Rutherford Postdoctoral Fellow, School of Earth and Environment, University of Canterbury, Christchurch, New Zealand; Postdoctoral Researcher, Department of Earth Sciences, University of Oregon Eugene, OR, USA.
N Z Med J. 2022 Apr 1;135:89-100.
The New Zealand Government has transitioned from the Alert Level framework, which relied on Government action and population level controls, to the COVID-19 Protection Framework, which relies on vaccination rates and allows for greater freedoms (for the vaccinated). Under the COVID-19 Protection Framework with and current widespread community transmission of Omicron, there is significant interest in understanding the relative risk of spreading COVID-19 posed by unvaccinated, vaccinated, and boosted individuals.
A stochastic branching process model is used to simulate the spread of COVID-19 for outbreaks seeded by unvaccinated, vaccinated, or boosted individuals. The likelihood of infecting or getting infected with COVID-19 is calculated based on vaccination status. The model is applied to both the Delta and Omicron variants.
For the Delta variant, a vaccinated traveller infected with COVID-19 is 9x less likely to seed an outbreak than an unvaccinated traveller infected with COVID-19; however, for the Omicron variant, there is little difference between outbreaks seeded by unvaccinated and vaccinated individuals (boosted individuals are slightly less likely to seed large outbreaks). For the Delta variant, unvaccinated individuals are responsible for 87% of all infections whereas only 3% of infections are from vaccinated-to-vaccinated when normalised by population. Therefore, a vaccinated individual is 6.8x more likely to be infected by an unvaccinated individual than by a vaccinated individual. For the Omicron variant, unvaccinated individuals are responsible for 45% of all infections compared to 39% for vaccinated (two doses) and 15% for boosted (three doses) individuals when normalised by population. Despite the vaccine being less effective at preventing breakthrough transmission for Omicron, only 3% of all infections are from boosted-to-boosted individuals when normalised by population, indicating that three doses of the vaccine provides good protection from infection and breakthrough transmission.
This work demonstrates that most new infections are caused by unvaccinated individuals, especially for the Delta variant. These simulations illustrate the importance of vaccination in stopping individuals from becoming infected with COVID-19 and in preventing onward transmission. For Omicron, individuals vaccinated with two doses are only slightly less likely to spread COVID-19 than those who are unvaccinated. This work suggests that for the current Omicron outbreak the COVID-19 Protection Framework be updated to distinguish between those who have received two primary doses of the Pfizer-BioNTech vaccine (vaccinated individuals) and those who have received three doses (boosted individuals).
新西兰政府已从依赖政府行动和人群层面管控的警戒级别框架过渡到依赖疫苗接种率的 COVID-19 保护框架，从而为（已接种疫苗的）人群提供更多自由。在 COVID-19 保护框架下，随着奥密克戎在社区的广泛传播，人们对了解未接种、接种和加强免疫人群传播 COVID-19 的相对风险产生了浓厚兴趣。
使用随机分支过程模型模拟由未接种、接种或加强免疫个体引发的 COVID-19 传播。根据疫苗接种状况计算感染或感染 COVID-19 的可能性。该模型应用于德尔塔和奥密克戎变体。
对于德尔塔变体，感染 COVID-19 的接种旅行者引发疫情的可能性比感染 COVID-19 的未接种旅行者低 9 倍；然而，对于奥密克戎变体，未接种和接种个体引发的疫情之间几乎没有差异（加强免疫个体引发大规模疫情的可能性略低）。对于德尔塔变体，未接种人群导致 87%的所有感染，而正常化到人群后，仅 3%的感染来自接种疫苗的人群。因此，接种个体被未接种个体感染的可能性是被接种个体感染的 6.8 倍。对于奥密克戎变体，正常化到人群后，未接种个体导致 45%的所有感染，而接种两剂疫苗的个体占 39%，接种三剂疫苗的个体占 15%。尽管疫苗在预防奥密克戎突破性传播方面的效果较差，但正常化到人群后，所有感染中仅有 3%来自加强免疫个体，这表明三剂疫苗可提供良好的感染和突破性传播保护。
本研究表明，大多数新感染是由未接种人群引起的，尤其是对于德尔塔变体。这些模拟说明了疫苗接种在阻止个体感染 COVID-19 和阻止传播方面的重要性。对于奥密克戎，接种两剂疫苗的个体传播 COVID-19 的可能性略低于未接种个体。本研究表明，对于当前的奥密克戎疫情，需要对 COVID-19 保护框架进行更新，以区分已接种辉瑞-生物科技疫苗两剂（接种个体）和三剂（加强免疫个体）的人群。
