New modelling by Covid-19 Modelling Aotearoa suggests that testing with Rapid Antigen Tests before ending isolation for confirmed cases of Covid-19 would significantly decrease the risk of onward transmission, while allowing many people to isolate for shorter periods.

Executive summary

  • Isolating confirmed cases while infectious is an important way of reducing transmission of infectious diseases. Because the infectious period of most diseases varies between people, matching this period to an ideal isolation period is difficult. Doing so requires balancing the impact that isolation has on individuals and the community with the risk of avoidable onward infections if people are released from isolation while still infectious. Rapid antigen tests (RATs) have been suggested as a way to help identify cases who are still infectious and to better target isolation requirements. We extend the approach used by the UKHSA to investigate the impact of different isolation periods and test-to-release conditions for SARS-CoV-2.
  • Our results suggest that introducing a test-to-release criterion, in conjunction with minimum and maximum isolation lengths, offers the opportunity to appreciably reduce the risk of onward transmission, while resulting in only minor increases in the average time spent in isolation. Adding a one (or two) test-to-release policy to Aotearoa New Zealand’s current 7 day isolation policy (with a maximum isolation period of 10 days), is expected to lead to a reduction in the number of cases still infectious after release of 40% (or 60%), while the average isolation period will increase by only by 0.3 (or 0.6) days.
  • Alternatively, for a scenario with a minimum isolation period of only 5 days, but using a two test-to-release policy with a maximum isolation period of 10 days, results in an expected decrease in risk, relative to the current policy, as well as a decrease in the overall time spent in isolation for confirmed cases. This policy results in a 40% reduction in the number of cases infectious at release and hours infectious post-release. It is also expected to deliver a 8% decrease in the total hours spent in isolation for confirmed cases, but a 20% decrease in the total number of excess hours spent in isolation by cases that are no longer infectious.
  • The results above use a conservative 75% value for the assumed sensitivity of rapid antigen tests, and a short value for the assumed duration of mean infectious period. If the simulations are repeated with parameter values that use a higher rapid antigen test sensitivity and a longer mean infectious period — in line with recent literature — then we find that the benefits of test-to-release as part of an isolation regime are even greater.