The disease-induced herd immunity level for Covid-19 is substantially lower than the classical herd immunity level
The disease-induced herd immunity level for Covid-19 is substantially lower than the classical herd immunity level
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DOI：
10.1101/2020.05.06.20093336
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发表时间：
2020-05
期刊：
影响因子：
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通讯作者：
T. Britton;F. Ball;Pieter Trapman
中科院分区：
文献类型：
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作者：
T. Britton;F. Ball;Pieter Trapman
尽管采取了不同程度的预防措施，但大多数国家仍受到正在进行的新冠肺炎大流行的严重影响。一个常见的问题是一个国家或地区是否以及何时能够达到群体免疫h。经典的群体免疫水平Hc被定义为Hc=1-1/R0，其中R0是基本繁殖次数，根据国家和地区的不同，新冠肺炎估计在2.23.5的范围内。这里显示，在一个国家/地区发生疫情并采取了一套预防措施后，由疾病引起的群体免疫水平HD实际上比HC小得多。作为例证，我们表明，在一个年龄结构的社区中，如果R0=2.5，混合率适合于社会活动研究，并将个体分为三类：低活跃、中等活跃和高活跃，并且预防措施按比例影响所有混合率，则疾病诱导的群体免疫水平为HD=43%，而不是Hc=1-1/2.5=60%。因此，需要较低的感染比例才能产生群体免疫。其根本原因是，当疾病传播引发免疫时，高接触率人群中感染的比例高于低接触率人群。因此，疾病诱导的免疫比经典的群体免疫水平上的免疫在社区中均匀分布时更强。
Most countries are suffering severely from the ongoing covid-19 pandemic despite various levels of preventive measures. A common question is if and when a country or region will reach herd immunity h. The classical herd immunity level hC is defined as hC=1-1/R0, where R0 is the basic reproduction number, for covid-19 estimated to lie somewhere in the range 2.2-3.5 depending on country and region. It is shown here that the disease-induced herd immunity level hD, after an outbreak has taken place in a country/region with a set of preventive measures put in place, is actually substantially smaller than hC. As an illustration we show that if R0=2.5 in an age-structured community with mixing rates fitted to social activity studies, and also categorizing individuals into three categories: low active, average active and high active, and where preventive measures affect all mixing rates proportionally, then the disease-induced herd immunity level is hD=43% rather than hC=1-1/2.5=60%. Consequently, a lower fraction infected is required for herd immunity to appear. The underlying reason is that when immunity is induced by disease spreading, the proportion infected in groups with high contact rates is greater than that in groups with low contact rates. Consequently, disease-induced immunity is stronger than when immunity is uniformly distributed in the community as in the classical herd immunity level.