The Test, Trace and Isolate (TTI) strategy is now central to the UK Government’s approach to managing Covid-19. Through TTI, the Department of Health and Social Care (DHSC) has access to a rich source of information that – if properly utilised – has the potential to improve dramatically our understanding of how to reduce transmission of the SARS-CoV-2 virus that causes Covid-19 disease.
Through TTI, those who have been in close contact with people who have tested positive for the virus – and are therefore at high risk of contracting it themselves – are traced and required to isolate for fourteen days. There are two types of high-risk contact:
- Household-members, who have been in isolation since the onset of symptoms in an index case;
- External close contacts, identified by a symptomatic case, who are traced and asked to self-isolate.
Each week, TTI quarantines over 4,000 individuals because they live in the same household as an index case and up to 40,000 who are external close contacts of symptomatic index cases. This large number of people isolating means that for TTI to learn about transmission of the virus to these two high-risk groups (including asymptomatic infections) in a cost-effective way requires the efficient application of two statistical methods: record-linkage and random sampling. This would also capture information on adherence to quarantine, which can be useful for evaluating policy impact.
Here, record-linkage just means that TTI checks its own records to find out how many of the quarantined persons in each of high-risk group tested positive for the virus during (or soon after) their quarantine period. Random sampling is a fair method of selecting representative sub-samples, such as of quarantined high-risk households or external close contacts.
Our Covid-19 Task Force has issued a statement (PDF) with three recommendations for how these statistical methods should be deployed in respect of high-risk contacts who are tracked through the TTI process.
- Use record-linkage within TTI to establish the proportion of high-risk individuals who tested positive for the virus during or soon after the end of their quarantine period.
TTI holds demographic information about individuals in the two groups described above who are at high risk of contracting the disease. TTI also holds demographic information about persons who tested positive for the virus. TTI should therefore cross-link the two to determine what proportion of high-risk individuals tested positive for the virus within their quarantine period or a few days thereafter.
- Assess the level of infection, with or without prior symptoms, among those who are self-isolating.
Testing everybody who has been in close contact with someone who has tested positive for the virus is not efficient when prevalence estimates can be more cost-effective by visiting a random sub-sample. For example, by visiting a 50% random sample, or 2,000 high-risk households, on randomly-selected days during the household’s quarantine and offering a swab-test to household-members. A different random-sampling scheme can be used to select a quarter of the quarantined external close contacts to be offered a swab-test on random day.
Through efficient random sampling, we can build an understanding of the prevalence of asymptomatic cases as well as the likelihood of the virus being transmitted both within a household and to external close contacts.
- Monitor adherence to the instruction to stay at home.
To gauge the effectiveness of TTI and to understand if people need more support to isolate, it is essential to understand the level of adherence to the instruction to stay at home.
Through the random sampling of households and external close contacts, the TTI programme would also learn about people’s adherence to the stay at home instruction: by their being at home (or not) on their assigned day for being offered a test. This would be intended to inform policy-makers and the public about adherence to the instruction to isolate and to help determine if more support needs to be offered to enable people to isolate – it is not intended to direct enforcement agencies to individuals who have not adhered to the instruction to isolate.
Find out more about our Covid-19 Task Force.