Five approaches to the suppression of SARS‑CoV‑2
without intensive social distancing

Here we investigate the potential effectiveness of five non-pharmaceutical approaches to suppression of SARS-CoV-2 without intensive social distancing measures such as school and workplace closures, shelter-in-place orders, and prohibitions on the gathering of people. We developed models for these five approaches to illustrate the similarities and differences among them and help to identify their distinctive strengths and weaknesses. We find that targeted approaches aimed at either infected individuals (widespread testing, tracing, quarantine) or uninfected individuals (certification) can be effective, but that these will have to be used in combination with generalized interventions such as mask wearing and limiting gatherings.

Interventions

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.

Targeting infected persons

Targets infected people to limit transmission risk

Each approach in this strategy represents an escalation of intervention.

  1. Active case finding. All efforts that actively seek to identify cases. Equivalent to widespread testing.
  • Testing health care workers and others with high occupational exposures
  • Testing contacts of cases
  • Adopting minimally exclusive testing criteria

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.

  1. Contact tracing. Identification, communication with, and monitoring of possible exposures of known cases.
  • Interviewing cases or family members of cases
  • Technological aids like cell phone apps

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.

  1. Quarantine. Isolating traced contacts to the same degree that known cases are isolated.

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 uninfected persons

Targeting healthy people to limit exposure

  1. Certification. Certification is an approach that relaxes social distancing in stages. Under this approach, individuals are certified to be infection free before returning to daily routines such as school, work, and shopping. Certification can be durable (valid for an extended period of time, for instance based on an antibody test) or temporary (valid for a short period of time, for instance because one has recently tested negative by RNA test). Durable certification doesn’t lead to a reduction in transmission, but may be essential for the provision of essential goods and services during periods of high transmission, as conceived by the “shield immunity” concept of Weitz et al.1

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.

Generalized interventions

Aimed at reducing transmission or exposure broadly in a population. May be used in combination with targeted interventions

  1. Generalized interventions. Behavioral or environmental interventions that are adopted broadly within a population.
  • wearing face masks
  • improved hand hygiene
  • improved cleaning and disinfection of surfaces
  • greater provision of sick leave
  • increased enforcement of school/workplace guidelines for staying home when sick
  • contactless transactions
  • use of infection barriers in stores, restaurants, and waiting areas
  • distribution of hand sanitizer in public places
  • behavioral change
  • use of personal rather than public transport
  • micro-social-distancing (e.g. limiting physical contact, queue spacing)
  • public policies that limit aggregations of people (number of people allowed in stores, large events.)

May be added to either targeted strategy without modifying the topology of the flow diagram.

Summary of findings

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.

Models

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.

Questions

  1. How much might generalized interventions (without targeted interventions) reduce the total outbreak size compared with reference scenarios?
  2. When are contact tracing and quarantine most beneficial?
  3. What benefit does quarantine add to contact tracing?
  4. When can certification be effective?
  5. How does the extent of presymptomatic transmission affect the choice of intervention strategy?

Model for Strategy targeting infected persons

Interventions:
1. Active case finding 2. Contact tracing 3. Quarantine