Experts at the Harvard School of Public Health are betting on the latter, and have built a mathematical model to show why.
"At best, a containment policy will only postpone the emergence of a pandemic, 'buying time' to prepare for its effects," write Christina Mills, James Robins, and Marc Lipsitch, of the Harvard School of Public Health (HSPH), along with Carl Bergstrom, a collaborator from the University of Washington, in a paper that appeared in the February 20 online issue of PLoS Medicine.
For a virus to spark a pandemic, it must be a new subtype, infect humans, and spread easily and sustainably among humans. So far, AH5N1 meets the first two criteria.
The HSPH team's conclusion tempers the hopes for containment recently expressed by other researchers in studies in Nature and Science that assume a single outbreak.
Containment entails quickly identifying a cluster of human-to-human transmissions, then applying public health tools like vaccination, antivirals, and quarantine to curb a pathogen's spread. The HSPH researchers advocate containment--but up the ante, arguing also for new vaccines, stepped-up surveillance, and exposure-reduction methods such as limiting human-bird contact.
The researchers' differences depend on the risk of introduction of a version of AH5N1 that might spread easily among humans. If the risk is low, a single outbreak is possible and containment may postpone a pandemic for decades. However, if the risk is high, or if it becomes high in the future (due to changes in the prevalence or genetic makeup of the virus), multiple outbreaks are more likely, and containment may buy much less time.
"We think we're at a relatively high risk for a virus to emerge that could cause a pandemic," says Lipsitch, associate professor of epidemiology at HSPH. He cites trends to support this: Since 2003, AH5N1 has felled millions of birds--both domesticated and wild--in at least 48 countries. That means the amount of virus in the world is immense. As of April 27, there were 205 confirmed human cases of AH5N1 in nine countries--stark evidence of bird-to-human spread--and 55 percent of those infected had died.
"If we squash the virus in people in one location, nothing will change in the remaining infected birds--or the possibly infected people in another location, or at another time--to lower the risk of a pandemic," says Lipsitch. "So if we're at high risk now, we'll still be at high risk after the initial containment effort."
Christina Mills, who will receive her ScD in infectious disease epidemiology from HSPH in June, provides an historical perspective. "Based on what little is known about how pandemic flu emerges, the risk appears higher than it was in, say, 1968, when the last pandemic hit," she says. The reasons: Birds and people are now more numerous; they commingle more frequently; and AH5N1 has a wider geographic range than its predecessors.
"Containment, like all other strategies, is imperfect, and therefore we should try it--but try knowing we're likely to fail," says Lipsitch. Detection capabilities vary around the world, he notes, and given how rapidly flu spreads, even small delays could translate into outbreaks of explosive proportions.
"Overall, we found that containment will do no more than double the time to a pandemic," says Mills. "Our argument is: Let's take a broad look at all possible scenarios."
Thea Singer is the senior writer for the Review within HSPH's Office for Resource Development.
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