COVID-19 is an acute respiratory tract infection. Understanding where exactly the SARS-CoV-2 virus replicates and infects within the body is important in trying to contain the virus. The paper reports, based on review of nine cases, that the SARS-CoV-2 virus can replicate in upper respiratory tract tissues. Active replication in the throat was confirmed by the presence of viral replicative RNA in throat samples. Significant amounts of virus can still be found in the sputum (phlegm) of patients even after symptoms disappear and antibodies develop.
Throat swabs of the nine patients were taken on the first day of the onset of COVID symptoms through the 28th day post onset. Viral load, or copies of SARS-CoV-2 RNA per swab, was measured after each swab. From day 1-5, the average viral load was 6.76x10^5 copies per swab, and after the fifth day, the average viral load was 3.44x10^5 copies per swab. Within 7 days of onset, infectious virus could be readily isolated (grown in culture) in swab and sputum samples, suggesting that the virus can replicate in upper respiratory tract tissues. The virus could not be isolated in any sample after 8 days of onset despite high viral loads. Despite symptoms declining for most patients after the first week, viral RNA could be detected in throat samples well into the second week, and sputum samples of most patients were positive for SARS-CoV-2 RNA into the 3rd and 4th week, despite full disappearance of symptoms. Seroconversion, or the development of antibodies to the virus, occurred on average after 7 days of the onset of symptoms.
Despite all patients displaying only mild symptoms of SARS-CoV-2, simple throat swabs were still sufficient to measure viral load, which is not the case for the SARS virus. Also, while it only took five days for SARS-CoV-2 to reach peak concentration of viral RNA, it took 7 days for the SARS virus, and even then its peak concentration was 1000 times less than that of SARS-CoV-2. Furthermore, while live virus isolation could be obtained from throat swab samples of COVID-19 patients, this was not possible for SARS patients, suggesting that SARS-CoV-2, unlike SARS-CoV, can replicate in the upper respiratory tract as well as the lungs. Authors also propose based on current findings that hospitalized patients can be discharged after 10 days of symptoms onset if viral load is less than 100,000 copies per mL, as there is little risk of infectivity at this point. It is also important to note that, even after the development of SARS-CoV-2 antibodies and the disappearance of symptoms, there is not a rapid decline in viral load. However, because of the inability to isolate infectious virus after 8 days, viral emission in sputum at this point is likely not as infectious as it is within the first 7 days.