|
Are vaccines effective?
In the ideal
world, vaccines would by totally effective in everyone. However, most
vaccines do not induce immunity in 100% of the people who
receive them.
In order for
vaccines to be approved and recommended for the public they must first
prove that they are effective in the majority of people who
receive the vaccine. Most vaccines are effective in 85-95% of the
population. There are a variety of reasons that a given individual may
not respond to a vaccination. There is no way to know ahead of time
whether or not a given individual will respond to a give vaccine.
Milder
disease:
Because of
this incomplete coverage, it is possible for an individual who received
all of the recommended vaccinations for an agent to still come down with
that disease. For example, some children who have received the
varicella (chicken pox) vaccine still contract cases of chicken pox.
Their cases however are generally mild and far fewer serious
complications of the disease result. Thus, although the vaccine did not
entirely prevent the disease, did prevent the serious complications. As
a result of such epidemiology, the recommendation for varicella
vaccination has been modified to include a second dose in an attempt to
increase coverage.
If vaccines work, why is the
majority of people who contract disease during an outbreak people who
have been vaccinated?
As stated earlier, there is a
certain percentage of the population who do not form immunity even after
the recommended vaccination protocol. There are also a number of people
who loose their immunity after a number of years. Thus there are a
number of vaccinated people who have been vaccinated who are still
susceptible to the disease.
Consider this hypothetical
situation:
In a high school of 1,000
students, 970 have been vaccinated against measles. None of the students
has had measles before. Given the failure rate of the vaccine, about 97
vaccinated students would still be susceptible. When the student body is
exposed tomeasles, 90% of susceptible
student becomes infected because measles ishighly contagious. Then 27
unvaccinated students will become infected and 97 vaccinated students
will also become infected. From these statistics, one can say that 78%
of the people who got infected were previously vaccinated. The
existence of 97 vaccinated students who still got measles doesn’t
disprove the effectiveness of the vaccine. This is a factor of the much
greater number of students who had been vaccinated versus those who did
not. Relatively, 90% of the unvaccinated student contracted measles
while only 10% of the vaccinated ones did. The measles vaccine
protected 90% of vaccinated students. Without a vaccination program, all
1,000 students could have got measles.
Link:
CDC:
National Immunization Program Home Page
|
