To provide the best protection, some vaccines require an additional dose or doses as we get older.
For example, the pertussis vaccine offers a good level of protection within the first two years of getting vaccinated, but then protection decreases over time. This is known as waning immunity, which is the loss of protective antibodies over time. Similarly, actually getting the disease and developing antibodies through the infection may only protect you for a few years.
This is why it’s so important to follow the Centers for Disease Control and Prevention (CDC) recommended immunization schedule. For example, the pertussis vaccine includes shots at 2, 4, and 6 months of age, with booster shots at between 15 and 18 months old and between 4 and 6 years old, in order to be fully vaccinated. In addition, adults are encouraged to stay up-to-date on their vaccinations, especially pregnant women, parents and people interacting with infants and children. See the CDC’s recommended immunization schedule for adults.
Vaccines also are not perfect. Depending on the vaccine, about 1 percent to 5 percent of children who are vaccinated fail to develop immunity. If these children are exposed to that disease, they could get sick. Sometimes giving an additional vaccine dose will stimulate an immune response in a child who didn’t respond to the first dose. For example, a single dose of measles vaccine protects about 95 percent of children, but after two doses, almost 100 percent are immune.
Sometimes a child is exposed to a disease just prior to being vaccinated, and gets sick before the vaccine has had time to work. Sometimes a child gets sick with something that is similar to a disease they have been vaccinated against. This often happens with flu. Many viruses cause symptoms that look like flu, and people even call some of them flu, even though they are really something else. Flu vaccine doesn’t create immunity to these viruses (like the common cold).
Sources:
CDC: Pertussis FAQ
CDC: Parents’ Guide to Childhood Immunizations FAQ