Antso Raherindrasana, C Jessica Metcalf, Jean-Michel Heraud, Simon Cauchemez, Amy Winter, Amy Wesolowski, Richter Razafindratsimandresy, Lea Randriamampionona, S A Rafalimanantsoa, Yolande Masembe, Charlotte Ndiaye, Julio Rakotonirina
MEASLES AND OUTBREAK RISK
Measles vaccination is often referred to as a ‘best buy’ in public health, because of the high case fatality rate associated with infection, alongside the existence of a safe and inexpensive vaccine. The current WHO recommendation is that all children have access to two doses of the measles vaccine.1 In 2011, countries in the WHO African region adopted a measles elimination goal to be reached by 2020. In the last decades, substantial gains have been made in numbers of cases and deaths averted. Yet, an important feature of measles epidemiology is that large outbreaks can occur following years of (apparently) successful control. This phenomenon is known as a ‘posthoneymoon period’ outbreak.2 The ‘honeymoon’ consists of the period following vaccine introduction where cases drop substantially. This ‘honeymoon’ is at risk of ending in an outbreak if vaccination coverage is subsequently suboptimal. Every year, children born into the population that are left unvaccinated are also unlikely to experience immunisation by natural infection, because measles incidence is low. These children, thus, remain susceptible to measles, and over the years, as children are born and left unvaccinated, susceptible individuals accumulate in the population, across the range of ages reflecting cohorts born during the low incidence years. Once the size of the susceptible pool exceeds the threshold for herd immunity (defined as the proportion susceptible in the population exceeding 1/R0 where R0 is the number of new infections per susceptible individual in a completely susceptible population, which may be as high as 20 for measles3 ), a new outbreak can take hold, and it will grow at a speed defined by the effective reproductive number RE=R0 S where S is the proportion of the population that is susceptible.