The Centers for Disease Control and Prevention updated its guidance for people who are isolating at home with Covid-19 to prevent transmission of the virus.
They offer one strategy based on time and symptoms, and another approach based on testing.
Someone who has tested positive for Covid-19 and has symptoms may discontinue isolation 10 days after the symptoms first appeared so long as 24 hours have passed since the last fever without the use of fever-reducing medications, and if symptoms such as coughing and shortness of breath have improved.
People with Covid-19 symptoms isolating at home and with access to tests can leave isolation if a fever has passed without the use of medication, if there is an improvement in symptoms, and if tests taken more than 24 hours apart come back negative, according to the guidelines.
The revised guidelines were posed online Friday. The CDC also updated guidance for people who are in isolation after testing positive for Covid-19 but who don't have symptoms. The agency recommended two options: a time-based strategy and a test-based strategy.
A person without symptoms can discontinue isolation 10 days after the first positive test and if they have not subsequently developed symptoms.
"Because symptoms cannot be used to gauge where these individuals are in the course of their illness, it is possible that the duration of viral shedding could be longer or shorter than 10 days after their first positive test," the CDC warned.
Viral shedding means a person can pass the virus to someone else.
If a person develops symptoms, then the symptom-based or test-based strategy should be used, according to the guidelines.
People who have tested positive for Covid-19 and are asymptomatic can also discontinue isolation if the results of two tests taken more than 24 hours apart come back negative.
The decision of ending isolation "should be made in the context of local circumstances," the CDC advised. Health care workers who are in close contact with vulnerable populations and people who are immunocompromised — which could prolong viral shedding after recovery — are recommended to isolate for longer.
The CDC noted the updated guidance may "appear in conflict" with the recommendations for people known to have been exposed to the virus. The agency recommends a 14-day quarantine after exposure, based on the time it takes to develop illness from the virus.
It is possible that a person known to be infected could leave isolation earlier than a person who is quarantined because of the possibility they are infected," the agency explained.
Previous guidance from May 3 had extended the home isolation period from seven to 10 days since symptoms first appeared or after the first positive test. The CDC said this update was made "based on evidence suggesting a longer duration of viral shedding."
The agency warned these recommendations "will prevent most, but cannot prevent all, instances of secondary spread."
"The risk of transmission after recovery is likely substantially less than that during illness; recovered persons will not be shedding large amounts of virus by this point, if they are shedding at all," the agency said.