In 1972, long before smallpox was eradicated worldwide, vaccination for the
disease was halted in the United States, where the disease had not been seen
for years. As smallpox was isolated to smaller and smaller regions on the
planet, vaccination was halted in the disease-free parts of the world until
finally mass smallpox vaccination was eliminated entirely, and vaccination
of contacts and in concentric area rings was used to control the few
remaining outbreaks until the disease was finally declared to be eliminated
completely.
It has been proposed here and elsewhere, that perhaps this model suggests
that it would be reasonable to cease vaccination for polio, now that that
disease, too, is on the eve of its eradication. After all, the Americas,
Europe, China and East Asia, and the South Pacific have all been certified
free of wild polio. The disease has been isolated to just seven countries,
with more than 90% of the cases in just Nigeria, Afghanistan, Pakistan and
parts of India. Isn't it time the rest of the world can cease the program of
vaccination, which, after all, carries known health risks?
The answer is NO.
Smallpox and polio are very different diseases, and the vaccines are
different as well. Smallpox virus is transmitted via the airborne route, and
does not survive long in the environment. This means that the disease can
only be transmitted directly from person to person. Polio is transmitted by
the oral-fecal route, and can survive for weeks or more in contaminated
waste streams. Smallpox causes obvious symptoms in its victims, even in its
mildest forms, so that it is relatively easy to identify an index case and
trace contacts. Polio is so mild as to go undetected in as many as 95% of
infections, so that an infectious traveler could spread the disease over a
wide area, unknown to health authorities. Smallpox vaccine is the live virus
of a related disease, vaccinia, so the public is not exposed to any actual
smallpox virus in the vaccination program. Oral Polio Vaccine (OPV) consists
of mutated live viruses of the three strains of polio. These can
occasionally mutate back into the pathogenic forms of the virus and cause
paralytic polio in the vaccinated persons and in persons exposed to viruses
shed from their intestinal tracts. Smallpox vaccine confers immunity
quickly, and can offer protection even after one is exposed to the disease
virus. Polio vaccination takes longer to be effective and cannot overpower
the pathogenic virus after exposure. Immune suppressed persons, such as HIV
patients and organ donor recipients may be able to carry polio viruses in
their intestines for extended periods of time. One of these "carriers" could
spread polio far and wide in an unvaccinated population. It has been
estimated that as many as 7000 cases of paralytic polio would occur in an
unvaccinated city from one infected person before a vaccination program
could catch up and halt the spread.
Smallpox virus, at the time of the eradication of the wild disease, was
believed to exist only in two or three level-four containment research labs
in the world. (The virus has probably gotten into the hands of terrorist
organizations since then.) There was hope that even the research stocks
could be eliminated at that time. In contrast, polio virus is common in labs
all over the world, and no attempt has been made to document and track where
stocks of it exist. Since a reservoir of the virus is necessary for the
production of vaccine, it will never be practical to eliminate it entirely.
For this reason, there will always be a risk that polio will escape into an
unvaccinated population and start a major epidemic.
Even after the whole world is certified polio free, our only protection
against the re-emergence of the disease will still be mass vaccination.
Whether cessation of polio vaccination will EVER be prudent is controversial
among epidemiologists, but they all agree that we must continue vaccination
until at least several years after the last wild polio is eradicated.
--Rich