Insights Into How COVID-19 Spreads in Households
(Representational image)
New modelling research suggests the coronavirus (SARS-CoV-2) that causes COVID-19 may spread more easily among people living together and family members than severe acute respiratory syndrome (SARS) or Middle East respiratory syndrome (MERS). The estimates are the first of their kind to quantify symptomless transmission.
The analysis was published in The Lancet Infectious Diseases journal. It was based on contact tracing data from 349 people with COVID-19 and 1,964 of their close contacts in Guangzhou (the most populated city in southern China), found people with COVID-19 were at least as infectious before they developed symptoms as during their actual illness, and that older people (aged 60 years or more) were most susceptible to household infection with SARS-CoV-2.
The study of people living together and family members (not living at the same address), and non-household contacts (eg, friends, co-workers, passengers) suggests that breaking the chain of transmission within households through timely tracing and quarantine of close contacts, in addition to case finding and isolation, could have a huge impact on reducing the number of COVID-19 cases.
While the model has been updated to reflect the current knowledge about the transmission dynamics of COVID-19, the authors caution that it is based on a series of assumptions, for example about the length of incubation and how long symptomatic cases are infectious, that are yet to be confirmed, and might affect the accuracy of the estimates.
“Our analyses suggest that the infectiousness of individuals with COVID-19 before they have symptoms is high and could substantially increase the difficulty of curbing the ongoing pandemic”, says Dr Yang Yang from the University of Florida in the USA who co-led the research. “Active case finding and isolation in conjunction with comprehensive contact tracing and quarantine will be key to preventing infected contacts from spreading the virus during their incubation periods, which will be crucial when easing lockdown restrictions on movement and mixing.”
Household transmission of COVID-19 is suspected to have contributed substantially to the rise in cases in China following the introduction of lockdown measures. But little research has assessed the spread of disease at the household level. Previous estimates of household infections are specific to the setting where the data were obtained, and represent the proportion of infections among all traced contacts, which does not fully account for the difference in individual exposure history, or the fact that infections may not necessarily be secondary, and could be tertiary—ie, the possibility of transmission among contacts themselves, or infection risks from objects such as clothes, utensils, and furniture.
In the study, researchers developed a transmission model that accounted for individual-level exposure, tertiary transmission, potential exposure to untraced infection sources, and asymptomatic infections. Using data gathered by the Guangzhou Center for Disease Control and Prevention (CDC) on 215 primary COVID-19 cases (ie, with no known source of exposure, or assumed to have been infected outside Guangzhou), 134 secondary/tertiary cases, and 1,964 of their close contacts between January 7 and February 18, 2020, the study estimated the secondary attack rate (the probability that an infected person transmits the disease to a susceptible individual) among people living together and family members, and non-household contacts. Close contacts—unprotected individuals who had been within a metre of a person with COVID-19 less than 2 days before their symptoms developed—were traced, quarantined, and tested for SARS-CoV-2 on days 1 and 14.
The study also modelled the effects of age and sex on the infectivity of COVID-19 cases and susceptibility of their close contacts. For the primary results, researchers assumed an average incubation period of 5 days and a maximum infectious period of 13 days (including up to 5 days before illness onset). Among the 349 laboratory-confirmed primary and secondary COVID-19 cases, 19 (5%) reported no symptoms during the follow-up period.
“Family members such as parents and older children may not be living at the same address, which might explain why they appear at less risk of secondary infections than those living in the same household as the COVID-19 case”, says co-author Dr Natalie Dean from the University of Florida, USA. “While the likelihood of transmitting COVID-19 in households may seem quite low, it is around twice what has been estimated for SARS (4.6–8%) and three times higher than for MERS (4–5%), although these data are only based on a small number of studies.”
The analysis was published in The Lancet Infectious Diseases journal. It was based on contact tracing data from 349 people with COVID-19 and 1,964 of their close contacts in Guangzhou (the most populated city in southern China), found people with COVID-19 were at least as infectious before they developed symptoms as during their actual illness, and that older people (aged 60 years or more) were most susceptible to household infection with SARS-CoV-2.
The study of people living together and family members (not living at the same address), and non-household contacts (eg, friends, co-workers, passengers) suggests that breaking the chain of transmission within households through timely tracing and quarantine of close contacts, in addition to case finding and isolation, could have a huge impact on reducing the number of COVID-19 cases.
While the model has been updated to reflect the current knowledge about the transmission dynamics of COVID-19, the authors caution that it is based on a series of assumptions, for example about the length of incubation and how long symptomatic cases are infectious, that are yet to be confirmed, and might affect the accuracy of the estimates.
“Our analyses suggest that the infectiousness of individuals with COVID-19 before they have symptoms is high and could substantially increase the difficulty of curbing the ongoing pandemic”, says Dr Yang Yang from the University of Florida in the USA who co-led the research. “Active case finding and isolation in conjunction with comprehensive contact tracing and quarantine will be key to preventing infected contacts from spreading the virus during their incubation periods, which will be crucial when easing lockdown restrictions on movement and mixing.”
Household transmission of COVID-19 is suspected to have contributed substantially to the rise in cases in China following the introduction of lockdown measures. But little research has assessed the spread of disease at the household level. Previous estimates of household infections are specific to the setting where the data were obtained, and represent the proportion of infections among all traced contacts, which does not fully account for the difference in individual exposure history, or the fact that infections may not necessarily be secondary, and could be tertiary—ie, the possibility of transmission among contacts themselves, or infection risks from objects such as clothes, utensils, and furniture.
In the study, researchers developed a transmission model that accounted for individual-level exposure, tertiary transmission, potential exposure to untraced infection sources, and asymptomatic infections. Using data gathered by the Guangzhou Center for Disease Control and Prevention (CDC) on 215 primary COVID-19 cases (ie, with no known source of exposure, or assumed to have been infected outside Guangzhou), 134 secondary/tertiary cases, and 1,964 of their close contacts between January 7 and February 18, 2020, the study estimated the secondary attack rate (the probability that an infected person transmits the disease to a susceptible individual) among people living together and family members, and non-household contacts. Close contacts—unprotected individuals who had been within a metre of a person with COVID-19 less than 2 days before their symptoms developed—were traced, quarantined, and tested for SARS-CoV-2 on days 1 and 14.
The study also modelled the effects of age and sex on the infectivity of COVID-19 cases and susceptibility of their close contacts. For the primary results, researchers assumed an average incubation period of 5 days and a maximum infectious period of 13 days (including up to 5 days before illness onset). Among the 349 laboratory-confirmed primary and secondary COVID-19 cases, 19 (5%) reported no symptoms during the follow-up period.
“Family members such as parents and older children may not be living at the same address, which might explain why they appear at less risk of secondary infections than those living in the same household as the COVID-19 case”, says co-author Dr Natalie Dean from the University of Florida, USA. “While the likelihood of transmitting COVID-19 in households may seem quite low, it is around twice what has been estimated for SARS (4.6–8%) and three times higher than for MERS (4–5%), although these data are only based on a small number of studies.”
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