People without symptoms spreading Covid-19, study reveals

Tuesday March 24 2020

A health worker wearing a protective gear prepares to disinfect the matatu in which the first Kenyan Covid-19 patient travelled to Ongata Rongai. PHOTO | YASUYOSHI CHIBA | AFP


Infected people without symptoms might be driving the spread of the Covid-19 coronavirus, a new study has shown.

While emphasis has been put on the fact that the virus is spread mainly by people who are already showing symptoms, such as fever, cough or difficulty breathing, a team of infectious disease researchers at the University of Texas found that the germ that causes Covid-19 is most contagious before and during the first week of symptoms.

Since the outbreak of the disease — which has been declared a pandemic — in Wuhan, China in December, a number of studies conducted to understand how the virus behaves have shown that people without symptoms are causing substantial amounts of infection.


The scientists also found that about 10 per cent of patients are infected by someone who has the virus but does not show symptoms yet.

"The data suggest that this coronavirus may spread like the flu. That means we need to move quickly and aggressively to curb the emerging threat," said Lauren Ancel Meyers, a professor of integrative biology at UT Austin.


Researchers studying the infection have found that over 10 per cent of patients are infected by someone who has the virus but does not show symptoms yet.

READ: Coronavirus: Kenya confirms 2 more cases - VIDEO

In a study published in the Emerging Infectious Diseases journal, a team of scientists from the United States, France, China, and Hong Kong were able to calculate what’s called the serial interval of the coronavirus by measuring the time it takes for symptoms to appear in two people with the virus — the person who infects another, and the infected second person.

The speed of an epidemic depends on two things — how many people each case infects and how long it takes cases to spread. The first quantity is called the reproduction number; the second is the serial interval. The short serial interval of Covid-19 means emerging outbreaks will grow quickly and could be difficult to stop, the researchers said.


According to the researchers, including those from the University of Texas at Austin, the average serial interval for the novel coronavirus in China was approximately four days.

"Our findings are corroborated by instances of silent transmission and rising case counts in hundreds of cities worldwide. This tells us that Covid-19 outbreaks can be elusive and require extreme measures," Prof Meyers said.

Having examined more than 450 infection case reports from 93 cities in China, Prof Meyers’ team said the findings were the strongest evidence yet that people without symptoms must be transmitting the virus, known as pre-symptomatic transmission. According to the paper, more than one in 10 infections were from people who had the virus but did not yet feel sick.

Previously, the scientists had some uncertainty about asymptomatic transmission with the coronavirus.

"This provides evidence that extensive control measures including isolation, quarantine, school closures, travel restrictions and cancellation of mass gatherings may be warranted," Meyers said. "Asymptomatic transmission definitely makes containment more difficult."


The study provides an insight into why it took a week to identify Kenya’s first confirmed case of the disease. It took nine days after the 27-year-old woman jetted into the country for her to present to hospital with symptoms mimicking those of Covid-19.

It has emerged that she may have been infected aboard the plane to Nairobi from London after having sat next to a coughing passenger. Upon her arrival at JKIA, where she did not show any symptoms, she shared a cab together with three other passengers in order to cost-share. She then alighted in the city centre and boarded a different taxi to her boyfriend’s place in Rongai, where seven other people were living.

As she went about her business in the city, she may have passed on the virus unknowingly, putting an unknown number of people at risk.

Known as Patient Zero, the Kenyan student living in the US tested positive for Covid-19 was immediately isolated at Kenyatta National Hospital’s Infectious Disease Unit (KNH-IDU) marking the onset of public health officers’ nightmare to trace every person she had come into contact with.