US President Joe Biden tested positive again in what his doctor described as a "rebound" case (Photo: Getty)
Joe Biden tested positive for Covid at the weekend.
While he’s far from the only world leader to contract the disease over the last few years, this is the second time he’s tested positive in a matter of days – despite testing negative for a brief spell in between.
The US President first tested positive on July 21, and was experiencing very mild symptoms, before appearing to make a full recovery.
The White House physician Kevin O’Connor has since explained: “After testing negative on Tuesday evening, Wednesday morning, Thursday morning and Friday morning, the President tested positive late Saturday morning, by antigen testing.
“This in fact represents ‘rebound’ positivity.”
The note claimed Biden felt no reemergence of symptoms, and continues to “feel quite well”, so treatment was not resumed. The president is still under close observation and isolating.
But just what is “rebound” positivity, and how common is it?
So what is a “rebound infection”?
This is when patients appear to recover from Covid, test negative, before experiencing the return of symptoms and then testing positive again for two to eight days later.
It is not particularly common though, as Biden pointed out in his tweet from Saturday: “Folks, today I tested positive for Covid again. This happens with a small minority of folks.”
What does it have to do with antivirals?
In Biden’s case, his doctor noted this can occur in people who took the antiviral medication Paxlovid to treat the initial infection.
The drug, created by Pfizer, is used to prevent severe illness in newly infected at-risk patients (age can raise your risk – Biden is 79).
Tablets are taken twice a day for five days.
It’s been made available to people in the US under emergency use authorisation by the Food and Drug Administration for those aged 12 and older, and particularly vulnerable people in the UK can receive the medication through the NHS.
The CDC said there was “currently no evidence” anyone who experienced rebound cases needed a second course of Paxlovid.
Rebound infections can also occur in those who do not take Paxlovid, and appear to impact both the unvaccinated and vaccinated.
Is the second infection better or worse?
According to the British Medical Journal, there have been no reports of severe illness in any of those who experience this Covid rebound.
Most appear to recover and stop testing positive around three days later, without additional treatment.
The US Centres for Disease Control and Prevention (CDC) said: “A brief return of symptoms may be part of the natural history of SARS-CoV-2 infection in some people, independent of treatment with Paxlovid and regardless of vaccination status.”
How is it different to reinfection?
Previously, patients who make a full recovery from Covid appear to be protected for around three months after initially contracting the illness through natural immunity.
The CDC currently believe these are not cases of reinfection.
Therefore it seems more likely to be the tail-end of the first infection.
What do you do if you think you have a rebound infection?
Patients have been advised to re-isolate for at least five days if they test positive again, or start to experience more symptoms. The CDC have also urged people to wear a mask for 10 days should they test positive again, too.
This article originally appeared on HuffPost UK and has been updated.