In epidemiology, a virus’s basic reproduction number— alternatively referred to as its Ro, denotes the estimated quantity of cases generated by a single carrier in a susceptible population.
Determining Ro as early as possible is an important step in suppressing a pathogen’s spread.
It has been previously determined that the majority of SARS-CoV-2 transmissions are staffed by young people and travel-associated clusters, and now a new study has located the approximate length of time that subjects remain highly contagious.
“Based on the present findings, early discharge with ensuing home isolation could be chosen for patients who are beyond day 10 of symptoms with less than 100,000 viral RNA copies per ml of sputum,” the authors explained in a media release.“There is little residual risk of infectivity, based on cell culture.”
The researchers found that patients shed high levels of viral debris linked to Covid-19 early on in their infection. This even appears to be true of patients who are asymptomatic.
By the same token, carriers who evidence mild forms of Covid-19 may test positive 10 days after symptoms subside, but are not likely very infectious.
“This is a very important contribution to understanding both the natural history of Covid-19 clinical disease as well as the public health implications of viral shedding,” said Michael Osterholm, director of the University of Minnesota’s Center for Infectious Diseases Research and Policy.
In order to test their hypothesis, the researchers based out of Berlin and Munich attempted to grow the virus out of secretion samples submitted by infected participants. After the eighth day of illness, the authors were unable to successfully grow SARS-CoV-2 viruses from throat swabs or sputum specimens. Viral load was calculated via fragments of RNA found in urine, blood, septum, and stool.
The current Covid-19 tests look for tiny sections of the virus’s RNA as well. Testing positive is a good indicator of whether or not a patient is shedding viral material, but it isn’t a flawless measure of their infectious potential.
All but two of the patients involved in the paper stop emitting viral debris after day five of infection. The remaining two, who continued to shed passed day 10, evidenced more aggressive forms of Covid-19, which supports the theory that severity reliably predicts rate of transmission.
This unusual pathology has complicated surveillance tactics and relief efforts alike. People with Covid-19 emit roughly 1,000 times more virus early on compared to peak-transmission observed in patients with SARS infection. Moreover, SARS carriers tend to become gradually more infectious as the ensuing disease progresses. This means patients with Covid-19 are most contagious before symptoms begin, or while the symptoms are extremely mild.
The vast majority of people infected with SARS-Cov-2 develop antibodies within twelve days of contracting the virus, which precludes the need for medical intervention.
“The present study shows that COVID-19 can often present as a common cold-like illness. SARS-CoV-2 can actively replicate in the upper respiratory tract, and is shed for a prolonged time after symptoms end, including in stool. These findings suggest adjustments of current case definitions and re-evaluation of the prospects of outbreak containment,” the authors conclude.