All vaccines, just like all medicines, have the risk of causing an allergic reaction. For example, you might be allergic to any one of the ingredients in the vaccine, such as the gelatin or preservative. Fortunately, severe allergic reactions that cause breathing or swallowing problems are very rare, occurring at an estimated rate of less than one in a million doses. In addition, these severe allergic reactions usually occur quickly, within minutes or maybe a few hours after the vaccine. So if you don’t have a reaction by then, you are unlikely to have a severe allergic reaction at all.
There are a number of other severe reactions that are known to occur with certain vaccines. The DTaP VIS has listed some uncommon risks such as seizure (one child in fourteen thousand), nonstop crying for over three hours (one child in one thousand), and a high fever over 105ºF (one child in sixteen thousand). While scary, these reactions are not noted to cause permanent damage. The chickenpox VIS also lists seizures as a risk (less than one child in one thousand) and pneumonia (“very rare”).
The MMR VIS has several moderate risks listed. These include seizures (one child in three thousand) and temporary joint pain and stiffness (one out of every four recipients for teenage or adult women). It also lists thrombocytopenia, or a low platelet count, as a risk of the vaccine, occurring in one recipient in thirty thousand. Platelets are necessary to help your blood clot when you are cut, so low platelets can lead to a temporary bleeding disorder.
Another side effect that seems to be linked to some vaccines is Guillain-Barré syndrome (or GBS). GBS is an autoimmune syndrome in which the body’s immune system attacks the body’s own nerves. Without the use of certain nerves, your muscles will not function and paralysis sets in. In a population of people who have not recently received vaccines, GBS randomly occurs at a background rate of about one in a million people. (The background of a disease is the average rate the disease occurs over several years.)
In the 1970s, GBS was clearly linked to a specific type of flu vaccine, the swine flu vaccine. Interestingly, the actual risk only changed from a prevaccine rate of approximately one case of GBS in a million people to a postvaccine rate of eleven to twelve cases of GBS per million people. So even though you were eleven or twelve times more likely to develop GBS in the first six weeks after the swine flu vaccine, your actual risk of GBS was still very low, around one in one hundred thousand recipients of the vaccine.
GBS is less clearly linked to other vaccines, such as Tdap, the nasal influenza vaccine, and the meningitis vaccine. For example, several adolescents have had GBS within a month of having received the meningitis vaccine, but it is not clear if the GBS is linked to the vaccine. Specifically, experts are unsure if the rate of GBS after that vaccine is higher than the one in a million background rate. As the meningitis vaccine is still new, more research and monitoring will need to be done before a better estimate of the risk of GBS is available.
Finally, the MMR and DTaP VIS mention that other serious problems have been reported after administration of the given vaccine. These problems include long-term seizures, coma, or lowered consciousness and permanent brain damage (both MMR and DTaP) and deafness (MMR only). These problems occur so rarely that it is difficult for experts to be sure if they are related to the vaccine or not.
You might notice that one problem we am not listing as a side effect of vaccines is autism. Despite concerns linking autism to both the MMR vaccine and to the thimerosal preservative in vaccines, in our opinion, the preponderance of the evidence does not show any connection.