Contact tracing data from the first pandemic wave of COVID-19 in Shandong Province, China

In a recent study published in the latest issue of Epidemicsresearchers assessed the transmission risks of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in different contact settings to prioritize disease control by potential super-spreaders and their contacts.

Study: Variability in risk of transmission of SARS-CoV-2 in close contacts: a contact tracing study in Shandong Province, China. Image Credit: elenabsl/Shutterstock


Contact tracing is an excellent tool for assessing the relative transmissibility of SARS-CoV-2 in different settings, including household, healthcare, and air travel. When combined with an accurate assessment of the heterogeneity of SARS-CoV-2 infectivity for different durations of exposure at the individual level, it could early predict the possibility of super-spreading events to design a strategy appropriate intervention.

study design

In the present study, researchers applied an individual Bayesian transmission model to contact tracing data from the first pandemic wave of coronavirus disease 2019 (COVID-19) in Shandong Province, China. They estimated secondary attack rates (SAR) in different contact settings and assessed potential risk factors for infection and transmission. The SAR reflects the transmissibility of a pathogen by measuring its susceptibility to contracting a viral infection, including COVID-19 from an infectious person through close contact.

They collected demographic, clinical and laboratory data on symptomatic and asymptomatic SARS-CoV-2 infections (index cases) from municipal centers for disease control and prevention (CDC) in the cities of Jinan, Jining and Qingdao in the province. from Shandong, China. They retrospectively retrieved the study’s surveillance database between January 22n/a and May 30e, 2020.

During the study period, they identified reverse transcription-polymerase chain reaction (RT-PCR) confirmed COVID-19 cases and traced their close contacts who were quarantined for 14 days. Additionally, the team collected their nasopharyngeal swab specimens on days 1, 4, 7, and 14 for RT-PCR testing.

According to the case definitions of the study, close contacts were people who had unprotected contact within one meter with a suspected or confirmed case of SARS-CoV-2 in the two days before the onset of the symptoms. symptoms or, if asymptomatic, the date of collection of the first RT-PCR positive sample. A group of close contacts may have several primary cases, called co-primary cases.

First, the researchers calculated a raw SAR, the average of secondary cases among close contacts across all close contact groups with a single primary case. Assuming that all secondary cases were infected by their primary case, they called it the data-driven SAR. However, not all secondary cases of COVID-19 were infected by a single primary case, as there could also be tertiary transmissions, or the infection could be acquired from an outside group. Fortunately, the Bayesian model accounted for and removed this discrepancy while predicting daily transmission dynamics.

Covariates in the study included each close contact’s age, sex, city and occupation, as well as each case’s severity of illness and symptom status during incubation and illness. The main results of the study considered an average incubation period of five days and a maximum infectious period of 21 days.

Study results

Among the 97 laboratory-confirmed index cases included in the transmission modeling analysis, there were eight clusters with a single primary case and two co-primary cases; in addition, there were 3158 close contacts in the remaining 81 clusters. The average size of these 89 close contact groups was 23. Overall, on average, one primary case generated 1.05 secondary cases in Shandong Province.

The female to male ratio among primary and secondary cases was 65% versus 35% and 59% versus 41%, respectively. Compared to secondary cases, primary cases were more likely to be severe (14% versus 5%) and less likely to be asymptomatic (6% versus 10%).

The overall DAS based on the data was 3.53%; moreover, it was highest (8.64%) among close contacts over the age of 60. Of the contact parameters examined during the study, household was associated with the highest data-based DAS of 10.1%, while air travel was associated with the lowest DAS of 0.43% .

The estimated daily transmission probability of infected persons of transmitting the infection to close contacts during their incubation period was 0.044 within households, 0.032 within health facilities, 0.023 within workplaces, 0.004 during air travel and 0.002 in all other settings. It was slightly lower during the disease period, and the estimated relative infectivity was highest when incubation and infectious period were longer.

In addition, multivariate analysis revealed that:

i) close contacts under the age of 60 had a 36-49% lower risk of infection than those over the age of 60;

ii) healthcare professionals were 65% less likely to be infected than non-medical contacts;

iii) the risk of infection was much lower during air travel, odds ratio (OR) 0.08 compared to within households; and

iv) the risk of infection in workplaces and health facilities was slightly lower, with ORs of 0.52 and 0.73, respectively.

The study model predicted the possibility of super-spreading events in two households and one healthcare facility (three groups of close contacts), each with a single primary case.


In summary, the estimated household SARs in Shandong, calculated for the entire 22-day infectious period, were higher than most household transmission studies in China. SARS-CoV-2 was more transmissible in homes than in workplaces and healthcare settings, and the risk of transmission was much lower during air travel or other contact settings. In addition to the highest SAR, household metrics favored the highest number of secondary cases per primary case. Additionally, age, medical profession, and city were risk modifiers.

Simulation of transmission dynamics among close contacts of primary cases, assuming an average incubation of five days and a maximum infectious period of 22 days, showed that 64% of cases did not generate secondary transmissions and that 20 % of cases resulted in 80% secondary transmissions.

In conclusion, the study emphasizes that vaccination and non-pharmaceutical interventions should be prioritized for large groups of close contacts in households. While the risk of transmission was lower during air travel, the consideration of viral exposure in such a large number of air passengers and the implication of long-distance dissemination also warrant prevention efforts in this area. context.

Comments are closed.