Contact tracing is particularly challenging for COVID-19 because transmission often occurs before symptoms appear and some individuals can transmit who never develop symptoms at all.īefore the widespread deployment of vaccination, symptomatic testing followed by contact tracing, set in the context of ongoing widespread distancing measures, had been the primary means of controlling COVID-19 in many jurisdictions in Europe, the UK and North America. Several delays in the process make rapid contact tracing challenging: the time to develop symptoms, to seek a test, to get test results, and for contact tracing teams to reach contacts. If contact tracing teams reach an individual’s contacts only toward the end of their infectious period, very few further infections will be prevented. The effectiveness of contact tracing for any infectious disease is limited by how quickly contacts can be informed. These considerations are important both in regions where widespread vaccination has been deployed, and in those where still few residents have been immunized. Only by ramping up testing of asymptomatic individuals can we avoid the inherent delays that limit the efficacy of contact tracing. We propose embracing approaches to COVID-19 control that broadly test individuals without symptoms, in whatever way is economically feasible – either with fast cheap tests that can be deployed widely, with pooled testing, or with screening of judiciously chosen groups of high-risk individuals. Even when used effectively with other measures, occasional bursts in call loads can overwhelm contact tracing systems and lead to a loss of control. Without other measures in place, contact tracing alone is insufficient to prevent exponential growth in the number of cases in a population with little immunity. Even if delays are minimized, contact tracing can only prevent a fraction of onward transmissions from contacts. Contact tracing is made ineffective, however, by delays in testing, calling, and isolating. Contact tracing has played a central role in COVID-19 control in many jurisdictions and is often used in conjunction with other measures such as travel restrictions and social distancing mandates.
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