Coronavirus can be transmitted 3 to 5 days before symptoms show making it harder to control than SARS: Study
An infected person can spread the virus three to five days before they realize they are sick. This makes it difficult for experts to outsmart the virus.
This is according to a study, where scientists looked at 243 cases of COVID-19 in Singapore between January 23 and March 16. They found 10 of these patients may have transmitted the disease to three to five people before noticing symptoms of the disease -- fever or cough, for instance. These findings suggest that diagnosing and isolating people with symptoms alone might do little to stem the spread.
This mode of spreading is called a presymptomatic spread, which is different from the transmission occurring between people who show no symptoms, called the asymptomatic spread.
Some patients take time to develop symptoms. “What we know from reports, what we know from published literature, is that the predominant way in which transmission is occurring is among symptomatic individuals. We know people might transmit [the disease] in the few days before they become symptomatic in their presymptomatic phase," the WHO expert Dr. Maria Van Kerkhove, said during a press briefing.
Regardless of whether they show symptoms or not, the virus spreads between people through droplets from the nose and mouth, added Van Kerkhove. What started in China's Wuhan, has infiltrated 184 countries or regions around the world. The virus has sickened more than 1,348,628 people and killed 74,856.
What does the study say?
The Centers for Disease Control and Prevention (CDC) study throws light on how the virus spreads before symptoms appear. The scientists then identified six clusters where presymptomatic spread may have had a part to play. Each cluster had two to five patients each.
The team then looked at 243 patients. And 157 of them contracted the disease in the country itself. Of the 157, 10 cases can be attributed to presymptomatic spread.
"Presymptomatic transmission of SARS-CoV-2, the virus that causes coronavirus disease 2019 (COVID-19), might pose challenges for disease control," the scientists wrote in their report.
This makes it more dangerous than its close relative, the SARS virus. Though SARS had a higher death rate, it could only spread between people showing symptoms, allowing experts to control it. Controlling the new coronavirus needs more effort.
The study makes the case for social distancing ."Early detection and isolation of symptomatic COVID-19 patients and tracing of close contacts is an important disease containment strategy; however, the existence of presymptomatic or asymptomatic transmission would present difficult challenges to contact tracing."
The study is not without its limitations. The authors acknowledge that their findings might be erroneous largely because it relies on the memory of the patients. This is called recall bias. For instance, people with mild disease may not remember when their symptoms began.
Other studies have also suggested that the virus spreads from someone showing delayed symptoms. "The evidence of presymptomatic transmission in Singapore, in combination with evidence from other studies, supports the likelihood that viral shedding can occur in the absence of symptoms and before symptom onset," they wrote.