The Centers for Disease Control (CDC) recommends against giving vaccines to people who have a contraindication, which is any condition that increases the risk for serious vaccine side effects.
For example, DTaP vaccine is contraindicated in patients with neurological disorders like cerebral palsy, uncontrolled seizures, progressive encephalopathy, or a history of Guillain-Barré Syndrome within 6 weeks of a previous vaccine against tetanus.
There are also precautions to administering vaccines. A precaution means the patient has a medical condition that increases the risk of side effects. If the patient has a precaution, vaccination should be delayed unless a doctor determines that the benefits outweigh risks.
All vaccines carry precautions for patients with a moderate or severe illness, with or without a fever. There are a few reasons why. One is that it will be hard to tell the difference between a vaccine side effect and the illness. The illness might also worsen vaccine side effects.
For children who have already experienced a major vaccine side effect, doctors and parents must carefully weigh the risks and benefits.
A common concern is the measles vaccine, which is associated with a mild decrease in platelet count within a few days of administering the vaccine. This causes a bleeding and bruising disorder known as thrombocytopenia in roughly 1 in 30,000 doses of the MMR vaccine.
According to Dr. H. Cody Meissner, a professor of pediatrics at Tufts Medical Center:
“The decision to vaccinate such a child with MMR depends on the benefits of immunity to measles, mumps and rubella and the risks for recurrence or exacerbation of thrombocytopenia after vaccination.”
Women who are pregnant or breastfeeding must tell their doctor before receiving any vaccine because the risks depend on the type. Live virus vaccines can sometimes pass the virus to a fetus or a breastfeeding baby. If infection does occur, it is usually well-tolerated because the virus is attenuated (weakened) — but not always.
Allergic reactions are another serious vaccine side effect, although the risk is usually estimated at fewer than 1 in a million doses. All vaccines are contraindicated in anyone who has had a severe allergic reaction (e.g. anaphylaxis) to a previous dose of the vaccine or any ingredient.