COVID-19 Patients May Be Most Infectious in First Week of Symptoms, Study Finds

Research on nine adults with mild coronavirus symptoms found the virus actively replicating in the throat and lungs, implying high levels of infectious viral 'shedding'

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A digital sign in Las Vegas displays a message about coughing and sneezing due to the continuing spread of the coronavirus across the United States on March 28, 2020.
A digital sign in Las Vegas displays a message about coughing and sneezing due to the continuing spread of the coronavirus across the United States on March 28, 2020.Credit: AFP
Ruth Schuster
Ruth Schuster

A study on mild cases of COVID-19 found proof of active replication of the coronavirus SARS-CoV-2 in the patients’ throats and lungs during their first week of symptoms, implying high levels of infectious viral "shedding" during that time. Shedding is when the host – people in this case – is replicating a virus progeny. The findings of the research, carried out in Munich, Germany, were published in the scientific journal “Nature” on Wednesday.

Viruses cannot reproduce without a host, as the viral particle is inert when isolated. A virus that infects a cell essentially enslaves the cell's mechanisms to produce more infectious virus particles.

Dr. Christian Drosten, the director of the Institute of Virology at Berlin's Charité hospital, and his colleagues conducted the virological analysis on nine adults showing comparatively mild symptoms of COVID-19 respiratory disease, testing for viral shedding in throat and lung samples, sputum (mucus from the respiratory tract), stool, blood, and urine.

The researchers found high levels of virus replication in the upper respiratory tract - from the nose to the bronchi - during the first seven days of symptoms, and especially at the beginning. They also detected high levels of viral shedding in the patients' upper respiratory tract.

Medical employees wait to carry out tests at a coronavirus test center for public service employees in Munich, Germany, March 2020.Credit: Matthias Schrader/AP

The viral RNA concentrations in sputum samples seemed to decline more slowly than in throat swab samples, they report.

An infectious form of the coronavirus could be detected from samples taken from the patients' throat and lungs until day eight of symptoms if the patients' symptoms were waning, the researchers report. But two of the patients showing some early signs of pneumonia continued to produce high levels of the virus in their sputum until day ten or eleven of symptoms, they say.

Some viral RNA remained detectable in the sputum after the end of symptoms; and in six of the patients, it could be detected in sputum and stool samples for as long as three weeks, the team reported.

The good news is that no virus was detected in the patients' blood or urine samples. The stool samples did contain high concentrations of viral RNA during the first week but the researchers report that they detected active virus in only one case.

This study has its caveats, chiefly the small sample size and the fact that it was confined to patients with mild symptoms. But it indicates that in mild cases of the disease, the first week is a critical time for infectiousness, with the viral reproduction at its most efficient, and that shedding from the nose and throat also at its prime.

More research needs to be done, including on patients manifesting more severe forms of COVID-19; and whether an increase in a patient's viral load after the first week of symptoms could signal that their condition will worsen, the authors posit.

The symptoms of COVID-19 usually involve a dry cough, fever, shortness of breath, as well as headache, muscle pain and fatigue. Some report diarrhea as well. The symptoms may only show up five days to two weeks after the infection, which is known as the incubation period.

It bears adding that in contrast to a conventional wisdom making the rounds, a person with symptoms may test positive for flu – and have coronavirus as well. In other words, a positive test for flu doesn't mean you don't have COVID-19 as well.

Coronavirus typically spreads when an infected person coughs or sneezes, producing respiratory droplets containing active virus particles, or by touch. The virus can penetrate the body through the mouth, nose or eyes.

Experts warn that it can survive (meaning remain infectious) on surfaces, for different amounts of time depending on the material of the surface, though the risk of infection diminishes over time. In any case, that is why one should wash one's hands with soap and water for the prescribed 20 seconds after going outside and touching anything, even if the only thing one touched was the doorknob.

Another study published this week based on research done in China reported that the eyes and tears of coronavirus patients may be infectious. 

Various studies done to date have deduced that the SARS-CoV-2 causing COVID-19 particularly attacks the upper respiratory tract, where the epithelial cells have a higher concentration of the molecular sites to which the virus attaches. In the case of the 2003 epidemic SARS (caused by SARS-CoV), it was the lower respiratory tract that had the receptors for the virus. But in severe cases of COVID-19, the virus has spread to the lower respiratory tract as well. There is some evidence that the virus may even attack the kidneys, especially in severe cases. 

COVID-19 emerged in China in late 2019 and causes an acute respiratory tract infection. Its manner of transmission is still being intensely studied. The new study does not prove that it is not transmitted in stool. Other studies found viral shedding in some patients' fecal matter.

In any case, the study definitely demonstrates why people who suspect they may have the condition should quarantine themselves without compromise. As of writing, the confirmed global infection rate had passed 870,000 people, of whom 43,275 died. In Israel, the number of cases had reached 5,591, of whom 21 died.

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