining compliance will be both important and difficult.
Scholars Bradley Condon and Tapen Sinha found that in Mexico City this spring, when the government advised wearing masks on public transportation, compliance peaked at 65 percent three days later -- but declined to 26 percent only five days after that. This decline came even as the government was taking the extreme measure of closing all nonessential services and businesses. Such behavior does not portend well for sustained compliance with any measure.
The most important human intervention is, of course, a vaccine.