Optimising social mixing strategies achieving COVID-19 herd immunity while minimising mortality in six European countries

Strategies are needed to minimise the impact of COVID-19 in the medium-to-long term, until safe and effective vaccines can be used. Using a mathematical model in a formal optimisation framework, we identified contact mitigation strategies that minimised COVID-19-related mortality over a time-horizon of 15 months while achieving herd immunity in six or 12 months, in Belgium, France, Italy, Spain, Sweden, and the UK. We show that manipulation of social contacts by age can reduce the impact of COVID-19 considerably in the presence of intense transmission. If immunity was persistent, the optimised scenarios would result in herd immunity while causing a number of deaths considerably lower than that observed during the March-April European wave in Belgium, France, Spain and Sweden, whereas the numbers of deaths required to achieve herd immunity would be comparable to somewhat larger that the past epidemics in Italy and the UK. Our results also suggest that countries’ herd immunity thresholds may be considerably lower than first estimated for SARS-CoV-2. If post-infection immunity was short-lived, ongoing contact mitigation would be required to prevent major epidemic resurgence. ### Competing Interest Statement The authors have declared no competing interest. ### Funding Statement RR was supported by a Project Grant (APP1144570) from the Australian National Health and Medical Research Council. JMT was supported by an Early Career Fellowship (APP1142638) from the Australian National Health and Medical Research Council. ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: Not applicable All necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines and uploaded the relevant EQUATOR Network research reporting checklist(s) and other pertinent material as supplementary files, if applicable. Yes The study was conducted using publically available data only. The references are presented in the manuscript.