Challenges in creating herd immunity to SARS-CoV-2 infection by mass vaccination.
Challenges in creating herd immunity to SARS-CoV-2 infection by mass vaccination.
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DOI：
10.1016/s0140-6736(20)32318-7
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发表时间：
2020-11-21
期刊：
影响因子：
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通讯作者：
Collyer BS
中科院分区：
文献类型：
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作者：
Anderson RM;Vegvari C;Truscott J;Collyer BS
评论：柳叶刀。COM VOL 396 2020年11月21日16点15分发布新冠肺炎疫苗许可证。因此，疫苗接种对SARS-CoV-2传播的影响将缓慢开始，并在几年内积累，以达到目标覆盖率水平。确定人群所需的疫苗数量将取决于新冠肺炎疫苗第三阶段有效性试验的证据，以及对疫苗保护平均持续时间的假设-这将是一个假设，直到关于预防感染和严重疾病保护持续时间的第四阶段试验结果公布为止。对于提供终身保护的100%有效的疫苗，阻断传播所需的群体免疫水平PC是[1-1/R0]，其中R0是基本繁殖数。16考虑到大多数国家封锁前的R0值在2.5到3.5之间，我们估计所需的群体免疫力约为60%-72%。如果考虑比例疫苗效力ε，则pC的简单表达式为[1-1/R0]/ε。如果我们假设ε为0.8(80%)，则在定义的R0值范围内，所需的群体免疫力为75-90%。为了降低疗效，所有人都必须接种疫苗。这些总体估计忽略了可能使特定地区的这些数字更低或更高的异质性。如果我们假设免疫力是短暂的，这些计算就会变得更加复杂。19根据SARS-CoV-2的传播模型，可以计算出每年需要接种具有特定性质的新冠肺炎疫苗的人口比例(附录)。覆盖率PC的简单等式变成了一个更复杂的表达式，它涉及到人们接种疫苗的速度、ε、R0的大小和疫苗提供的平均保护期限(图)。图中绘制的曲面图显示了第一年必须接种疫苗的人口百分比，以及几年后系统平衡后必须接种疫苗的百分比的类似曲线图。通过对模型的数值评估，给出了这一时间的大致概念，并在第二年结束时达到平衡(附录)。第一年必须接种疫苗的人口比例远远高于几年后系统稳定后必须接种的百分比，因为随着大规模免疫接种的开始，大多数人口将容易受到影响，但几年后，希望将有较高比例的人接种疫苗，从而产生有效的群体免疫。根据我们基于以下假设的估计：疗效令人满意(80%)，但保护期很短(1-2年)，如果要有任何机会获得群体免疫以阻止SARS-CoV-2的继续传播，就需要为总人口的很大一部分人接种疫苗。如果疫苗在比自然感染更长的持续时间内具有保护性，那么我们的估计将过于悲观。一种新冠肺炎疫苗的免疫期限只有在社区范围的疫苗接种计划取得进展后才能解决。第三阶段试验将告诉我们有效性和安全性，但设计良好的第四阶段试验是基于具有代表性的和大量的疫苗接种者并随着时间的推移进行跟踪的关键。这些研究将记录任何严重的不良事件，并确定反复接触者是否感染冠状病毒，特别是SARS-CoV-2，如果感染，疾病的严重程度。这些基于队列的纵向研究将需要仔细的规划和持续的资金，可能来自拥有行业和…的政府
Comment www. thelancet. com Vol 396 November 21, 2020 1615 post licensure of a COVID-19 vaccine. As such, the impact of vaccination on the transmission of SARS-CoV-2 will start slowly and build up over a few years to reach target coverage levels. The amount of vaccine required for a defined population will depend on evidence from phase 3 COVID-19 vaccine trials on efficacy and what can be assumed about the average duration of vaccine protection—it will be an assumption until the findings of phase 4 trials on duration of both protection against infection and severe disease are reported. For a vaccine with 100% efficacy that gives life-long protection, the level of herd immunity as a proportion of the population, pc, required to block transmission is [1–1/R0], where R0 is the basic reproduction number. 16 Given an R0 value before lockdowns in most countries of between 2· 5 to 3· 5, we estimate the herd immunity required is about 60–72%. If the proportional vaccine efficacy, ε, is considered, the simple expression for pc becomes [1–1/R0]/ε. If we assume ε is 0· 8 (80%), then the herd immunity required becomes 75–90% for the defined range of R0 values. For lower efficacies, the entire population would have to be immunised. These overall estimates ignore heterogeneities that can make these figures lower or higher in specific locations. 17, 18 These calculations become more complicated if we assume immunity is short lived. 19 Calculations of the proportion of the population that will need to be immunised year by year with a COVID-19 vaccine of defined properties can be derived from transmission models of SARS-CoV-2 (appendix). The simple equation for coverage pc becomes a more complicated expression that involves the rate at which people are immunised, ε, the magnitude of R0, and the average duration of protection provided by the vaccine (figure). The surface plotted in the figure shows the percentage of the population in year 1 that must be vaccinated and a similar plot of the percentage that must be vaccinated once the system equilibrates after a few years. A rough idea of this time is given by numerical evaluations of the model and gives equilibration by the end of year 2 (appendix). The percentage of the population that must be vaccinated in year 1 is much larger than the percentage that must be vaccinated once the system has stabilised after a few years, since most of the population will be susceptible as mass immunisation starts, but after a few years, hopefully, a high proportion will be immunised such that effective herd immunity is created. What is clear from our estimates based on the assumptions that efficacy is satisfactory (> 80%) but duration of protection is short (1–2 years), is that a large proportion of the total population would need to be vaccinated if there is to be any chance of getting herd immunity to block the continued transmission of SARS-CoV-2. If the vaccine is protective over a longer duration than natural infection, then our estimates will be too pessimistic. What the duration of immunity is for a given COVID-19 vaccine will only be resolved once community-wide vaccination programmes progress. Phase 3 trials will tell us about efficacy and safety, but well designed phase 4 trials are essential based on representative and large numbers of those vaccinated and follow up over time. These studies will record any serious adverse events and identify whether repeatedly exposed individuals acquire coronavirus infections, particularly SARS-CoV-2, and if they do, what is the severity of disease. These cohortbased longitudinal studies will need careful planning and sustained funding, probably from governments with industry …