Delaying Contact Tracing Can Have Serious Ramifications

Contact tracing is one of the best tools we have in the public health arsenal against infectious diseases and outbreaks. This cornerstone of outbreak response is one that requires resources and adequate testing to ensure measures can begin rapidly. A backlog in testing, which is what we’re seeing in the United States, can severely impact the ability to perform contact tracing, let alone have it be effective.

In some states, COVID-19 testing is so backlogged that patients might not be notified of their results for 7-10 days. If we look at the lower end of this range, imagine a patient becomes sick and opts to be tested 2 days into their illness.

If it takes 7 days to learn the results and then a handful more to report it to the public health authorities and arrange contact tracing, this could easily put the date of such efforts either beyond or exceedingly close to the end of the incubation period. In essence, the effectiveness of contact tracing is wholly reliant on adequate timing and testing.

A new study in the Lancet Public Health sought to evaluate the impact of these delays on the effectiveness of contact tracing efforts. Ultimately what it showed was the for each day delayed by testing, that left even less time for contact tracing. The authors noted that “A testing delay of more than 1 day requires the tracing delay to be at most 1 day or tracing coverage to be at least 80% to keep R_CTS below 1.

With a testing delay of 3 days or longer, even the most efficient strategy cannot reach R_CTS values below 1. The effect of minimising tracing delay (eg, with app-based technology) declines with decreasing coverage of app use, but app-based tracing alone remains more effective than conventional tracing alone even with 20% coverage, reducing the reproduction number by 17·6% compared with 2·5%.”

Reviewing conventional and mobile app efforts, they opted not to consider hybrid approach. Moreover, the authors noted the realities that a 100% testing and testing coverage is extremely difficult to achieve, so the best-case scenario was defined at 80% testing and tracing coverage.

Modeling the impact of delays was performed at a population level using a model developed by Kretzschmar and colleagues that was modified for SARS-CoV-2. They assumed that without physical distancing, most individuals would have four close contacts and nine casual contacts per day. Symptomatic and asymptomatic cases were treated as having the same level of infectiousness.

The authors emphasized that minimizing those delays due to testing had perhaps the largest impact on reducing transmission — preventing those onward transmission per index cases can be up to 79% effective if there’s no testing delays. A 3-day testing delay then brings down this efficacy to 41.7% and as low as 4.9% when there’s a 7-day testing delay. They also noted that for interventions without contact tracing, physical distancing and isolation were extremely helpful in reducing secondary cases (reproductive number).

Overall, this study couldn’t be more relevant to the extreme testing challenges we’re facing in the United States. Not only does this contribute to frustrations to the patient and potential breaches in isolation, but the delay in contact tracing means that our ability to reduce transmission is severely impacted. Speedy testing and thus contact tracing are critical to outbreak control.