Coronavirus patients might be most infectious two or three days before symptoms appear, says study
People with COVID-19 might be most contagious two or three days before they begin to feel sick, helping the virus spread far and wide, a new study suggests.
"We showed substantial transmission potential before symptom onset," the Chinese scientists wrote in their study published in Nature. They estimate that about 44% of COVID-19 cases may spread from person to person before symptoms appear.
The possibility of the new coronavirus jumping from seemingly healthy people to others suggests that control measures such as tracing contacts of infected persons need revising, the experts suggested.
Tracing contacts, however, help control its closest cousin, SARS, but it might not be enough to slow the new coronavirus down without revision. The reason: people infected with SARS were most infectious between 7 and 10 days after symptoms began, which is not the case with COVID-19. Globally, the virus has infected more than 2 million people.
In the study, the experts looked at 94 patients admitted to China's Guangzhou Eighth People’s Hospital from January 21 through February 14. The team then collected their throat swabs until day 32 after symptoms began.
They found the amount of viral RNA in the patient throats changed with time. They found a high amount of virus in the throats when symptoms first appeared. "We observed the highest viral load -- the amount of virus -- in throat swabs at the time of symptom onset," the researchers wrote.
Finding such high amounts immediately as soon as symptoms appeared indicates that the virus was at work two or three days before. It quietly multiplied inside patients to reach those levels. With time, the virus levels appeared to decline, the study suggests.
The study suffers from a few limitations. They suspect that some patients might not remember when exactly they began feeling ill. This is called recall bias and it could have a bearing on the findings. "Symptom onset relies on patient recall after confirmation of COVID-19," they said. They also add that these patients received treatments, which could have a bearing on the amount of virus released from the body.
Nevertheless, the study has implications for control measures such as contact tracing, which means tracking contacts of individuals who crossed paths with an infected person. But health workers might have to tailor it to fight the new virus. To effectively slow it down, the authors propose, identifying people who may have come into contact two or three days before an infected individual fell sick.
However, the experts think contact tracing is important even as the world shifts towards mitigation. "Even when the control strategy is shifting away from containment to mitigation, contact tracing would still be an important measure, such as when there are super-spreading events that may occur in high-risk settings including nursing homes or hospitals," they wrote in their study.
"With a substantial proportion of presymptomatic transmission, measures such as enhanced personal hygiene and social distancing for all would likely be the key instruments for community disease control," the study authors wrote.