Weekly newsletter:
December 14-20 2020
Level difficulty:
SARS-CoV-2 transmission mode
Although we know that the clinical severity of infection by SARS-CoV-2 increases with age, little information exists concerning the virus’ mode of transmission. Having a better understanding of the factors which may contribute to transmission is very important if we want to minimize the impact of the pandemic on our social life.
To this end, epidemiological data was collected from 1 178 individuals infected by SARS-CoV-2 as well as 15 648 of their close contacts, between January and April 2020. All of these individuals came from Huwan, a Chinese province close to Hubei where the COVID-19 pandemic began.
Firstly, it was possible to study the mode of transmission in 14 622 individuals in contact with 870 SARS-CoV-2 positive patients, that is, 74% of reported cases in Hunan. 5 types of exposure to the virus were noted: at home, in the wider family, with friends and associates, in society in general and while undergoing medical care. The length of exposure to the virus was calculated according to the initial and final dates of exposure.
In studying the risk levels of each group, bearing in mind that lockdown in Hunan began on the 25th of January 2020, it was possible to determine that contact which took place at home presented the greatest risk of contamination, followed by contacts with the wider family, general social interaction and contact with associates. Health personnel were less affected, which suggests that protective sanitary measures had been well observed in the province’s hospitals and more generally by the wider population when leaving home.
This information can be illustrated thus:
Secondly, the kinetics of transmission were studied between contagious and yet to be infected individuals in order to determine the impact of human activity on infection by the SARS-CoV-2 virus. Thus it was possible to determine an average of 5,3 days between the appearance of symptoms in contagious individuals and individuals infected by those contagious patients. It was also estimated that 63.4% of infectious events occurred before symptoms appeared (in the context of lockdown). The speed of detection of an infection also had an important effect on transmission kinetics since the rapid self-isolation of a contact case could influence the spreading of the virus. It’s interesting to note that if an individual self-isolates just 2 days after the appearance of symptoms in the person with whom he had contact, the proportion of pre-symptomatic transmission is 87,3%. This proportion changes to 47,5% if individuals self-isolate at least 6 days after the appearance of symptoms (the number of people in contact with infected individuals increases as days pass, but not the number of people testing positive, thereby drastically reducing the contamination percentage). This data suggest that transmission generally occurs during the asymptomatic phase of the infection, but at a moment close to the appearance of symptoms).
Using this data it was possible to model different pandemic scenarios. If we suppose that the basic reproduction number of SARS-CoV-2 is represented by “R0”, this figure could potentially rise to 2,19 in an urban context with no sanitary measures being imposed. The various phases of the pandemic in Wuhan after lockdown were modelled as follows. Each time the R number went down, the degree of infection with SARS-CoV-2 diminished, as did its duration:
Different infection scenarios were then worked out. In the most optimistic model, with an R=1,56, if individuals infected by SARS-CoV-2 self-isolate at an average of 2 days after the appearance of symptoms, a reduction of 25% in the transmission of the virus coupled to 42% of isolated cases would be enough to reduce the R number to 0.
This study also showed that SARS-CoV-2 does not have a single transmission mode and that measures put in place by governments such as mask-wearing, enforced working from home or lockdown can play a major role in preventing the virus from spreading.
