Asymptomatic COVID-19 cases have been considered to be pertinent contributors to the spread of the disease, with its patients commonly referred to as silent carriers. However, with time, researchers now have a better understanding of the phenomenon.
New meta studies and analyses now say that asymptomatic cases may not be swelling in number – one in five infected persons is likely to be asymptomatic. However, researchers are divided on the number of silent carriers and whether they are a major driving force behind the pandemic in reality.
An earlier estimate said that asymptomatic cases could be as high as 81% of the total number of infections. However a meta-analysis published in the Official Journal of the Association of Medical Microbiology and Infectious Disease, Canada, last month, said that the rate rests at 17%. The study looked 13 other studies which involved 21,708 people.
The study defined asymptomatic patients as those who have not shown any of the typical symptoms of the disease and included studies which tracked participants for at least 7 days. This was done given that a normal person takes between seven to thirteen days to develop symptoms.
Another important aspect about asymptomatic patients has been ‘transmissibility’ – how many more persons can an asymptomatic patient further infect. The study sheds some new light on that as well. According to it, asymptomatic people are 42% less likely to transmit the virus as compared to those with symptoms.
To understand how frequently asymptomatic people can transmit the virus to others, a large population study was conducted in Geneva, Switzerland. Researchers modelled the spread of the virus in people that live together, say, in a house. In the study, published in the pre-print server MedRxiv earlier this month, the researchers said that the risk of an asymptomatic person spreading the virus is one-fourth that of the transmission risk associated with a symptomatic person.
The findings raise a question: Are asymptomatic people the real drivers of COVID-19 progression? Researchers stand divided. Some, like Andrew Azman, an assistant scientist at the Johns Hopkins Bloomberg School of Public Health, believe that asymptomatic people are silent carriers who pose a significant public health risk. However, researchers like Oyungerel Byambasuren, a biomedical researcher at the Institute for Evidence-Based Healthcare at Bond University in Gold Coast Byambasuren and the lead author of the earlier paper, is of a different view. “These people are not the secret drivers of this pandemic. They are not coughing or sneezing as much, they’re probably not contaminating as much surfaces as other people,” he told Nature.
Certain studies have shown that even an asymptomatic person has the same initial viral load (the number of viral particles in a throat swab) as someone with symptoms. However, the asymptomatic person could be free of the infection much faster than the symptomatic cases and they are infected for a shorter period of time.
A research team led by Muge Cevik, a physician from the St. Andrews University in the UK, conducted a systematic meta-analysis of other studies on viral dynamics and transmissibility. It was published in the pre-print repository SSRN (Social Science Research Network). The researchers also found that symptomatic patients have greater transmissibility than asymptomatic people.
As researchers engage further with COVID-19, they come across newer findings which topple our previous assumptions and estimations. However, a clearer picture is further into the future.