Quarantine and testing strategies in contact tracing for SARS-CoV-2: a modelling study

Abstract

Background In most countries, contacts of confirmed COVID-19 cases are asked to quarantine for 14 days after exposure to limit asymptomatic onward transmission. While theoretically effective, this policy places a substantial social and economic burden on both the individual and wider society, which might result in low adherence and reduced policy effectiveness. We aimed to assess the merit of testing contacts to avert onward transmission and to replace or reduce the length of quarantine for uninfected contacts. MethodsWe used an agent-based model to simulate the viral load dynamics of exposed contacts, and their potential for onward transmission in different quarantine and testing strategies. We compared the performance of quarantines of differing durations, testing with either PCR or lateral flow antigen (LFA) tests at the end of quarantine, and daily LFA testing without quarantine, against the current 14-day quarantine strategy. We also investigated the effect of contact tracing delays and adherence to both quarantine and self-isolation on the effectiveness of each strategy.Findings Assuming moderate levels of adherence to quarantine and self-isolation, self-isolation on symptom onset alone can prevent 35% (95% uncertainty interval [UI] 10-59) of onward transmission potential from secondary cases. 14 days of post-exposure quarantine reduces transmission by 48% (95% UI 18-79). Quarantine with release after a negative PCR test 7 days after exposure might avert a similar proportion (50%, 95% UI 23-80; risk ratio [RR] 1•02, 95% UI 0•88-1•41) to that of the 14-day quarantine period, as would quarantine with a negative LFA test 7 days after exposure (49%, 95% UI 20-78; RR 1•00, 0•82-1•28) or daily LFA testing without quarantine for 5 days after tracing (50%, 95% UI 24-79; RR 1•04, 0•69-1•79) if all tests are returned negative. A stronger effect might be possible if individuals isolate more strictly after a positive test and if contacts can be notified faster.Interpretation Testing might allow for a substantial reduction in the length of, or replacement of, quarantine in the control of onwards transmission from contacts of SARS-CoV-2-infected individuals. Decreasing test and trace delays and increasing adherence will further increase the effectiveness of these strategies. Further research is required to empirically evaluate the potential costs (increased transmission risk, false reassurance) and benefits (reduction in the burden of quarantine, increased adherence) of such strategies before adoption as policy.

High household transmission of SARS-CoV-2 in the United States: living density, viral load, and disproportionate impact on communities of color

“…Thus, public health messages to wear masks and self-isolate at onset of symptoms, while prudent, are unlikely to eliminate household spread, even if they were feasible in all households. Early and frequent testing, combined with agents for post-exposure prophylaxis, would be needed to substantially mitigate the impact of the virus on families that have been inoculated and not yet vaccinated [27]. Otherwise, mask wearing within a household at all times is preferable in households with unvaccinated members who are vulnerable to severe COVID-19.…”

“…However, compliance is difficult, especially for families with young children, those with limited resources, and those unable to work from home. If the quarantine period is decreased, the risk of onward transmission is increased, but the size of this risk remains an active subject of investigation [20,27]. One approach has been to reset the ‘quarantine clock’ for the entire household by 14 days each time a new household member is diagnosed, but this has further increased the burden and decreased compliance.…”

Time variation in the probability of failing to detect a case of polymerase chain reaction testing for SARS-CoV-2 as estimated from a viral dynamics model

“…PCR tests have been extensively used in SARS-CoV-2 research because of their high sensitivity and specificity compared with other tests such as antibody and antigen tests. However, this does not undermine the value of other tests, and appropriate tests should be chosen depending not only on their sensitivity and specificity but also on the purposes of testing and the cost [25][26][27]. For example, frequency has been suggested to be more important than sensitivity for screening purposes [27].…”

The importance of timely contact tracing — A simulation study

“…The probability that an agent infects their close contacts at time t is given by the maximum secondary attack rate of 0.35 multiplied by the relative infectiousness at time t. The detection probability can be thought of as the sensitivity of diagnostic testing. The detection probability by LFA testing is assumed to depend on the relative infectiousness (Quilty et al, 2021). Although the true detection probability may remain high even after the agent is no longer infectious (Wölfel et al, 2020), this assumption does not significantly alter our results as we are mainly interested in case detection during the infectious phase.…”

“…Although LFA is less sensitive than PCR, this is not so important if the goal is to detect actively infectious patients (Mina et al, 2020). While LFA testing is inexpensive and quick, its effect in reducing transmission may be comparable to a 14-day quarantine, when used as daily serial testing accompanied by isolation of positive individuals Q9 (Quilty et al, 2021). LFA testing is a critical component in the response against COVID-19 pandemic especially in resource-limited settings (Boum et al, 2021).…”