Demographic transition and the dynamics of measles in six provinces in China: A modeling study

Abstract

BackgroundIndustrialization and demographic transition generate nonstationary dynamics in human populations that can affect the transmission and persistence of infectious diseases. Decades of increasing vaccination and development have led to dramatic declines in the global burden of measles, but the virus remains persistent in much of the world. Here we show that a combination of demographic transition, as a result of declining birth rates, and reduced measles prevalence, due to improved vaccination, has shifted the age distribution of susceptibility to measles throughout China.Methods and findingsWe fit a novel time-varying catalytic model to three decades of age-specific measles case reporting in six provinces in China to quantify the change in the age-specific force of infection for measles virus over time. We further quantified the impact of supplemental vaccination campaigns on the reduction of susceptible individuals. The force of infection of measles has declined dramatically (90%–97% reduction in transmission rate) in three industrialized eastern provinces during the last decade, driving a concomitant increase in both the relative proportion and absolute number of adult cases, while three central and western provinces exhibited dynamics consistent with endemic persistence (24%–73% reduction in transmission rate). The reduction in susceptible individuals due to supplemental vaccination campaigns is frequently below the nominal campaign coverage, likely because campaigns necessarily vaccinate those who may already be immune. The impact of these campaigns has significantly improved over time: campaigns prior to 2005 were estimated to have achieved less than 50% reductions in the proportion susceptible in the target age classes, but campaigns from 2005 onwards reduced the susceptible proportion by 32%–87%. A limitation of this study is that it relies on case surveillance, and thus inference may be biased by age-specific variation in measles reporting.ConclusionsThe age distribution of measles cases changes in response to both demographic and vaccination processes. Combining both processes in a novel catalytic model, we illustrate that age-specific incidence patterns reveal regional differences in the progress to measles elimination and the impact of vaccination controls in China. The shift in the age distribution of measles susceptibility in response to demographic and vaccination processes emphasizes the importance of progressive control strategies and measures to evaluate program success that anticipate and react to this transition in observed incidence.

Epidemiological changes in measles infections in southern China between 2009 and 2016: a retrospective database analysis

“…Relatively high proportions of adult patients were also observed after the SIAs were introduced in other studies [20,21]. The decreasing incidence in the overall population, declining birth rate, and increasing vaccination coverage among infants and children compared to the low coverage rate in the 1980s and earlier [37,38] all contributed to the increasing mean age of infection and build-up of susceptible individuals among adults [39]. In China, infections among adults were mainly due to susceptibility caused by missed vaccinations and decreased opportunities for acquiring immunity through natural infection [38,40].…”

Estimation and prediction for a mechanistic model of measles transmission using particle filtering and maximum likelihood estimation

“…Measles vaccination programs employ multiple doses of vaccine, either through a routine second dose or SIAs. Administrative estimates of coverage for these second dose opportunities do not explicitly account for the fraction of the population likely to be previously immunized through either vaccination or natural immunity . Here, we include a dynamic demographic model of the age distribution of susceptible individuals, which is depleted both through natural infection and prior vaccination.…”

“…Furthermore, we assume, for simplicity, that the second dose opportunities for measles vaccination (via routine or SIAs) are independent of the first dose. However, it is possible that individuals who receive the first dose are more likely to also receive the second dose because of higher access to vaccination services, which would mean that coverage estimates for second dose opportunities are an overestimate of the resulting immunity . As with alternative assumptions about reporting rate, it is straightforward to alter the statement of Equation to account for an assumption of non‐independent second dose opportunities.…”

“…The targeted ages vary from country to country and campaign to campaign based on logistical capacity and expected risk. While the coverage of supplemental campaigns describes the number of children vaccinated, the impact of these campaigns on increased rates of immunization depends on the rate of immunization through the routine system (a child successfully immunized through the routine system receives no additional benefit through a supplemental dose) and rate of natural infection (a child that has previously been infected with measles is immune and also receives no benefit from a supplemental dose) . The model of Chen et al did not explicitly represent the age distribution of the susceptible population, thus the impact of these campaigns on increased immunization was represented implicitly.…”

Evaluating Measles Incidence Rates Using Machine Learning and Time Series Methods in the Center of Iran; 1997-2020

“…Greater regulation in vaccines has decreased the rate of this infection during the recent years worldwide [5]. However, the United Nations (UN) has warned countries about the increase in the amount of measles reports worldwide by 48.4 percent in 2019 which is also growing steadily due to inadequate monitoring of the vaccinations, development in anti-vaccination campaigns, economic and political issues surrounding health-care programs [4,6].…”