Measles is a highly contagious viral disease that spreads easily through the air. Before the widespread use of the Measles, Mumps, and Rubella (MMR) vaccine, measles was a common childhood illness often leading to severe complications. The MMR vaccine has drastically reduced case numbers globally. Despite its high effectiveness, a vaccinated person can still contract the disease, though this is an exceptionally rare event. This infection after vaccination is known as a breakthrough infection and presents differently than the disease in an unvaccinated person.
Quantifying Vaccine Protection
The level of immunity provided by the vaccine depends on the number of doses received. A single dose of the MMR vaccine protects against measles approximately 93% to 95% of the time. This high rate of protection leads to the first dose being administered in early childhood. However, a small percentage of individuals do not develop sufficient immunity from this initial shot.
Public health authorities recommend a second dose to address this gap, which significantly boosts the protective rate. The two-dose schedule provides effectiveness rates reaching about 97% to 99% against measles. The second dose completes the primary series, ensuring those who failed to respond to the first dose have a second opportunity to develop immunity. Very few people who complete the full two-dose series will develop measles upon exposure.
Mechanisms of Vaccine Failure
Breakthrough infections are typically due to one of two immunological pathways known as vaccine failure. The first and most common is primary vaccine failure. This occurs when the immune system fails to mount a protective antibody response following the initial vaccination, meaning the person never became immune. This non-response happens in a small percentage of people after the first dose, which is why a second dose is administered to convert these non-responders into protected individuals.
Primary Failure Causes
The reasons for primary failure are complex and relate to either the host or the vaccine itself. Host-related factors include an individual’s genetic makeup, immune status at the time of vaccination, or the presence of residual maternal antibodies in infants. Vaccine-related issues, such as improper storage leading to a loss of potency or errors in administration technique, are also potential causes.
Secondary Vaccine Failure
The second pathway, secondary vaccine failure, is less common with the measles vaccine and involves a waning of immunity over time. In this case, the person successfully developed a protective immune response, but antibody levels gradually decreased to a non-protective level years later. This decline leads to susceptibility, particularly when exposure to the virus is high, and is characterized by a milder illness. Secondary vaccine failure is estimated to occur in about 2% to 10% of vaccinated people 6 to 26 years after their last dose.
Characteristics of Post-Vaccination Measles
When a fully or partially vaccinated person contracts measles, the illness is significantly altered compared to the classic presentation in unvaccinated cases. This milder form is commonly referred to as “modified measles” or “attenuated measles.” Prior immunity softens the disease’s impact because the presence of pre-existing antibodies, even if not fully protective, allows the immune system to fight the virus more effectively.
The classic symptoms of high fever, persistent cough, runny nose, and inflamed eyes may be less pronounced or absent in modified measles. The characteristic generalized red rash is typically sparse, discrete, and lasts for a shorter duration. The incubation period before symptoms appear can also be prolonged compared to the standard 10 to 12 days. Vaccinated individuals who experience breakthrough infections have lower rates of complications and hospitalization. These cases often involve lower viral loads, which reduces the likelihood of transmitting the virus to other people.