Waning immunity and subclinical measles infections in England.

Following the introduction of vaccination against measles, levels of clinical infection have dropped markedly. As we move further into the vaccine era, increasingly many individuals owe their measles immunity to vaccination and have had few (if any) exposures to wild virus. A number of recent reports suggest that vaccinated individuals with low levels of immunity may be at risk of subclinical measles infection. We explore the interplay between levels of infection and immunity over time using a mathematical model that simulates infection, waning and boosting of immunity. We focus particularly on the situation in England, where vaccination has been in place since 1966. Simulations of our model demonstrate a rise in the levels of subclinical measles infection over time, and a corresponding rise in clinical measles infections if vaccination levels are too low. We compare the impact of intervention strategies, and find that the rise in cases is most effectively reduced by 'catch-up' vaccination of children. In recent years, vaccination levels in England have dropped from above 90% in the 1990s to 84% in 2001/2002. We discuss the impact of declining vaccination levels on clinical and subclinical infections.