The Incubation Period of The New Coronavirus SARS-CoV-2

The new coronavirus SARS-CoV-2 (previously known as 2019-nCoV) caused the 2019 coronavirus disease (COVID-19), which is now raging globally. According to three new studies, after infection with the new coronavirus  SARS-CoV-2, it takes an average of 5 days to develop the disease, and patients lose the most coronavirus particles at the beginning of the disease.

According to media reports and the first study published in the JAMA journal last month, researchers from the Central South Hospital of Wuhan University in China studied 4 patients. These four patients were exposed to  SARS-CoV-2 infection as medical professionals during their work, and three of them developed fever, cough, or both. Initially, one patient was asymptomatic, but a thin-layer CT examination was performed due to contact with a person infected with  SARS-CoV-2. The RT-PCR results of these 4 patients were all positive, and CT images showed ground-glass opacity or ground-glass opacity accompanied by pulmonary consolidation. The severity of the disease is mild to moderate.

Provide antiviral therapy to these 4 patients (75 mg oral oseltamivir every 12 hours). For 3 of them, all clinical symptoms and CT imaging abnormalities have disappeared. The CT image of the fourth patient showed slight ground glass turbidity. All 4 patients had negative results in two consecutive RT-PCR tests. The time from symptom onset to recovery is 12 to 32 days.

After being discharged from hospital or discontinuing isolation, the four patients were asked to continue isolation at home for 5 days. The RT-PCR test was repeated after 5 to 13 days, and all tests were positive. All patients underwent 3 repeated RT-PCR tests within the next 4 to 5 days and they were all positive. Additional RT-PCR tests were performed using kits from other manufacturers and the results were positive for all patients. Through clinician examination and chest CT examination, these patients are still asymptomatic, and there is no change compared with previous chest CT images. They did not report contact with anyone with respiratory symptoms. No family members are infected.

Given that after recovering from  coronavirus infection and being discharged from the hospital, the pharyngeal swabs from these 4 patients tested positive for RT-PCR. This phenomenon has raised concerns that patients may continue to infect others after the disease has passed for a long time.

However, a second study published on the preprint server medRxiv on March 9, 2020 suggested that patients with milder symptoms will shed viral particles within 10 days or less of onset.

Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota (not involved in these three studies), said, "This is a very important contribution to understanding the natural history of Covid-19's clinical disease and the impact of viral shedding on public health. "

The second study was conducted on 9 patients in Germany. Its authors found that in the early stages of the disease, these patients had a high load of infectious virus particles in their throat and lung samples, peaking four days after the onset of symptoms. However, although viral RNA can still be observed in these samples after the symptoms have resolved, for those with milder symptoms, there are no detectable infectious virus particles eight days after the onset of symptoms. No infectious virus particles were detected in feces, blood or urine.

The authors write, "Based on the current findings, patients with symptoms over 10 days and less than 100,000 copies of viral RNA per milliliter of sputum can choose to be discharged early and isolated at home." Review.

In the third study published in the Annals of Internal Medicine journal on March 10, 2020, the researchers analyzed news reports and news articles concerning the dates of contact and symptoms of  COVID-19 patients. In the 181 cases they found, the median time between exposure and symptom onset was 5.1 days, and 97.5% of the sick patients were sick within 11.5 days of exposure.

Co-author of the paper, Justin Lessler, an epidemiologist at the Bloomberg School of Public Health at Hopkins University in the United States, said in a press release, "Based on our analysis of public data, we are currently proposing to monitor or isolate for 14 days. It is reasonable, although some cases will be missed for a long time during this period. "

Graham Cooke, an infectious disease expert at Imperial College London, said, "It is reassuring that by the 14th day, although it may not be 100%, it will also be very close." Cooke reminded that if no symptoms appear after five days, Then people who have a history of contact should not consider themselves harmless. He said, "This is definitely the wrong interpretation. Five days later, half of the people have not yet developed symptoms."