Long, thin transmission chains of Severe Acute Respiratory Syndrome Coronavirus (SARS-CoV-2) may go undetected for several weeks at low to moderate reproductive numbers: Implications for containment and elimination strategy

Killeen, G. F.

Especially at low to moderate reproductive numbers, the generally mild, non-specific symptomology of Severe Acute Respiratory Syndrome Coronavirus (SARS-CoV-2) allows long, thin transmission chains to go undetected by passive surveillance over several weeks. This phenomenon has important implications: (1) Surveillance becomes less sensitive and reliable as an indicator of freedom from infection at the low reproductive numbers required to achieve elimination end points, passive surveillance systems may need to document an absence of new cases for at least a month to establish certainty of elimination. (2) Reproductive numbers should be kept as low as possible throughout such follow up periods without confirmed cases, to ensure such long, thin, undetected transmission chains all collapse before restrictions are eased and reproduction numbers are allowed to rebound. (3) While contact tracing systems may be highly effective when applied to large clusters in foci of elevated transmission where wide, rapidly expanding transmission chains are detected within two viral generations, large fractions of community transmission occurring through thinner, more extended transmission chains at lower reproductive numbers are often be too long to trace retrospectively and will be underrepresented in surveillance data. (4) Wherever surveillance systems are weak and/or younger age groups with lower rates of overt symptoms dominate transmission, containment effectiveness of contact tracing and isolation may be more severely limited, even at the higher reproduction numbers associated with larger outbreaks. While, contact tracing and isolation will remain vital for at least partially containing larger outbreaks, containment and elimination of SARS-CoV-2 will have to rely primarily upon the more burdensome and presumptive population-wide prevention measures that have proven so effective thus far against community transmission. Furthermore, these will have to be sustained at a much more stringent level and for longer periods after the last detected case than was necessary for SARS-CoV-1.