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Although many adults in the US and other
countries received the smallpox vaccine as children, it is
unclear if those shots would confer any protection in the
event of a bioterrorism-related outbreak.
Some experts expect that people who were
inoculated decades ago still might have at least partial immunity
against smallpox, which would reduce the risk of catching
the disease or at least lessen its severity.
In
the US, the smallpox vaccine was required for school entry
prior to 1972, and about 60%
of the US population has been vaccinated.
The eradication of smallpox was officially
declared in 1980, and according to the World Health Organization
(WHO), national vaccination programs had stopped in all countries
by the early 1980s, although the last reported case was in
Somalia in 1977.
"We expect that there is some protection
that continues," according to Dr. David Heymann, who
heads the communicable diseases section of WHO in Geneva,
Switzerland. However, he cautioned that it is nearly impossible
to be certain of how well previously vaccinated people would
be protected in the event of a smallpox outbreak.
The initial signs of smallpox infection
include headache, vomiting and fever. Then pus-filled lesions
form on the head and face, and may also appear on other parts
of the body.
There is no treatment for the disease,
which is fatal 20% to 40% of the time in unvaccinated people.
However, if an individual has been exposed to the virus and
is vaccinated within the next 4 days, it can reduce symptoms
or prevent the disease.
In those who have been vaccinated years
ago, however, the duration of immunity against the disease
is uncertain, according to the WHO's Heymann. Vaccination
provides "high rates of protection for at least 10 years,"
he said.
Officially, smallpox exists in only two
laboratories -- one in the US and the other in Russia. However,
there are some concerns that other governments or terrorist
groups may have samples of the deadly virus.
Finding out whether a vaccinated person
is still protected from smallpox is difficult due to the type
of immunity caused by the vaccine. It is possible to gauge
immunity induced by many types of vaccines by measuring levels
of antibodies to an infectious agent.
But the absence of antibodies to smallpox
on a blood test does not mean that there is not some lingering
immunity to the virus, since a person could still have a type
of immunity called cellular immunity. Even in the absence
of antibodies, cells may be sensitized to smallpox and capable
of triggering a protective reaction when exposed to the virus.
Evaluating a person's immunity against
smallpox is difficult. In the past, a process called "rechallenging"
was the only way to know for sure that a person was still
protected.
Smallpox vaccination causes a small lesion
to form at the site of the inoculation. When a vaccinated
person was "challenged" with a second dose of vaccine
in another location and did not develop the tell-tale scar,
then that person was thought to be immune to the virus. The
formation of a second lesion indicated a lack of immunity.
Despite the inability to measure immunity
with any accuracy without rechallenging the vaccinated, previous
studies of people exposed to natural smallpox after being
vaccinated suggest that some protection against the disease
may linger for decades.
In a 1972 study, researchers analyzed
680 cases of smallpox that were imported from other regions
into Europe and Canada from 1950 to 1971. About half of unvaccinated
people who were exposed to the virus died from the disease.
But as would have been expected, less than 2% of those who
had been vaccinated within the previous 10 years died.
But vaccination still reduced the risk of death 20 years after
the fact among people aged 10 to 49. The results show that
past vaccination leads to a "significant decrease in
mortality rate and severity of disease.
About
half of the unvaccinated died compared with about 4% of those
who had been inoculated 20 years earlier.
A study, published in 1913, found that
people who had been vaccinated against smallpox as children
were much less likely to die during an outbreak of the disease
in Liverpool, England, from 1902 to 1903. What this study
tells you is that there is indeed long-lived immunity.
Although the protection of the vaccine
waned with age, among people
older than 50, only about 5% of people who had been vaccinated
as a child died compared with about 50% of unvaccinated people.
The public needs to be reassured that
if you were vaccinated as a child, you might have some immunity.
A spokesperson for the CDC told Reuters
Health that it would be "uncertain" whether people
who had been vaccinated in the past were still immune to smallpox,
and it would be "fair to assume" that they would
need another vaccination to guarantee immunity.
The threat of bioterrorism has spurred
the US government to build up a stockpile of smallpox vaccine
to have on hand in case of an outbreak of the often fatal
disease.
The supply of 300 million doses of vaccine
-- enough to inoculate every one in the country -- will be
ready by next year. However, there are no plans to reinstate
a mass vaccination program, since the proven risk of serious
and even fatal side effects of the vaccine outweigh the potential
risk of bioterrorism.
Historically,
the smallpox vaccine caused serious reactions in about 1 in
every 4,000 persons and death in about 4 per 1 million.
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