Primary vaccine failure occurs when an individual does not mount a sufficient antibody response after vaccination. This occurs in 2-10% of vaccinated healthy individuals. This is more common in those >50 years old suggesting that an aging immune system contributes to reduced responsiveness to vaccination. Secondary vaccine failure occurs when individuals who previously responded to vaccination lose protective antibodies over time. A study published in
Clinical Infectious Diseases evaluated the clinical characteristics of measles infection in vaccinated versus unvaccinated patients. Among cases of confirmed measles, the majority were in unvaccinated subjects, however there were cases of confirmed measles in individuals who received 1 dose or 2+ doses of the measles vaccine. It was unclear, however, if these individuals who had received at least 1 dose of the vaccine represented primary or secondary vaccine failure. Those who received 2 or more doses of the vaccine were noted to have milder clinical courses with lower rates of hospitalization, cough, coryza, conjunctivitis, and fever than those who had 1 dose or were unvaccinated. Overall those who are infected with the measles virus despite vaccination seem to do better than those who contract the virus and are unvaccinated.
References: Primary and Secondary Vaccine Failure