Researchers in China and the United States have conducted a study assessing the household transmissibility of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) that has important implications for managing the coronavirus disease 2019 (COVID-19) pandemic.
The retrospective study of Wuhan households found that children and adolescents were less vulnerable to infection than adults, but more likely to pass the infection on to others. Infants aged 0 to 1 year were at a greater risk of infection than those aged 2 to 12 years.
As reported in The Lancet Infectious Diseases, people aged 60 years or older were at a greater risk of infection than any other age group.
The study also found that symptomatic individuals were much more likely to infect others than asymptomatic individuals and were more likely to do so prior to symptom onset than after.
“These findings have implications for devising interventions for blocking the household transmission of SARS-CoV-2, such as timely vaccination of eligible children once resources become available,” writes Shun-Qing Xu from Huazhong University of Science and Technology in Wuhan and colleagues.
Wuhan was the world’s first epicenter of the pandemic
When the COVID-19 outbreak first began in late 2019, Wuhan was the world’s first epicenter of the pandemic, accounting for around 80% of all cases in China.
Since then, the unprecedented spread of the novel SARS-CoV-2 virus has led to more than 96 million people being infected and caused more than 2 million COVID-19-related deaths worldwide.
The resumption of normal day-to-day life and economic activity depends heavily on the scientific community’s understanding of key SARS-CoV-2 transmission venues such as households and workplaces, as well as drivers of transmission and the effectiveness of control measures.
“In resource-limited areas, including Wuhan in China early on in the epidemic, isolation of cases and quarantine of close contacts often occurred at home, enabling onwards transmission within households,” says Xu and colleagues. “Households are ideal settings for assessing transmissibility of a pathogen and associated determinants of susceptibility and infectivity.”
Although children are known to be less susceptible to severe disease than adults, their ability to infect household members is not well characterized. However, this question is highly relevant with regard to preventing SARS-CoV-2 transmission in schools and households.
Compared with that of symptomatic individuals, the relative infectivity of pre-symptomatic individuals during the incubation period is also unclear.
What did the researchers do?
The researchers conducted a retrospective analysis of all households where members had COVID-19 or asymptomatic SAR-COV-2 infection confirmed by the Wuhan Center for Disease Control and Prevention between 2nd December 2019 and 18th April 2020.
Household secondary attack rates and risk factors associated with infectivity and susceptibility were estimated using a statistical transmission model that accounted for confounding variables and individual-level exposure history.
The household secondary attack rate is defined as the probability that an infective individual will transmit the virus to a susceptible household member.
The researchers also assessed the effectiveness of isolation and quarantine intervention policies at reducing household transmission in Wuhan.
What did the study find?
The team identified 27,101 households with 29,578 primary cases and 57,581 household contacts.
The overall secondary attack rate within households was estimated to be 15·6%.
Children and adolescents were less vulnerable to infection than older individuals, but were more likely to pass the virus on once infected. When the same exposure time was assumed, individuals aged younger than 20 years were 58% more likely to infect others than adults aged 60 years or older.
People aged 60 years or older were at a greater risk of infection than all other age groups.
Infants aged 0 to 1 year were more than twice as likely to be infected than children aged 2 to 5 years and 53% more likely to be infected than children aged 6 to 12 years.
Children and adolescents were as likely to develop symptoms as adults were, but were much less likely to develop severe disease.
Symptomatic individuals were around 80% more likely to infect others than asymptomatic individuals. They were also 42% more likely to transmit the virus before they had developed symptoms than afterwards.
Finally, the researchers report that isolating cases, quarantining household contacts, and implementing movement restrictions reduced household transmission by 52% among primary cases and by 63% among secondary cases.
What are the implications of the study?
The researchers say the findings have important implications for forecasting and controlling the COVID-19 pandemic.
“The high infectivity of children with SARS-CoV-2 infection highlights the need for careful planning of school reopening,” writes Xu and colleagues. “Additionally, the susceptibility of infants supports caregivers of infants being prioritized for vaccination.”
Finally, “when feasible, cases could be isolated and household contacts quarantined away from their homes to prevent household transmission, particularly when pre-symptomatic,” concludes the team.
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