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Household Secondary Attack Rate of COVID-19 and Associated Determinants

Our take —

This contact tracing study, published as a preprint and thus not yet peer-reviewed, in Guangzhou, China found that the infectivity of the virus was the same between the incubation period and the symptomatic periods. Attack rates (secondary and tertiary) among those in the household was nearly 13%, decreasing to 3% among non-household contacts; attack rates among susceptible persons increased by age of the susceptible contacts. Quantification of infectivity from asymptomatic cases was a limitation.

Study design


Study population and setting

Researchers in Guangzhou, China used contact tracing data reported to the Guangzhou Municipal Centers for Disease Control as of February 17, 2020. This included 349 confirmed COVID-19 cases that formed 195 clusters with 212 index cases and 137 secondary or tertiary cases. The sample also included 1938 uninfected contacts from these cases.

Summary of Main Findings

Investigators found that most of the primary and non-primary cases reported recent travel. The attack rate for household contacts from index cases was 12.6% while it was 3.1% for non-household contacts. The household attack rate was lower among contacts <20 years (5.3%, 95% CI: 2.4, 9.8%) compared to older age groups (13.7% among 20-59 year olds [95% CI: 10.7, 17.2%] and 17.7% among those 60 years or older [95% CI: 11.9, 24.8%]). Secondary attack rates excluding tertiary cases, and any cases that had untraced exposures were 13.8% for household contacts (95% CI: 11.1-17.0%) and 7.1% for non-household contacts (95% CI: 4.7-10.6%), with a mean incubation period of 4 days and maximum infection period of 13 days. Overall, the secondary attack rates were similar between the incubation period and infection period (OR: 1.13 [95% CI: 0.59-2.18]). The estimated effective R0 (average number of individuals infected by a case given the proportion of Guangzhou that were susceptible) was 0.48 (95% CI: 0.39, 0.58).

Study Strengths

The provides further evidence of the importance of early transmission prior to symptom onset.


Although the researchers included asymptomatic cases in the analysis, these individuals were few, underrepresented, and a proxy date for symptom onset was used, which may not be a valid comparison to symptomatic cases.

Value added

The was one of the first studies to utilize contact tracing data to estimate infectivity during the incubation period.