Testing is essential to "slowing the spread"
Increasing testing capacity and ensuring rapid turnaround time (from symptoms, to testing, to test result delivery to both the patient and the health department) are key to “slowing the spread”.
Polymerase Chain Reaction (PCR) testing is the current gold standard for COVID-19. For contact tracing to be most effective, PCR testing should be widespread and available to all people showing symptoms. The CDC provides detailed recommendations for testing protocol and best practices.
There is increasing evidence of transmission from asymptomatic and pre-symptomatic cases, and it is likely that a significant proportion of patients would test positive for COVID-19 without symptoms (e.g., 20% in South Korea; 50% in Iceland).
Contact tracing will not prevent transmission from untested asymptomatic cases. Because asymptomatic testing accuracy and feasibility rates are still preliminary, even if communities do test asymptomatic contacts, negative results would not change quarantine recommendations. The CDC provides guidance on testing asymptomatic contacts of people diagnosed with COVID-19.
Serology testing for COVID-19 to identify past infection and presumed immunity is still being developed, and its role and potential use in contact tracing is still unclear. In particular, sensitivity and specificity of serology tests vary widely and must be further validated before guiding decision-making.