In a recent study at the University of Florida and elsewhere, researchers found the COVID-19 virus 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 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).
It 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 is published in The Lancet Infectious Diseases. One author is Dr. Yang Yang from the University of Florida.
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 researchers caution that it is based on a series of assumptions.
For example, the length of incubation and how long symptomatic cases are infectious are yet to be confirmed, and this might affect the accuracy of the estimates.
The team says 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.
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.
It 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.
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