Breach Risk Under Vaccine Efficacy Scenarios

Figure 1: Breach risk across transmissibility levels.

Problem description

The emergence of more transmissible SARS-CoV-2 variants has put increasing pressure on legacy quarantine systems. Against this backdrop, we ask two questions: (RQ1) how does vaccine efficacy (VE) change breach risk under a fixed quarantine protocol, and (RQ2) how does higher transmissibility (R₀) shift that risk. To answer these questions, we analyze a hotel-based system with a defined testing schedule and isolation rules. We therefore quantify performance by breach events—instances where an infectious traveller or quarantine worker has uncontrolled contact with the public. Operationally, we report breaches per 1,000 arrivals and the 30-day probability of at least one outbreak, and we judge success when both remain below a predefined threshold. These definitions link directly back to RQ1 and RQ2, allowing us to compare scenarios consistently and attribute changes in risk to VE and transmissibility.

Analysis

To address Q1, we vary VE from low to high while holding the protocol fixed, then estimate breaches/1000 and 30-day break out probability.

Figure 2: Breach risk across transmissibility levels.

As shown in Figure 2, breach risk decrease as VE rises; importantly, the decline is non-linear beyond moderate VE. This figure answers Q1 by showing that higher VE substantially reduces breach pressure, but does not eliminate it.

Building on this, for Q2 the article increased the transmissibilitiy while keeping VE and protocal constant.

Figure 3: Outbreak-free time vs vaccine efficacy.

As the Figure 3 show, we could see that more transmissible variants erode the safety margin, so quarantine alone cannot compensate without strong vaccination and tight operations.

Conclusions

Taken together, breach risk falls with VE and rises with R₀. Vaccination meaningfully counteracts variant pressure, yet stringent quarantine remains necessary—even for vaccinated travellers—when facing highly transmissible variants.

Policy inspirations

In practical terms, pairing high community vaccination with tight quarantine controls (testing cadence, PPE, isolation timing) helps keep breach rates and 30-day outbreak probability under target. However, if arrival prevalence spikes or a substantially more transmissible variant emerges, today’s thresholds may no longer be met. Next, track variant characteristics, re-calibrate test schedules, and re-estimate risk on a monthly cycle to maintain an adequate safety margin.

Reference

1.Zachreson, C., Shearer, F. M., Price, D. J., Lydeamore, M. J., McVernon, J., McCaw, J., & Geard, N. (2022). COVID-19 in low-tolerance border quarantine systems: Impact of the Delta variant of SARS-CoV-2. Science Advances, 8(14), eabm3624. https://doi.org/10.1126/sciadv.abm3624