In this Cape Town outbreak study, the authors state, "The epidemiology of measles
in Cape Town has thus changed as evinced in this epidemic, with an increase in the number of cases occurring in older, previously vaccinated children
. The possible reasons for this include both primary and secondary vaccine
Coetzee N, Hussey GD, Visser G, Barron P, Keen A.Department of Community health, University of Cape Town. The 1992 measles epidemic in Cape Town--a changing epidemiological pattern. S Afr Med J. 1994 Mar;84(3):145-9.
Over the last 6 years there has been a decline in the incidence of measles in Cape Town. However, during August 1992 an outbreak occurred, with cases reported at many schools in children presumably immunised. The objectives of this study were to characterise the epidemic in Cape Town and to determine possible reasons for the outbreak. The investigation consisted of two components--a description of the epidemic and an investigation of an outbreak at one primary school. Results indicate that during the last 4 months of the year, 757 cases were notified in Cape Town, compared with 144 in the first 8 months. The epidemic affected mainly white and coloured children over 5 years of age (P < 0.001). In contrast, during the period before the epidemic most cases occurred in black children and in those aged less than 1 year (P < 0.001). There was no significant increase in hospitalised cases. Investigation of the outbreak at one school revealed that the attack rate was 7.6% (25/329 children). Immunisation coverage (at least one dose of any measles vaccine) was 91% and vaccine efficacy was estimated to be 79% (95% CI 55-90); it was highest for monovalent measles (100%) and lowest for measles-mumps-rubella (74%). The epidemiology of measles in Cape Town has thus changed as evinced in this epidemic, with an increase in the number of cases occurring in older, previously vaccinated children. The possible reasons for this include both primary and secondary vaccine failure.
As we have mentioned in previous articles, vaccination, though it may prevent disease incidence, shifts the disease into a higher-age group and does not offer natural, long-term immunity.