A measles epidemic threshold in a highly vaccinated population
A measles epidemic threshold in a highly vaccinated population
复制标题
DOI：
10.1371/journal.pmed.0020316
复制
发表时间：
2005-11-01
期刊：
影响因子：
15.8
通讯作者：
Kretzschmar, M
中科院分区：
文献类型：
--
作者：
Walllinga, J;Heijne, JCM;Kretzschmar, M
大规模接种麻疹疫苗成功地降低了麻疹的发病率，并将流行模式从大约两年一次的周期转变为不规则的暴发序列。对这一系列疫情的一个可能解释是，接种疫苗的人群受到了坚实的群体免疫力的保护。如果是这样的话，我们希望看到易感个体的比例保持在流行阈值以下。另一种解释是群体免疫偶尔出现局部失误，使疫苗覆盖率低的地区发生重大疫情。在这种情况下，我们预计易感个体的比例将在疫情爆发前超过流行阈值。这两个解释麻疹疫情的不规则序列可以测试对观察的易感个体和感染攻击rates.Methods和结果的分数，我们估计在每个流行年的开始，每个流行年在荷兰超过28年的感染攻击率的易感个体的分数。在此期间，疫苗覆盖率平均为93%，没有持续的麻疹传播。在疫苗覆盖率低的社区发生了几次麻疹疫情，这些疫情在没有干预的情况下结束。我们发现，有一个明确的阈值的易感个体的比例，低于此只有轻微的爆发发生，并在此之上，无论是轻微的和重大的爆发发生。一个精确的，定量的关系之间存在的易感个体的分数超过这一阈值和感染攻击率在重大outbreaks.Conclusion在人群中具有高，但异质性疫苗覆盖率，麻疹传播可以中断没有建立坚实的群体免疫。当感染再次出现时，疫苗覆盖率低的社区可能会发生大规模疫情。在这样一次重大疫情期间，每增加一个超过阈值的易感个体，几乎就增加两次感染。这种数量关系为预测麻疹传播中断后未来重大疫情的可能性和规模提供了可能性。
Background Mass vaccination against measles has successfully lowered the incidence of the disease and has changed the epidemic pattern from a roughly biennial cycle to an irregular sequence of outbreaks. A possible explanation for this sequence of outbreaks is that the vaccinated population is protected by solid herd immunity. If so, we would expect to see the fraction of susceptible individuals remaining below an epidemic threshold. An alternative explanation is the occurrence of occasional localised lapses in herd immunity that allow for major outbreaks in areas with a low vaccine coverage. In that case, we would expect the fraction of susceptible individuals to exceed an epidemic threshold before outbreaks occur. These two explanations for the irregular sequence of measles outbreaks can be tested against observations of both the fraction of susceptible individuals and infection attack rates.Methods and Findings We have estimated both the fraction of susceptible individuals at the start of each epidemic year and the infection attack rates for each epidemic year in the Netherlands over a 28-y period. During this period the vaccine coverage averaged 93%, and there was no sustained measles transmission. Several measles outbreaks occurred in communities with low vaccine coverage, and these ended without intervention. We show that there is a clear threshold value for the fraction of susceptible individuals, below which only minor outbreaks occurred, and above which both minor and major outbreaks occurred. A precise, quantitative relationship exists between the fraction of susceptible individuals in excess of this threshold and the infection attack rate during the major outbreaks.Conclusion In populations with a high but heterogeneous vaccine coverage, measles transmission can be interrupted without establishing solid herd immunity. When infection is reintroduced, a major outbreak can occur in the communities with low vaccine coverage. During such a major outbreak, each additional susceptible individual in excess of the threshold is associated with almost two additional infections. This quantitative relationship offers potential for anticipating both the likelihood and size of future major outbreaks when measles transmission has been interrupted.