Our conceptual framework distinguishes between targeted and generalized interventions. Targeted interventions are interventions that are applied to specifically identified individuals in a population, typically based on infection or exposure status. Generalized interventions are behavioral or environmental interventions that are adopted broadly within a population. Targeted interventions are follow one of two strategies: targeting infected or uninfected individuals.
Targets infected people to limit transmission risk
Each approach in this strategy represents an escalation of intervention.
It is assumed that identified cases are isolated and that onward transmission is eliminated or greatly reduced upon isolation. Active case finding contrasts with passive case finding, which we define as the detection of cases among symptomatic patients who present to medical services for diagnosis of symptoms and receive a test only after meeting some criteria.
Contact tracing increases awareness among the subset of the population most likely to develop symptoms, decreases transmission from traced contacts who are encouraged to isolate, and increases the rate of case finding in the population. Prior to the 2020 COVID-19 pandemic, contact tracing had never been attempted at the scale that would be required to be effective in suppressing SARS-CoV-2.
Quarantine represents an escalation of intervention severity that amplifies the impact of contact tracing. The major effect of this approach is that it reduces the dependence on finding secondary cases (because secondary cases are already identified as contacts) and reduces or eliminates onward transmission from these cases (because the case is already in isolation when symptoms begin). Another effect is that it reduces the average contact rate within the population. Effectively, the portion of the population that is in quarantine is engaged in intensive social distancing, which can be thought of as a “partial lockdown” that is tunable based on the intensity of contact tracing.
Targeting healthy people to limit exposure
We note that these strategies have different political, philosophical, ethical and behavioral implications. For instance, Strategy 1 may disincentivize care-seeking because receiving a positive test could preclude one from working whereas Strategy 2 may incentivize care-seeking because a negative diagnostic test or positive antibody test is required to work. Similarly, Strategy 1 prioritizes a right to work whereas Strategy 2 prioritizes a duty to protect. In addition, Strategy 1 and Strategy 2 approaches could be combined. But, because they are structurally different, we do not consider such combinations here.
Aimed at reducing transmission or exposure broadly in a population. May be used in combination with targeted interventions
May be added to either targeted strategy without modifying the topology of the flow diagram.
Any of the preceding strategies may suppress transmission, but that suppression depends on achieving a certain level of effectiveness (reduction in transmission among isolated persons, intensity of contact tracing, frequency of certification, etc.) that varies according to the strategy.
Interventions targeting infected individuals (active case finding, contact tracing, quarantine) are expected to work only when case ascertainment is high.
Intervention targeting uninfected individuals (certification) is only expected to work in a narrow range of conditions (i.e. high frequency testing).
Regardless of the strategy adopted, a large testing capacity is required and success will depend on the effectiveness of generalized interventions.
Additionally, generalized interventions function as a “force multiplier.” In most realistic scenarios, generalized interventions will be essential to achieve suppression.
Below, we present conceptual models devised to be realistic for SARS-CoV-2, but they are not fit to data from any particular population. We studied the dynamics of active case finding, contact tracing, quarantine, and certification individually and in combination with generalized interventions after a “first wave” that infects a small fraction of the population. For comparison, we also consider the two limiting cases of maintaining intensive social distancing and doing nothing. The models are parameterized for a population of 10 million people, slightly larger than London (8.9 million) and New York City (8.3 million) and slightly smaller than the US state of Georgia (10.6 million), but they may be parameterized for a population of any size.
1. Active case finding 2. Contact tracing 3. Quarantine