May 20, 2020
Reviewed by Christopher J. Graber, M.D., MPH, FIDSA
As COVID-19 disease (infection from the SARS-CoV-2 virus) continues to spread worldwide, a greater understanding of transmission dynamics is necessary to inform strategies to control the pandemic. Contact tracing, in which public health officials determine with whom and when infected patients had contact, can inform this understanding.
In a recently published paper in JAMA Internal Medicine, investigators from Taiwan, a country that had 100 patients with laboratory-confirmed COVID-19 disease as of March 18, 2020, report results of contact tracing of these patients, yielding 2,761 close contacts. Close contacts were primarily defined as persons not wearing appropriate personal protective equipment (PPE) while having face-to-face contact with a confirmed case for more than 15 minutes or health care workers contacting a confirmed case within 2 meters without appropriate PPE (regardless of exposure time) for up to 4 days before symptom onset until date of COVID-19 disease confirmation. All close contacts were quarantined at home for 14 days following their last exposure to the index case; testing was done for close contacts with high-risk exposure or relevant symptoms, yielding 22 secondary cases that could be paired with index exposures (18 symptomatic, 4 asymptomatic).
Overall infection risk was 0.8% (95% confidence interval [CI], 0.5-1.2%) with a clinical attack rate of 0.7% (95% CI, 0.4-1.0%). All 22 cases had their first exposure within 6 days of symptom onset of their index case. Clinical attack rate was the same (0.7%, 95% CI, 0.2-2.4%) for contacts with exclusive presymptomatic exposure to index cases, though no transmissions were observed from index cases that remained asymptomatic. Clinical attack rate was approximately 5 to 6-fold higher among household and family contacts than health care contacts. Severe disease in the index case was also associated with higher transmission (risk ratio 3.76, 95% CI, 1.10-12.76).
These results of nationwide contact tracing provide important clues as to the risk of COVID-19 disease transmission. Though overall transmission numbers were low (such that the findings need to be confirmed in larger studies), they nonetheless suggest that most transmission of COVID-19 disease is either presymptomatic or early in disease course and that transmission in household/family settings is a prime area for prevention efforts.