08 Nov 2021
Work package 1: TTIQ
Australia’s test, trace, isolate, quarantine (TTIQ) approach has proven very effective at preventing transmission. As we transition to a new phase of the pandemic with high vaccination coverage but fewer restrictions, our TTIQ systems may need to adapt to higher caseloads than previously.
We consulted with TTIQ experts involved in the states and territories to identify the most important questions to answer to help plan how to adapt TTIQ, and used various modelling approaches to assess the impact on transmission potential of changing the management of cases and contacts in a highly vaccinated population. These questions mostly focussed on contact tracing and testing of symptomatic people and options for differential management of vaccinated individuals presenting either as index cases or contacts.
- For people identified as close contacts of cases, testing on Day 1 of quarantine ensures that downstream infections can be rapidly identified and isolated. For a range of explored strategies, the Day 1 test was found to always be the most important test to prioritise.
- Case-initiated contact tracing supports timely quarantine in times of system stress. Case-initiated contact tracing involves instructing confirmed cases to self-identify their primary close contacts and ask them to get tested and quarantine.
- When the TTIQ system is under stress, it may no longer be possible for public health units to complete all case interviews on the same day as case notification. Prioritising the most recently tested cases for contact tracing (rather than the ones that have been in the queue the longest) increases the impact of the TTIQ response.
- Reducing the quarantine period for vaccinated individuals from 14 days to 7 days is estimated to have no discernible impact on the performance of the TTIQ system.
- Completely removing isolation and/or quarantine for vaccinated individuals is estimated to increase transmission potential by 3–5%.
- To understand the epidemiological situation in each state and territory and be able respond effectively, we need to monitor the impact of TTIQ over time. To measure the overall impact of TTIQ on transmission, we need to estimate both the reduction in contagiousness of detection infections (via quarantine and isolation) and the fraction of all infections detected.
Out of scope:
- Our modelling only considered the effect of different strategies on TP – not on workforce planning or other reasons for testing.
- We considered cases detected in the community via testing of symptomatic individuals and via testing of close contacts identified through contact tracing. Other means of enhanced case finding such as routine asymptomatic screening of individuals in high-risk settings are not included in our analyses. Such strategies, focused on workforce continuity, have the potential to further reduce transmission by increasing the overall proportion of infections detected.
- All testing is assumed to be standard PCR testing. We did not consider the impact of potential TTIQ strategies which incorporate rapid antigen self-tests (though see work package 2 for rapid antigen self-tests in a school setting).
- We focused on quantifying the impact of individual alternate strategies on TP and not combinations of strategies. Analyses of the impacts of specific combinations of strategies will help to further inform TTIQ program implementation.
- We assumed, based on recent survey data, that a person with COVID-19 symptoms has a 50% chance of seeking a test - whether or not they are vaccinated. This probability may change as people learn to live with the virus and may be different for vaccinated and unvaccinated individuals.
- We assumed fixed delays for each component of the TTIQ system (e.g. times from symptom onset to getting tested, test turnaround times, etc.) based on case data from NSW and VIC. In reality, these delays will vary over time as a result of changing caseloads and component effectiveness.
Frequently Asked Questions:
What is transmission potential?
Transmission potential or TP is the population average ability of the virus to spread (whether or not it is present). See Australia’s national surveillance plan for details: https://www.health.gov.au/sites/default/files/documents/2021/04/australian-national-disease-surveillance-plan-for-covid-19.pdf
TP is estimated from time-series data on population behaviours and cases (when the virus is circulating). When TP is above 1, conditions are suitable for the virus to spread in the general population. When TP is below 1, conditions are not suitable to sustain transmission in the general population.
The national strategy is to target a TP at or around 1, which will constrain community transmission of the virus, maintaining the capacity of our healthcare system. We therefore evaluate TTIQ strategies by estimating the percentage change in TP.
How influential are the estimated changes in transmission potential between TTIQ strategies likely to be?
The impact of even minor changes in TP on the epidemic depends critically on how close TP is to the national strategic objective of maintaining a control threshold of 1. If TP at the population level is very close to 1, even a small increase in TP (by just a few percent) can drive a change from decreasing to increasing epidemic activity. The consequences of increasing epidemic activity will depend on caseloads at the time. Alternative TTIQ strategies that lead to an increase in the TP needs to be considered carefully as they may (ultimately) prompt the need for other measures, including the requirement for increased public health and social measures.
Vaccinated people are less likely to display symptoms if infected. Will it become more difficult to identify infections with increasing vaccination coverage?
TTIQ is limited by the ability to detect infections — people with undetected infections are not placed into isolation and so both they and their infected contacts will continue to contribute to transmission. Much of our ability to detect infections in the community relies on people coming forward for testing when they develop symptoms. Since COVID-19 vaccines reduce the fraction of infections displaying symptoms, we expect lower levels of detection with increasing vaccination coverage, for similar surveillance effort.
What is a primary close contact?
A person who has had close contact with a confirmed case of COVID-19 and therefore may have been infected but is not (yet) a confirmed case.
What is passive detection?
In our analyses, we consider passive detection to be the detection of cases in the community via testing of individuals who develop symptoms and seek a test through their own initiative.
What is active detection?
In our analyses, we consider active detection to be the detection of cases via testing of close contacts identified through contact tracing.
What is downstream contact tracing?
The process of identifying and managing individuals who had contact with a confirmed case during the time in which the case was likely to be contagious.
What is upstream contact tracing?
For cases with no known source of infection, identifying individuals who had contact with a confirmed case during the time in which the case was likely to have acquired their infection. The aim is to identify the source of their infection and therefore find previously undetected cases and/or chains of transmission.
We do not include upstream contact tracing in our simulations.
What is isolation?
Isolation separates people who are suspected (e.g. awaiting a test result) or confirmed cases of a disease from others for the duration of the contagious period.
What is quarantine?
Quarantine separates people who are contacts of confirmed cases of a disease for the duration of the incubation period. If a contact tests positive for the disease, they will then be directed to isolate.
Note that our findings on quarantine strategies are specific to the quarantining of primary close contacts of confirmed cases, not of overseas or interstate travellers.