In an attempt to maintain the elimination of Covid-19 in Aotearoa New Zealand, all international arrivals are required to spend 14 days in government-managed quarantine and to return a negative test result before being released. We model the testing, isolation and transmission of Covid-19 within quarantine facilities to estimate the risk of community outbreaks being seeded at the border. We use a simple branching process model for Covid-19 transmission that includes a time-dependent probability of a false-negative test result.

We show that the combination of 14-day quarantine with two tests is highly effective in preventing an infectious case entering the community, provided there is no transmission within quarantine facilities. Shorter quarantine periods, or reliance on testing only with no quarantine, substantially increases the risk of an infectious case being released. We calculate the fraction of cases detected in the second week of their two-week stay and show that this may be a useful indicator of the likelihood of transmission occurring within quarantine facilities. Frontline staff working at the border risk exposure to infected individuals and this has the potential to lead to a community outbreak. We use the model to test surveillance strategies and evaluate the likely size of the outbreak at the time it is first detected. We conclude with some recommendations for managing the risk of potential future outbreaks originating from the border.

Executive summary

  • Weekly testing of all workers who have direct or indirect contact with border arrivals and quarantine residents will greatly reduce the chance of another outbreak.
  • These tests should be conducted by a health professional with knowledge of Covid-19 symptoms to reduce false negatives and help diagnose probable cases.
  • If the first detected case of Covid-19 is in a frontline worker, there is a high probability that the outbreak is still very small and can be contained by fast case isolation and contact tracing.
  • If the first detected case of Covid-19 is in an individual without a direct link to arrivals or quarantine facilities, then an immediate local lockdown may be necessary to contain the outbreak.
  • If the first detected case of Covid-19 is in an individual who is a household contact of a frontline worker and does not themselves have direct contact with the quarantine process, this should be treated in the same way as community transmission and strong community-wide social distancing measures, possible a regional return to a high alert level, should be considered.
  • This applies even if the first detected community cases is a household contact of a frontline worker because it means there is a high risk the frontline worker has already infected others.
  • Well-managed 14-day quarantine, with minimal interactions between travellers beyond family groups, and tests on day 3 and day 12 provides a very good safeguard against infected travellers initiating community outbreaks.

An earlier version of this paper was leaked after being supplied confidentially to government and was reported on at https://www.stuff.co.nz/national/health/coronavirus/300075002/leaked-covid19-report-warns-border-isolation-and-quarantine-staff-carry-the-highest-risk-of-causing-an-outbreak.