Objectives To assess the risk of sustained community transmission of SARS-CoV-2/COVID-19 in Queensland (Australia) in the presence of high-transmission variants of the virus.
Design We used an agent-based model Covasim and the demographics, policies, and interventions implemented in the state. Using the calibrated model we simulated possible epidemic trajectories that could eventuate due to leakage of infected cases with high-transmission variants, during a period of zero community transmission.
Setting Model calibration covered the first-wave period from early March 2020 to May 2020. Predicted epidemic trajectories were simulated from early February 2021 to late March 2021.
Main outcomes A calibrated model of COVID-19 epidemiology in Queensland; the conditions that could lead to an outbreak; and how likely that situation is to occur.
Results Simulations showed that one infected agent with the ancestral (A.2.2) variant has a 14% chance of crossing a threshold of sustained community transmission (i.e., > 5 infections per day, more than 3 days in a row), assuming no change in the prevailing preventative and counteracting policies. However, one agent carrying a more infectious variant (e.g., B.1.1.7) has a 43% chance of crossing the same threshold; a threefold increase. Doubling the average number of daily tests results in a decrease of this probability from 43% to 23%.
Conclusions The introduction of even a small number of people infected with high-transmission variants dramatically increases the probability of sustained community transmission in Queensland.
The known The B.1.1.7 variant that emerged in the UK spreads faster than the ancestral COVID-19 strain of early 2020, with a reported transmissibility between 40%-90% higher. However, the probabilities of developing sustained community transmission in Queensland, which is currently a zero community transmission setting, are unknown.
The new Using an agent-based model, with the levels of testing observed in Queensland during February–March 2021, we found that as few as 3 agents infected with a highly-transmissible variant have an 80% chance of developing sustained community transmission.
The implications Until high vaccine coverage is achieved, a swift implementation of policies and interventions, together with high adherence rates, will be required to minimise the probability of sustained community transmission from high-transmission variants.