A modelling study

22 March 2021

Executive summary

  • Vaccination of New Zealand’s frontline border workforce is a priority in order to protect this high-exposure group from the health impacts of COVID-19.
  • 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 or reduce symptoms of COVID-19 but not prevent transmission. Counterintuitively, this means that vaccinating frontline border workers could increase the risk of a community outbreak.
  • In a scenario where the vaccine reduces transmission by 50%, vaccinating border workers could increase the risk of a significant community outbreak from around 7% per seed case to around 9% per seed case.
  • Until more is known about the effect of the vaccine on transmission, we recommend increasing the routine testing of vaccinated border workers to mitigate this risk. Regular saliva testing may be a good way to achieve this.
  • Careful attention should be paid to any groups, such as frontline workers’ family members, who may be vaccinated but who are not undergoing routine testing to ensure they do not become asymptomatic spreaders.

Abstract

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. Here, we 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. We 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. We 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.