Isolation, contact tracing reduced SARS-CoV-2 community spread

Contact tracing and isolation helped reduce community spread of the SARS-CoV-2 infection, according to a study from China.

“These results shed further light on how SARS-CoV-2 is transmitting … and how effective control measures can be in specific contexts. We provide a key piece of evidence supporting intensive contact tracing,” said the researchers.

“[However,] the overall impact of isolation and contact tracing … is uncertain and highly dependent on the number of asymptomatic cases.”

The findings were based on analysis of 391 SARS-CoV-2 cases (mean age 45 years, 52 percent female, 94 percent symptomatic) and 1,286 of their close contacts* (identified through contact tracing) detected by the Shenzhen Center for Disease Control and Prevention, China, between January 14 and February 12, 2020. Cases were identified via symptom- or contact-based surveillance. Symptomatic patients, regardless of RT-PCR results, were isolated and treated at designated hospitals. RT-PCR-positive asymptomatic patients, close contacts, and RT-PCR-negative travellers from Hubei were quarantined.

Most cases (91 percent) presented with mild to moderate disease. At cut-off (February 22, 2020), 225 cases had recovered (median time to recovery, 21 days), while three had died.

The median time between infection and symptom onset was 4.8 days. An estimated 95 percent of patients who developed symptoms did so within 14 days. Median estimated time to recovery was 20.8 days, which differed by patient age (median 19.2 vs 22.4 days in patients aged 20–29 vs 50–59 years) and baseline disease severity (41 percent longer time to recovery in severe vs mild cases).

Confirmation of disease occurred an average 5.5 and 3.2 days after symptom onset in the symptom- and contact-based surveillance groups, respectively, while isolation occurred an average 4.6 and 2.7 days after symptom onset in the respective groups. As such, contact-based surveillance reduced time to disease confirmation by 2.3 days and time to isolation by 1.9 days. [Lancet Infect Dis 2020;doi:10.1016/S1473-3099(20)30287-5]

Compared with other close contacts of the cases, the risk of infection was elevated in household contacts (odds ratio [OR], 6.27) and those travelling with SARS-CoV-2 cases (OR, 7.06). The risk of infection was increased with more frequent contact (OR, 8.8 vs moderate-frequency contact).

The household secondary attack rate** was 11.2 percent, higher than the secondary attack rate in the overall population (6.6 percent). While attack rates were comparable across ages, they appeared higher in older adults (eg, 15.4 and 9.7 percent in those aged 60–69 and ≥70 years, respectively). The risk of infection in children was similar to that of the overall population (eg, 7.4 percent for age <10 years).

Caveat: Asymptomatic spread

Study limitations included changing definition of a confirmed case and inability to identify all contacts of a confirmed case or asymptomatic individuals.

“[L]ow transmission levels [ie, each index case caused an estimated mean 0.4 secondary cases] might in part be due to the impact of isolation and surveillance, but it is equally likely that unobserved transmission has some role,” said the researchers. It remains possible that clusters can occur despite surveillance and isolation.

“[Previous studies have suggested] that in the household, around 40 percent of transmission occurs before symptom onset, the live virus is shed for at least 1 week after symptom onset, and there is high shedding in asymptomatic individuals. Crucially, the effectiveness of case isolation and contact tracing will depend on the fraction of transmission originating from asymptomatic and pre-symptomatic individuals,” agreed Drs Kaiyuan Sun and Cécile Viboud from the Fogarty International Center, National Institutes of Health, Bethesda, Maryland, US, in a commentary. [Lancet Infect Dis 2020;doi:10.1016/S1473-3099(20)30357-1]

Effective control of SARS-CoV-2 is dependent on other factors, they added. “Successful strategies include ample testing and contact tracing, supplemented by moderate forms of social distancing.” Additionally, the strict control measures enacted in Shenzhen could have led to the low transmission rate in this study, particularly among household contacts, said Sun and Viboud.