Vaccine Benefits, Vaccine Risks: 10 Basic Questions Answered
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4. If only to err on the side of caution, why can't all vaccines be thimerosal-free?
"The vaccines that are given to children in the U.S. are nearly thimerosal
free," says Schuchat. "All but the flu vaccines that are given to young
children do not contain thimerosal. Some formulations of flu vaccine are
thimerosal-free and some are not. So the pediatric vaccine supply is nearly
thimerosal-free. Thimerosal is a preservative that keeps germs from
overgrowing. So vials of vaccine that are multiple doses, 20 or 30 doses, have
a preservative to keep them from bacterial contamination. It was put in there
because people died from vaccines that were contaminated.
"To get thimerosal out of all vaccines would require additional construction
of facilities to be able to prepare vaccines differently. So there have been
major changes since 1999 in removing thimerosal from the vaccine supply, but it
is a many-year process. Although there is no compelling evidence that
thimerosal caused harm, the concerns that have been raised have led to this
"Some of the pharmaceutical companies are going through the transition. Not
looking for another preservative, but looking for production capacity to make
"But right now, for every childhood vaccine, there is a thimerosal-free
5. Can vaccines cause the diseases they are supposed to prevent? Every year you hear people say, "I got my flu shot and then I came down with the flu."
"Flu vaccine does not cause the flu," says Schuchat. "Many vaccines do not
provide 100% protection, but they decrease the chances of getting an infection.
Even so, they don't reduce the chances to zero. Sometimes, even a person who is
vaccinated can get an infection the vaccine was supposed to prevent. But the
vaccines used today don't cause the diseases they are supposed to prevent."
6. What harms do vaccines do?
"There can be side effects from each individual vaccine," says Schuchat.
"The most common side effect is pain or swelling at the site of injection. For
each vaccine recommended, the CDC is required to provide a
vaccine information statement to a parent or patient receiving one of the
vaccines. It lists background information on benefits and risks. Most vaccines
do not have serious adverse events associated with them.
"But some do. For instance, the flu vaccine is made from eggs. People with
egg allergies should not receive the flu vaccine because they can have a
serious reaction. So there are questions doctors use to screen people before
getting any vaccine to make sure they don't have special risks. For the vast
majority of childhood vaccines, there are very few of these.
"There is a possibility -- it is not yet clear -- but with the meningococcal
conjugate vaccine, there have been reports of Guillain-Barre syndrome. We do
not yet know if this is a true vaccine-related risk. But if it is there, it
happens at the rate of about one per million doses administered. And that has
been added to the vaccine information sheet."