In a new analysis of Guangzhou city contact tracing data, investigators examined secondary attack rates of SARS-CoV-2.
A Guangzhou Center for Disease Control and Prevention study team publishing in The Lancet Infectious Diseases used contact tracing data from the city in Guangdong province, China to estimate the secondary attack rate of SARS-CoV-2 among household and close contacts in Guangzhou, China.
The investigators created a statistical transmission model based on a comprehensive contact tracing dataset to estimate the secondary attack rate of coronavirus disease 2019 (COVID-19) among household and non-household close contacts, defined as the probability that an infected individual will transmit the disease to a susceptible individual. Findings point to pitfalls of the isolation of higher-risk older adults in multi-generational households.
“The provision of comfortable facilities for exposed contacts to quarantine or for mild cases to isolate away from their families could be a valuable strategy to limit onward transmission within households,” the Guangzhou CDC authors wrote.
The Lancet authors incorporated 2 additional definitions of household contacts into their analysis: individuals who were family members or relatives, regardless of address, and individuals living at the same address regardless of relationship. The study team also analyzed the demographic determinants of transmissibility and the infectivity of COVID-19 cases during their incubation period.
“The infectiousness of patients with COVID-19 during their incubation periods is high and could substantially increase the difficulty of curbing the ongoing pandemic. Active case finding and isolation in conjunction with comprehensive contact tracing and quarantine are useful for preventing infected contacts from spreading the virus during their incubation periods, which could be crucial when societal restrictions on human movement and mixing are lifted,” the authors explained.
The investigators estimated that the local R of SARS-CoV-2 could vary between 0∙5, consistent with the mean Rt . On this basis, they calculated that without isolation of cases or quarantine of their contacts and assuming a mean incubation period of 5 days, the local reproductive number would have ranged from 20—50% higher.
Most patients in the analysis group were adults between 20-59 years of age.
Out of 215 primary cases, 158 (73%) and 46 of 134 secondary cases (46%) had traveled to or lived in Hubei province despite unprecedented efforts to restrict travel out of the region.
The study team calculated secondary attack rates of 13.2% (95% CI 10·9—15·7) among household contacts and 2.4% (1·6–3·3) among non-household contacts.
Within households, the estimated secondary attack rates were lower in the youngest age group (younger than 20 years than the 20—59 years age group and the oldest age group (≥60 years).
The team’s demographic analyses did not point to a significant difference in secondary attack rate between sexes.
By February, secondary attack rates within and outside households decreased in the city. When household was defined by residential address, the data-based secondary attack rate among household contacts increased to 17.2% relative to 2.6% among non-household contacts.
“SARS-CoV-2 is more transmissible in households than SARS-CoV and Middle East respiratory syndrome coronavirus. Older individuals (aged ≥60 years) are the most susceptible to household transmission of SARS-CoV-2. In addition to case finding and isolation, timely tracing and quarantine of close contacts should be implemented to prevent onward transmission during the viral incubation period,” the authors concluded.
The study was supported by the United States National Institutes of Health, the Science and Technology Plan Project of Guangzhou, and the Project for Key Medicine Discipline Construction of Guangzhou Municipality.