Centre for the Mathematical Modelling of Infectious Diseases, London School of Hygiene & Tropical Medicine, Keppel Street, London, UK.
Department of Infectious Disease Epidemiology, London School of Hygiene & Tropical Medicine, Keppel Street, London, UK.
BMC Med. 2019 Sep 25;17(1):180. doi: 10.1186/s12916-019-1413-7.
Vaccination has reduced the global incidence of measles to the lowest rates in history. However, local interruption of measles virus transmission requires sustained high levels of population immunity that can be challenging to achieve and maintain. The herd immunity threshold for measles is typically stipulated at 90-95%. This figure does not easily translate into age-specific immunity levels required to interrupt transmission. Previous estimates of such levels were based on speculative contact patterns based on historical data from high-income countries. The aim of this study was to determine age-specific immunity levels that would ensure elimination of measles when taking into account empirically observed contact patterns.
We combined estimated immunity levels from serological data in 17 countries with studies of age-specific mixing patterns to derive contact-adjusted immunity levels. We then compared these to case data from the 10 years following the seroprevalence studies to establish a contact-adjusted immunity threshold for elimination. We lastly combined a range of hypothetical immunity profiles with contact data from a wide range of socioeconomic and demographic settings to determine whether they would be sufficient for elimination.
We found that contact-adjusted immunity levels were able to predict whether countries would experience outbreaks in the decade following the serological studies in about 70% of countries. The corresponding threshold level of contact-adjusted immunity was found to be 93%, corresponding to an average basic reproduction number of approximately 14. Testing different scenarios of immunity with this threshold level using contact studies from around the world, we found that 95% immunity would have to be achieved by the age of five and maintained across older age groups to guarantee elimination. This reflects a greater level of immunity required in 5-9-year-olds than established previously.
The immunity levels we found necessary for measles elimination are higher than previous guidance. The importance of achieving high immunity levels in 5-9-year-olds presents both a challenge and an opportunity. While such high levels can be difficult to achieve, school entry provides an opportunity to ensure sufficient vaccination coverage. Combined with observations of contact patterns, further national and sub-national serological studies could serve to highlight key gaps in immunity that need to be filled in order to achieve national and regional measles elimination.
疫苗接种使麻疹的全球发病率降至历史最低水平。然而，要实现并维持麻疹病毒传播的局部中断，就需要持续高水平的人群免疫力，而这一点难以做到。麻疹的群体免疫阈值通常规定为 90-95%。这个数字并不容易转化为中断传播所需的特定年龄组的免疫水平。以前对这些水平的估计是基于基于高收入国家历史数据的推测性接触模式。本研究旨在确定特定年龄组的免疫水平，以确保在考虑到经验观察到的接触模式时消除麻疹。
我们将 17 个国家的血清学数据估算的免疫力水平与特定年龄组混合模式的研究相结合，得出了调整接触后的免疫水平。然后，我们将这些与血清学研究后的 10 年中的病例数据进行比较，以确定消除麻疹的接触调整免疫阈值。最后，我们将一系列假设的免疫概况与来自广泛社会经济和人口统计学背景的接触数据相结合，以确定它们是否足以消除麻疹。
我们发现，接触调整后的免疫水平能够预测各国在血清学研究后的十年内是否会爆发疫情，大约 70%的国家都符合这种情况。接触调整后的免疫阈值水平被发现为 93%，对应的基本繁殖数约为 14。使用来自世界各地的接触研究，我们以这个阈值水平测试了不同的免疫情景，发现 95%的免疫水平必须在 5 岁之前达到，并在年龄较大的人群中保持，以保证消除麻疹。这反映了 5-9 岁儿童所需的免疫水平比以前更高。
我们发现消除麻疹所需的免疫水平高于以前的指导意见。在 5-9 岁儿童中实现高免疫水平的重要性既是挑战，也是机遇。虽然实现如此高的水平可能具有挑战性，但入学提供了一个确保充分接种疫苗的机会。结合接触模式的观察结果，进一步的国家和次国家血清学研究可以突出需要填补的免疫空白，以实现国家和区域消除麻疹。
