Australia and New Zealand have a strategy to eliminate community transmission of COVID-19 and require overseas arrivals to quarantine in government-managed facilities at the border. In both countries, community outbreaks of COVID-19 have been sparked following infection of a border worker. This workforce is rightly being prioritised for vaccination. However, although vaccines are highly effective in preventing disease, their effectiveness in preventing transmission of COVID-19 is less certain. There is a danger that vaccination could prevent symptoms of COVID-19 but not prevent transmission.
In this article, the authors use a stochastic model of COVID-19 transmission and testing to investigate the effect that vaccination of border workers has on the risk of an outbreak in an unvaccinated community. They simulate the model starting with a single infected border worker and measure the number of people who are infected before the first case is detected by testing.
Findings show that if a vaccine reduces transmission by 50%, vaccination of border workers increases the risk of a major outbreak from around 7% per seed case to around 9% per seed case. The lower the vaccine effectiveness against transmission, the higher the risk. The increase in risk as a result of vaccination can be mitigated by increasing the frequency of routine testing for high-exposure vaccinated groups.