A remarkable number of coronavirus patients have no symptoms of the disease at all.
Analysis of public datasets by the Scripps Research team suggest that nearly 45 percent of people carrying SARS-CoV-2 are asymptomatic.
An asymptomatic person does not show any symptoms of infection but is able to pass the virus to others for more than two weeks.
The very high percentage of asymptomatic cases most likely played a key part in the early stages of the coronavirus disease pandemic by spreading the virus silently and vigorously.
Hence, the research points out the necessity for extensive testing and contact tracing in order to reduce the spread of Covid-19 in populations.
Professor Eric Topol, the director of the Scripps Research Translational Institute, said:
“The silent spread of the virus makes it all the more challenging to control.
Our review really highlights the importance of testing.
It’s clear that with such a high asymptomatic rate, we need to cast a very wide net, otherwise the virus will continue to evade us.”
Professor Topol and his colleague gathered this data from 16 studies worldwide.
The data also includes cruise ship passengers, nursing home residents, prison inmates, among others.
Mr Daniel Oran, study’s first author, said:
“What virtually all of them had in common was that a very large proportion of infected individuals had no symptoms.
Among more than 3,000 prison inmates in four states who tested positive for the coronavirus, the figure was astronomical: 96 percent asymptomatic.”
When the team reviewed the collected information, they noticed that asymptomatic people can spread the virus for an extended time, possibly lasting beyond 14 days.
Viral load is the amount of virus present in an infected person and generally higher viral loads lead to worse outcomes.
The study found that the SARS-CoV-2 viral load in asymptomatic persons was at the same level as symptomatic patients, therefore the chance of viral transmission could be similar.
Also, just because they don’t carry any symptoms, it does not mean that there won’t be any harm.
For example, CT scans on asymptomatic patients of the Diamond Princess cruise ship show subclinical abnormalities in the lungs.
This is probably due to the impact of the SARS-CoV-2 infection on lung function despite not showing immediately.
The authors, however, admit that it is difficult to differentiate between pre-symptomatic and asymptomatic patients since long-term data and repeated testing are not available.
A pre-symptomatic individual is a person who has been infected with the virus and will sooner or later show symptoms, whereas an asymptomatic person will have silent symptoms of COVID-19 even though they are similarly infected.
Mr Oran said:
“Our estimate of 40 to 45 percent asymptomatic means that, if you’re unlucky enough to get infected, the probability is almost a flip of a coin on whether you’re going to have symptoms.”
The study was published in Annals of Internal Medicine (Oran & Topol , 2020).