US President Joe Biden is back in isolation after testing positive for COVID-19 a few days after he was cleared of the infection following his treatment with antiviral medication Paxlovid.
His doctor, Kevin O'Connor, said Mr Biden didn't have a re-emergence of symptoms and was "quite well".
Rather, Dr O'Connor pointed to it being a so-called "rebound" case.
What's the timeline?
Mr Biden first tested positive for the virus on July 21 — less than a fortnight ago.
He tested negative on Tuesday evening.
But Dr O'Connor said Mr Biden kept up testing because of "the potential for so-called 'rebound' COVID positivity observed in a small percentage of patients treated with Paxlovid".
He continued to test negative on Wednesday, Thursday and Friday.
But Mr Biden went back into isolation after he tested positive again on Saturday morning.
"This in fact represents 'rebound' positivity," Dr O'Connor said.
Mr Biden returned another positive test on Sunday, but still felt well.
What is COVID rebound?
The US Centers for Disease Control and Prevention (CDC) describes it as:
"... a recurrence of COVID-19 symptoms or a new positive viral test after having tested negative."
There have been reports of this happening between two and eight days after recovery, according to a CDC advisory published in late May:
"Based on information from the case reports, COVID-19 rebound did not represent reinfection with SARS-CoV-2 or the development of resistance to Paxlovid."
These rebound cases usually clear up in an average of three days.
The CDC also said a brief resurgence of symptoms might "be part of the natural history" of the virus that causes COVID-19 for some people, regardless of their vaccination status or whether or not they've had Paxlovid.
How common is COVID rebound after Paxlovid?
Estimates vary.
In May, a post on the US Food and Drug Administration (FDA) website said Paxlovid clinical trial data showed that after completing their Paxlovid treatment course, 1 to 2 per cent of patients either:
- tested positive for COVID on a PCR after testing negative; or
- had an increase in the amount of the virus detected by a PCR test after finishing treatment
In June, a US study, which is yet to be peer-reviewed, found a rebound rate of 3.53 per cent of its participants in the seven days after Paxlovid treatment and 5.4 per cent in the 30 days after the treatment.
Researchers said the group of COVID rebound patients had a "significantly higher" prevalence of underlying medical conditions.
What is Paxlovid?
It's an antiviral treatment for COVID-19 which is approved for use in Australia — but only for people who meet certain criteria.
It has two active substances, nirmatrelvir and ritonavir, which are taken as separate tablets.
Patients take two doses a day for five days.
Who is eligible for antiviral treatments in Australia?
The eligibility criteria for antiviral treatments in Australia were expanded last month.
Here's who can access the treatments:
- People aged 70 and older who test positive for COVID-19 — with or without symptoms, regardless of risk factors
- People aged 50 and older with two or more risk factors for severe illness
- Aboriginal or Torres Strait Islander people aged 30 years and older with two or more risk factors for severe illness
- Anyone aged 18 and older who is moderately to severely immunocompromised, has a disability with multiple conditions and/or frailty or has cerebral palsy or down syndrome may also be eligible
