Polio isn’t hard to prevent. Beginning in the late 1980s, campaigns aimed at killing off the disease cut the number of new cases around the world to just a few hundred a year. But even if there were no new cases this year, a study out today concludes, it could take several more years before epidemiologists could declare polio truly dead.
The core reason is surprisingly simple, if not widely known: Poliovirus, the virus that causes the disease polio myelitis, doesn’t actually make most people sick. For every person who does get sick—and that can mean something as mild as a sore throat or as severe as paralysis and death—there are 19 infected who don’t get sick but do remain contagious, any one of whom could set off the next outbreak.
Polio remains common in a few places and may be returning to others, leading health officials to re-double efforts to wipe the virus off the face of the Earth. But even if institutions like the World Health Organization succeed in destroying poliovirus, how would we really know? How could we know when poliovirus only sickens five percent of the people it infects?
The team estimated that only one percent of infections are ever reported, owing in large part to the already small number of infections that manifest symptoms.
We can’t, argue University of Michigan researchers Micaela Martinez-Bakker, Aaron King, and Pejman Rohani. Instead, health officials need to keep close watch long after outward signs of the virus are gone.
The team reached that conclusion after a fresh look at polio outbreaks and reports of its best-known symptom, acute flaccid paralysis (AFP), in the United States during the 1930s, ’40s, and ’50s. Their analysis relied on a standard model in epidemiology but took account of population size, birth rates, state-to-state migrations, geography, the time of year, and infant responses to polio—maternal antibodies are thought to protect babies for roughly their first six months.
The team could draw a number of conclusions from their analysis, though the most significant ones may concern so-called silent infections—the 95 percent of infections that don’t turn in to active polio disease. For one thing, the team estimated that only one percent of infections are ever reported, owing in large part to the already small number of infections that manifest symptoms.
That raises the possibility that polio could survive for long periods of time without any reports of someone getting sick. To quantify that possibility, the researchers used computer simulations of their model with parameters they estimated from the U.S. data. Their simulations revealed that if there are fewer than 100 new infections per month, it could take perhaps two and a half years before anyone reports a new case of polio.
“In light of polio’s propensity for silent circulation, we conclude that AFP data can be misleading,” the team writes in PLoS Biology. They recommend instead using current data and epidemiological models to determine whether polio is truly gone.
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