Study suggests shortening quarantine for people infected by novel coronavirus to ten days would be risky | AGÊNCIA FAPESP

Study suggests shortening quarantine for people infected by novel coronavirus to ten days would be risky In October the CDC recommended ten instead of 14 days of isolation for patients with mild or moderate symptoms, but Brazilian researchers found viable viral particles in 25% of samples collected from patients on the tenth day of symptoms (Camila Romano with lab technician Anderson Vicente de Paula; photo: Virology Lab, LIMHC/USP)

Study suggests shortening quarantine for people infected by novel coronavirus to ten days would be risky

March 03, 2021

By Karina Toledo  |  Agência FAPESP – Research conducted at the University of São Paulo’s Institute of Tropical Medicine (IMT-USP) in Brazil suggests it may be risky to shorten the period of isolation recommended for people with mild and moderate cases of COVID-19 from 14 days to ten days, as recommended in October by the United States Centers for Disease Control and Prevention (CDC).

In the study, which was supported by FAPESP, the IMT-USP research team worked with 29 nasopharyngeal swab samples from patients with an RT-PCR confirmed COVID-19 diagnosis. The material was collected at a public primary health center in Araraquara, in the state of São Paulo, on the tenth day after the onset of symptoms, and inoculated into laboratory-cultured cells.

In 25% of the cases, the viruses present in the samples were capable of infecting the cells and replicating in vitro. In theory, therefore, people who came into contact with droplets of saliva expelled by 25% of these patients during the period in which the material was collected could still be contaminated. The complete research findings are published on the preprint platform medRxiv in an article that has yet to be peer-reviewed.

“Infected people with mild symptoms are advised to remain totally isolated at home, without contact with anyone, throughout the quarantine period. There’s strong pressure to reduce the period of isolation, in light of both economic and psychological factors, but if the point of isolation is to mitigate the risk of transmitting the virus, 25% [of patients with replication-competent viruses] is a very high proportion,” said Camila Romano, principal investigator for the study, told Agência FAPESP.

The 14-day quarantine was established early in the pandemic, she added, on the basis of the average time taken for the RT-PCR test to become negative after the onset of symptoms. Most of these initial studies involved people with moderate or severe COVID-19 who had to be treated in hospital.

“It was assumed that a viral load low enough not to be detected by an RT-PCR test, considered the gold standard for diagnosing COVID-19, made the risk of transmission very small. At that time there weren’t enough test kits to diagnose all suspected cases, let alone to waive the isolation requirement for patients with mild symptoms,” Romano said. “So 14 days became the standard period of isolation for people with the virus who didn’t need to go to hospital.”

Later research showed that viral RNA can be detected in the respiratory tract by the RT-PCR test for periods longer even than 14 days. However, according to the same studies, after the eighth or ninth day of symptoms, it was highly unusual to detect viable viral particles capable of replication in cells from patients with mild or moderate COVID-19.

As a result, in mid-2020 the CDC revised its isolation or quarantine recommendations. For people exposed to the virus but not diagnosed by RT-PCR, isolation for ten days was said to be sufficient to reduce the risk of transmission to 1%. For confirmed cases with mild or moderate symptoms, isolation could end ten days after symptoms began, provided the individual had no fever for at least 24 hours. The period should be extended, however, for severe patients with compromised immune systems or individuals who continued to experience symptoms.

“In Brazil, the rule is still 14 days, although some cities are thinking about reducing quarantine to ten days,” Romano said. “In Switzerland and some other countries, people with mild symptoms are released from isolation after only seven days. As more studies are conducted for different populations and with more sensitive methodologies, we can see that it’s early days to come to a final decision on the ideal duration of quarantine. Countries are experiencing fresh waves of the pandemic, with more and more plumping for a 14-day quarantine rule. It’s important to take the most recent data into account when reformulating policy on isolation.”


The study described in the article is part of an ongoing project designed to evaluate household transmission of SARS-CoV-2 in Araraquara, where lockdown was implemented on February 15, 2021, after the P1 variant was detected in patients there.

Thanks to a partnership with city managers, the IMT-USP team was able to contact patients with mild symptoms who tested positive by RT-PCR at a local health center and were not hospitalized.

Fifty-three people were invited to participate. They were aged between 17 and 60 and tested positive on the tenth day of symptoms. Only 29 of the 53 samples collected contained sufficient material that was well-conserved and could be used in the experiments.

In a laboratory certified for Biosafety Level III (BSL-3) at IMT-USP, the samples were incubated with Vero cells, derived from the kidney of an African monkey and widely used in cultures for coronavirus studies.

“The experiment consisted of offering the virus a suitable environment in which to replicate,” Romano said. “We inoculated the cultured cells with the nasopharyngeal secretion collected from the patients and monitored the cells for four or five days.” This was enough time to see if contact with the virus had a cytopathic effect, in which cells undergo pathological changes. The variation in viral load was measured using the RT-PCR technique.

In 25% of the cases analyzed, a significant cytopathic effect was observed, as well as an increase in viral load.

“A lab experiment can’t perfectly reproduce what happens in nature, of course, but our findings suggest there may be viable viral particles in patients’ secretions on the tenth day of symptoms,” Romano said.

The researchers are now conducting trials to see how the dynamics of transmission risk varies in any given patient. Samples are being collected every day between the ninth and 14th day of symptoms. This material will be inoculated into cell cultures and measurements taken to find out how fast the proportion of samples with viable virus declines over time.

According to Romano, the results obtained so far reinforce the importance of maintaining 14 days as the standard duration of quarantine. “Generally speaking, isolation needs to be intensified right now. Otherwise, the slow advance of vaccination will exert selective pressure on the virus and favor the emergence of resistant variants. Relaxing isolation is extremely dangerous at this time,” she warned.

The article “Discontinuation of isolation for persons with COVID-19: Is 10 days really safe?” can be read at:




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