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VOL. 71 (2), 321-339, 2005 HISTORY AND FUTURE OF POLIOVACCINATION...

disease is endemic. It was only by the end of the 19th century and the
beginning of the 20th century that poliomyelitis became a threat to
human health, and the first polioepidemics occurred.

What are the factors underlying this change in epidemiologic
behaviour of poliomyelitis? The main factor inducing polioepidemics
is paradoxically the improvement of sanitation and hygiene.

Poliovirus is in fact not so highly neurotropic as has been
supposed. The infection is primarily an inapparent one, involving the
alimentary tract and consequently poliomyelitis is actually a relatively
uncommon complication (poliomyelitis causes paralysis in one case
of every 200 to 1,000 infections). The shedding of the virus from the
throat and intestinal tract by asymptomatic persons (clinically
unrecognised cases) serves as the main source of spread of the
infection, and explains why a history of contact between patients is
usually lacking. Dissemination of the virus is facilitated by crowding
and poor standards of hygiene and sanitation (example: tropical
developing countries). Under these conditions, there is a continuous
circulation of the virus, immunizing infections early in childhood are
universal, sporadic paralytic cases are confined to the youngest age
group and consequently there are no epidemics. The disease remains
endemic, which has been the case for at least 3,000 years.

In contrast (and before vaccination became possible), in countries
with high standards of hygiene and sanitation, and practically always
with high socioeconomic levels, circulation of the virus was
intermittent and children were protected from exposure early in life.
As a consequence, large numbers of susceptible individuals built up,
and when virulent strains were introduced in a community, this
resulted in progressively more devastating epidemics that involved
increasingly older age groups. Moreover, there is a clear evidence for
a higher frequency of paralytic disease when infections occur in
susceptible older children and young adults (paralytic disease in one
case of every 75 infections) as compared with young children
(paralytic disease in one case of every 1,000 infections). Also this
fact substantially contributed to the emergence of severe outbreaks
and epidemics (8).

Although, it is now clear why polioepidemics occurred so late in
history, there are still a lot of questions to answer: (1) what are the

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