1.4 Epidemiologic features of measles

Despite a dramatic decrease in measles incidence since 2000, epidemics occur in areas with low vaccination coverage or pockets of susceptible groups. Measles is highly contagious and is transmitted from person to person by infectious aerosols, large respiratory droplets or direct contact with nasal or throat secretions from an infected person. The basic reproduction number, Ro, is the average number of secondary cases that are generated from a primary case in a population that is completely susceptible. Measles has an Ro of 12-18, the highest of any infectious communicable disease [18]. In the absence of measles vaccination, in a population with homogeneous contact patterns, nearly everyone will become infected.

The current measles vaccine, a live-attenuated strain of measles, is safe and highly effective at providing protection against measles. Measles vaccine is often administered as measles-rubella (MR) or measles-mumps-rubella (MMR) vaccine. Measles continues to disproportionally affect children in developing countries, where it is one of the leading causes of under-five mortality. Currently the WHO recommends administration of the first dose of measles-containing vaccine (MCV) at nine months of age where there is a high risk of mortality among infants due to ongoing transmission and at 12 months of age in areas with low rates of measles transmission [19].

After a single dose of MCV at 8-9 months of age, 89.6% of children develop protective immunity to measles. However, since maternal antibody can interfere with the response to vaccination, a higher rate of seroconversion is achieved if vaccination can be delayed until most maternal antibody has waned. Following administration of MCV at 11-12 months of age, 99% of children seroconvert [19]. To achieve the 95% coverage required to interrupt transmission of measles virus, a two-dose vaccination strategy is recommended. The second dose of MCV may be delivered as part of routine immunization services or as school entry requirements. In many countries with gaps in their routine immunisation programme, supplemental immunization activities (SIAs) are important to achieve high vaccination coverage.

Vaccination programmes have greatly reduced the worldwide incidence of measles, including the successful elimination of endemic measles in the region of the Americas. However, measles outbreaks can still occur in countries with high vaccination coverage. Prolonged outbreaks may indicate that an immunity gap exists in the population involved. After elimination has been achieved, continued introduction of measles will inevitably lead to small outbreaks. Most cases occur among individuals who were never vaccinated, those who failed to seroconvert following vaccination, or those persons who had a suboptimal response and lack full protection. Because pockets of susceptibility may exist in some communities, the maintenance of adequate herd immunity to contain outbreaks is critical to sustain elimination [17].