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Can vaccines make you sick?

Although vaccines are effective in reducing the likelihood of getting an illness, they are not always 100 percent effective and therefore some people will still get the illness against which they were vaccinated. Fortunately they often have a less severe case of infection than if they had not been vaccinated.

Some people believe that vaccines are not effective. There are a few of reasons for this misconception, some of which are quite understandable. For example, when an outbreak of a preventable disease occurs, if almost everyone has gotten the vaccine to prevent it, then more people who get sick will have gotten vaccinated simply because so few have not gotten vaccinated. Say the vaccine for the disease is 98 percent effective (no vaccine is 100 percent effective). In this example, the 2 percent for whom the vaccine has not worked can easily outnumber all the people who did not get the vaccine. This does not mean that the vaccine does not work. Most childhood vaccines are effective for more than 90 percent of recipients. Ironically, if immunization efforts were not so successful in the United States, the apparent paradox would not exist, because the unvaccinated group would be larger—and, during an outbreak of disease, a higher percentage of the total number of affected people would come from that group.

Many people will note that the annual influenza vaccine does not have the same high percentage of effectiveness, generally reducing the risk of getting the flu by no more than 60 percent. But unlike other vaccinatable illnesses, the flu virus is different every year, and the vaccines' effectiveness varies with the prediction of what flu viruses will circulate each year. Other illnesses, like mumps, measles and chickenpox, do not vary from year to year.

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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.