Parents may not understand why their children must
be vaccinated again and again, nor why, even after multiple vaccinations,
a child sometimes develops paralytic polio. These failures lend credence
to recent rumors circulating in northern Nigeria, where some mothers have
refused to permit their children to be vaccinated. According to Kew, the
parents have heard that the polio vaccine is a Western plot to sterilize
their children. Their resistance to the push for universal vaccination
is the reason the country remains a polio hot spot, and the consequences
extend beyond its borders. Infections in Nigeria have spread westward
to Ghana, the Ivory Coast, Mali and Guinea, and eastward to Sudan, Yemen
and Somalia.
Problems getting everyone vaccinated aren’t the only factor slowing
the eradication campaign. The poliovirus can survive in water or sewage
for many days, and a small number of immunocompromised polio patients
have excreted live virus for years. That means a community in which no
one is currently infected may still harbor active viruses. Worse, live
vaccine strains can recover neurovirulence and (even if only rarely) cause
paralytic disease. About one in a million people given the live oral virus
will develop crippling paralysis.
In recent years, circulating vaccine-derived poliovirus
has produced small outbreaks in areas, including the Dominican Republic
and Haiti, where wild poliovirus was thought to have been eradicated.
That, Wimmer explains, is because oral Sabin vaccines were originally
developed by passing them through a nonhuman host environment: monkey
kidney cells. Through natural selection, new strains adapted to that alien
environment. Once the vaccine virus finds itself in the human intestine,
the weakened viral strains adapt to their new conditions and move closer
to the original, virulent wild form. These better-adapted viruses can
travel in the community if everyone isn’t immunized, sometimes causing
paralytic disease. Such dangers posed by oral live vaccines have prompted
the United States and Europe to switch to IPV, Salk’s original vaccine.
Circulating vaccine-derived polioviruses tend to
grow progressively more neurovirulent, though the exact reason is another
mystery. Is it a matter of binding more efficiently to the receptors or,
as Kew believes, a re-evolution of the vaccine strain’s capacity
to replicate more effectively—the better to exploit a host’s
tissues and become more virulent? Does the virus’s neurotropism,
a tendency to target nervous system tissue, confer any advantage to the
virus, or is tropism, and the paralysis it causes, just an incidental
effect?
Whether we will ever solve those and all of polio’s other mysteries
is doubtful: In the world after the disease, most research on live poliovirus
will be restricted or banned, which seems a small price to pay for a world
free of polio.
 Dossier
1. A History of Poliomyelitis, by John R. Paul
(Yale University, 1971). An indispensable classic,
written by a physician who played a major role
in fighting epidemics in the mid-twentieth century.
2. Splendid Solution: Jonas
Salk and the Conquest of Polio, by Jeffrey Kluger (Putnam, 2004). A lively
narrative of Salk’s early life, research
career and sharp rivalry with Albert Sabin, as
well as a vivid history of the development of Salk’s
inactivated vaccine.
3. Polio: An American Story, by David M. Oshinsky
(Oxford, 2005). Sets the Sabin-Salk rivalry against
a broad canvas of polio in America and the terror
that the disease induced in ordinary Americans.
Oshinsky makes the scientific issues understandable
and compelling.
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