BEIJING: Researchers claim that half of the patients they treated for mild COVID-19 infection still had the novel coronavirus for up to eight days after symptoms disappeared, a finding that explains why controlling the spread of the disease has been difficult.The research, published in the American Journal of Respiratory and Critical Care Medicine, assessed 16 patients with COVID-19, who were treated and released from the Treatment Center of PLA General Hospital in Beijing between January 28 and February 9, 2020.In the study, researchers, including Indian-origin scientist Lokesh Sharma from Yale University in the US, analysed samples collected of throat swabs taken from all the patients on alternate days.They said the patients were discharged after their recovery and confirmation of negative viral status by at least two consecutive polymerase chain reaction (PCR) tests."The most significant finding from our study is that half of the patients kept shedding the virus even after resolution of their symptoms," said co-lead author Sharma."More severe infections may have even longer shedding times," he said.According to the research, the primary symptoms in these patients included fever, cough, pain in the pharynx, and difficult or labored breathing (dyspnea).They said the patients were treated with a range of medications.The time from infection to onset of symptoms -- incubation period -- was five days among all but one patient, the scientists noted, adding that the average duration of symptoms was eight days.They said the length of time patients remained contagious after the end of their symptoms ranged from one to eight days.Two patients also had diabetes and one had tuberculosis, neither of which affected the timing of the course of COVID-19 infection, the study said."If you had mild respiratory symptoms from COVID-19 and were staying at home so as not to infect people, extend your quarantine for another two weeks after recovery to ensure that you don't infect other people," suggested study co-author Lixin Xie from the Chinese PLA General Hospital in Beijing."COVID-19 patients can be infectious even after their symptomatic recovery, so treat the asymptomatic/recently recovered patients as carefully as symptomatic patients," the scientists said.The researchers said all of these patients had milder infections and recovered from the disease, and that the study looked at a small number of patients.They noted that it is unclear whether similar results would hold true for more vulnerable patients such as the elderly, those with suppressed immune systems, and patients on immuno-suppressive therapies."Further studies are needed to investigate if the real-time PCR-detected virus is capable of transmission in the later stages of COVID-19 infection," Xie added.