It isn’t very common, but it can happen. Depending on the vaccine, about 1 percent to 5 percent of children who are vaccinated fail to develop immunity. For very few people, underlying illnesses may affect their immune system response and in turn, they may require additional vaccination or protection.
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.
Some vaccines, like the pertussis (whooping cough) vaccines, are effective but not perfect. They typically offer good levels 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, natural infection may also protect you only 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. And the CDC recommends the tetanus-diphtheria-pertussis (Tdap) shot for everyone 11 years old and older, including pregnant women.