The Infectivity Of Covid-19 Patients Starts Two Or Three Days Before The Onset Of Symptoms

According to real-time statistical data from Johns Hopkins University in the United States, as of April 7th, Beijing time, nearly 21,199 cases of COVID-19 cases were diagnosed globally, and a total of 143,725 deaths.
For a long time, asymptomatic infections have been relatively difficult to find because of their special concealment and the subjectivity of symptoms, and have attracted much attention. Earlier, the US Centers for Disease Control and Prevention stated that in the United States, as many as one-quarter of COVID-19 patients have no symptoms.
Since April 1st, China has also established a daily reporting system for asymptomatic infected persons, and will announce to the society the newly-reported cases, the situation of reversion and the management situation every day. On April 15, the State Council of our country held a press conference to announce the first time the specific data of asymptomatic people in China. As of April 14, a total of 6,764 cases of asymptomatic infections have been reported nationwide.
There have been many studies on the infectivity and virus shedding time of asymptomatic patients. Recently, a study published in the journal “Nature Medicine” pointed out that COVID-19 patients are most infectious before symptoms appear, and they begin to fall off SARS-CoV-2 2-3 days before the first symptoms appear.
The study was led by the Eric Lau team of the University of Hong Kong and involved 94 patients with COVID-19 admitted to the Eighth People ’s Hospital of Guangzhou, China, 50% were male, with a median age of 47 years, and 61 of 93 patients were Mild. Specifically, the researchers collected pharyngeal swabs from 94 COVID-19 patients who were treated at the Eighth People’s Hospital of Guangzhou, China, from January 21 to February 14. A total of 414 throat swabs were collected from the onset of symptoms to 23 days after the onset of symptoms.
The results showed that the researchers detected high viral load shortly after the patient developed symptoms, and then gradually dropped to the detection limit around the 21st day. There was no significant difference in viral load among different genders, age groups and disease severity.
In addition, the researchers also conducted a sample of 77 “transmission pairs (transmitters and infected persons)” from public data to assess how much time has passed between each patient’s onset of symptoms. It assumes a median incubation period (the time from infection to symptoms) of 5.2 days. The research team concluded that infectivity started 2.3 days before the onset of symptoms and peaked 0.7 days before the onset of symptoms.
In the sensitivity analysis, the same estimation procedure is used, but assuming that the infectious attack is unchanged 1-7 days before the onset of symptoms, it is found that the infectivity peaks 0-2 days before the onset of symptoms, and the spread before the onset of symptoms The ratio is 46% to 55%.
The study also found that the virus may begin to fall off two to three days before the first symptoms appear. After the symptoms appear, the rate of viral load in the patient decreases faster. This is consistent with the results of two recent studies. On April 1, the teams in Munich, Berlin, and Cambridge in Germany published the article “Virological assessment of hospitalized patients with COVID-2019” on Nature. The study found that the peak of COVID-19 virus shedding occurred at an early upper respiratory tract infection, which indicates that virus carriers may be most infectious when they have mild symptoms or almost no obvious symptoms.
In addition, the Lancet published an observational cohort study on the temporal changes in viral load in deep-throat saliva and serum antibody responses during SARS-CoV-2 infection. The study found that patients with COVID-19 had the highest viral load in the deep throat saliva during the first week of onset, and then gradually declined.
Looking back, the latest study also has certain limitations. First, the time of onset of symptoms depends on the memories of COVID-19 patients, and there may be deviations. Second, the interval between reported consecutive cases may vary. Finally, the kinetics of viral shedding is based on data from patients who are treated according to a nationally promulgated program, including combinations of antiviral drugs, antibiotics, corticosteroids, immunomodulators, and traditional Chinese medicine preparations, which may change the kinetic pattern of shedding .
At present, there are many factors that affect the effectiveness of outbreak prevention and control measures, including the interval between successive cases in the chain of transmission, and the time from exposure to symptoms (latency). If the continuous interval is shorter than the incubation period, it means that the virus may have spread before the dominant symptoms. Therefore, the prevention and control measures taken at the time of the occurrence of symptoms may have a “shrinking” effect on the spread of infection.
The author of the article points out that if more than 30% of the cases are caused by pre-symptomatic or asymptomatic transmission, tracking only contacts and quarantined cases may not be able to successfully suppress the outbreak and prevent the outbreak. Urgent consideration should be given to adopting a more comprehensive contact tracking standard to track people contacted by COVID-19 patients 2-3 days before the onset of symptoms.