COVID-19 in low-tolerance border quarantine systems: impact of the Delta variant of SARS-CoV-2

TL;DR

This study models COVID-19 border quarantine effectiveness against the Delta variant, showing that higher vaccine efficacy significantly reduces outbreak risk and supports relaxing quarantine measures when combined with high vaccination coverage.

Contribution

It introduces a detailed transmission model incorporating vaccination effects and timing factors, specifically analyzing quarantine system resilience against the Delta variant.

Findings

Vaccine efficacy of 70-90% greatly extends outbreak-free periods.

Vaccination combined with quarantine reduces importation risk.

Higher vaccine efficacy is crucial for quarantine relaxation strategies.

Abstract

In controlling transmission of COVID-19, the effectiveness of border quarantine strategies is a key concern for jurisdictions in which the local prevalence of disease and immunity is low. In settings like this such as China, Australia, and New Zealand, rare outbreak events can lead to escalating epidemics and trigger the imposition of large scale lockdown policies. Here, we examine to what degree vaccination status of incoming arrivals and the quarantine workforce can allow relaxation of quarantine requirements. To do so, we develop and apply a detailed model of COVID-19 disease progression and transmission taking into account nuanced timing factors. Key among these are disease incubation periods and the progression of infection detectability during incubation. Using the disease characteristics associated with the ancestral lineage of SARS-CoV-2 to benchmark the level of acceptable risk, we examine the performance of the border quarantine system for vaccinated arrivals. We examine disease transmission and vaccine efficacy parameters over a wide range, covering plausible values for the Delta variant currently circulating globally. Our results indicate a threshold in outbreak potential as a function of vaccine efficacy, with the time until an outbreak increasing by up to two orders of magnitude as vaccine efficacy against transmission increases from 70% to 90%. For parameters corresponding to the Delta variant, vaccination is able to maintain the capacity of quarantine systems to reduce case importation and outbreak risk, by counteracting the pathogen's increased infectiousness. To prevent outbreaks, heightened vaccination in border quarantine systems must be combined with mass vaccination. The ultimate success of these programs will depend sensitively on the efficacy of vaccines against viral transmission.