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One study reported that children were

Concentrations of SARS-CoV-2 RNA in upper respiratory specimens decline after onset of symptoms.(6-11)
Several studies have found similar concentrations of SARS-CoV-2 RNA in upper respiratory specimens from children and adults.(12-18)
To date, most studies of SARS-CoV-2 transmission have found that children and adults have a similar risk of transmitting SARS-CoV-2 to others.
One study reported that children were more likely to transmit SARS-CoV-2 than adults >60 years old.(19)
Certain SARS-CoV-2 variants of concern are more transmissible than the wild type virus or other variants, resulting in higher rates of infection. People infected with the Delta variant, including fully vaccinated people with symptomatic breakthrough infections, can transmit infection to others. However, like other variants, the amount of virus produced by Delta breakthrough infections in fully vaccinated people decreases faster than in unvaccinated people. This means fully vaccinated people are likely infectious for less time than unvaccinated people.
The likelihood of recovering replication-competent (infectious) virus is very low after 10 days from onset of symptoms, except in severely ill or immunocompromised people.
For patients with mild-to-moderate COVID-19, replication-competent virus has not been recovered after 10 days following symptom onset for most patients.(8, 9, 20- 24) Outliers exist; in one case report, an adult with mild illness provided specimens that yielded replication-competent virus for up to 18 days after symptom onset.(25)
Recovery of replication-competent virus between 10 and 20 days after symptom onset has been reported in some adults with severe COVID-19; some of these individuals were immunocompromised.(7)However, in this series of patients, it was estimated that 88% and 95% of their specimens no longer yielded replication-competent virus after 10 and 15 days, respectively, following symptom onset.
Detection of sub-genomic SARS-CoV-2 RNA or recovery of replication-competent virus has been reported in severely ill or severely immunocompromised patients beyond 20 days, and as long as 144 days after a positive SARS-CoV-2 test result.(1-5)
Prolonged detection of replication-competent virus may be associated with other factors. For example, a 13-year-old immunocompetent male was hospitalized for injuries received in a motor vehicle crash. He required intubation, developed pulmonary infiltrates, and tested positive for SARS-CoV-2. Viral cultures of upper and lower respiratory tract specimens were positive for SARS-CoV-2 on days 47 and 54 of his hospitalization.(26)
The risk of SARS-CoV-2 transmission to others varies based upon several factors including time after symptom onset, virus variant, virus levels in the upper respiratory tract, and disease status (asymptomatic, pre-symptomatic, or symptomatic).
In a large contact tracing study, no contacts developed SARS-CoV-2 infection if their exposure to a COVID-19 case patient occurred 6 days or more after the case patient’s symptom onset.(27)
One study published in January 2021 reported that 59% of SARS-CoV-2 transmission originated from index cases that were asymptomatic or pre-symptomatic.(28)
A meta-analysis published in February 2021 found that the secondary attack rate for asymptomatic (never develop symptoms) index cases was 1.9%, but was 9.3% for pre-symptomatic and 13.6% for symptomatic index cases.29 Therefore, individuals with SARS-CoV-2 infection without symptoms pose a transmission risk and should isolate based upon CDC’s quarantine and isolation recommendations.
People who have recovered from COVID-19 may have prolonged detection of SARS-CoV-2 RNA.(30) However, prolonged detection of viral RNA does not necessarily mean that such people are a transmission risk. Studies of patients who were hospitalized and recovered indicate that SARS-CoV-2 RNA can be detected in upper respiratory tract specimens for up to 3 months (12 weeks) after symptom onset.(19, 24, 25)
Investigation of 285 “persistently SARS-CoV-2 RNA positive” adults, which included 126 adults who had developed recurrent symptoms, found no secondary infections among 790 contacts. Efforts to isolate replication-competent virus were attempted for 108 of these 285 case patients, and SARS-CoV-2 was not recovered in viral culture from any of the 108 specimens.”(24)
The probability of SARS-CoV-2 reinfection may increase with time after recovery, consistent with other human coronaviruses, because of waning immunity and the possibility of exposure to viral variants.(31-39) The risk of reinfection also depends on host susceptibility, vaccination status, and the likelihood of re-exposure to infectious cases of COVID-19. Continued widespread transmission makes it more likely that reinfections will occur.
Loss of taste and smell may continue for weeks or months after recovery.(40) The presence of these symptoms does not mean that the isolation period must be extended.
Limitations of Current Evidence
Studies referenced in this document may have differences compared to the current epidemiology of COVID-19 in the United States. Specifically, many of these references involve non-US populations, homogenous populations, virus transmission prior to the availability of vaccination for COVID-19, and infection prior to the known circulation of SARS-CoV-2 current variants of concern, such as the Delta variant. More studies are needed to fully understand virus transmission related to the Delta variant and other SARS-CoV-2 variants among the fully vaccinated.
Studies have used viral culture to attempt to grow SARS-CoV-2 from clinical samples from patients who tested positive for SARS-CoV-2 to determine infectiousness. Because viral culture must be done in very specialized laboratories, there is a limited number of these studies currently available.
Many studies that assessed the duration of SARS-CoV-2 infectiousness have been conducted in adults. More studies are needed, especially in children with SARS-CoV-2 infection.
More data are needed to understand the frequency and duration of infectious SARS-CoV-2 shedding among the spectrum of mild to severely immunocompromised people, including both asymptomatic and symptomatic people.

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