By ZEKE MILLER | AS­SO­CI­AT­ED PRESS

WASH­ING­TON (AP) — Pres­i­dent Joe Biden test­ed pos­i­tive for COVID-19 again Sat­ur­day, slight­ly more than three days af­ter he was cleared to ex­it coro­n­avirus iso­la­tion, the White House said, in a rare case of “re­bound” fol­low­ing treat­ment with an an­ti-vi­ral drug.

White House physi­cian Dr. Kevin O’Con­nor said in a let­ter that Biden “has ex­pe­ri­enced no re-emer­gence of symp­toms and con­tin­ues to feel quite well.”

In ac­cor­dance with Cen­ters for Dis­ease Con­trol and Pre­ven­tion guide­lines, Biden will re-en­ter iso­la­tion for at least five full days. The agency says most re­bound cas­es re­main mild and that se­vere dis­ease dur­ing that pe­ri­od has not been re­port­ed.

Word of Biden’s pos­i­tive test came — he had been neg­a­tive Fri­day morn­ing — just two hours af­ter the White House an­nounced a pres­i­den­tial vis­it to Michi­gan this com­ing Tues­day to high­light the pas­sage of a bill to pro­mote do­mes­tic high-tech man­u­fac­tur­ing. Biden had al­so been sched­uled to vis­it his home in Wilm­ing­ton, Delaware, on Sun­day morn­ing, where first la­dy Jill Biden has been stay­ing while the pres­i­dent was pos­i­tive. Both trips have been can­celled as Biden has re­turned to iso­la­tion.

Biden, 79, was treat­ed with the an­ti-vi­ral drug Paxlovid, and test­ed neg­a­tive for the virus on Tues­day and Wednes­day. He was then cleared to leave iso­la­tion while wear­ing a mask in­doors. His pos­i­tive tests puts him among the mi­nor­i­ty of those pre­scribed the drug to ex­pe­ri­ence a re­bound case of the virus.

Both the Food and Drug Ad­min­is­tra­tion and Pfiz­er point out that 1% to 2% of peo­ple in Pfiz­er’s orig­i­nal study on Paxlovid saw their virus lev­els re­bound af­ter 10 days. The rate was about the same among peo­ple tak­ing the drug or dum­my pills, “so it is un­clear at this point that this is re­lat­ed to drug treat­ment,” ac­cord­ing to the FDA.

While Biden was test­ing neg­a­tive, he re­turned to hold­ing in-per­son in­door events and meet­ings with staff at the White House and was wear­ing a mask, in ac­cor­dance with CDC guide­lines. But the pres­i­dent re­moved his mask in­doors when de­liv­er­ing re­marks on Thurs­day and dur­ing a meet­ing with CEOs on the White House com­plex.

Asked why Biden ap­peared to be breach­ing CDC pro­to­cols, Press Sec­re­tary Karine Jean-Pierre said, “They were so­cial­ly dis­tanced. They were far enough apart. So, we made it safe for them to be to­geth­er, to be on that stage.”

Reg­u­la­tors are still study­ing the preva­lence and vir­u­lence of re­bound cas­es, but the CDC in May warned doc­tors that it has been re­port­ed to oc­cur with­in two days to eight days af­ter ini­tial­ly test­ing neg­a­tive for the virus.

“Lim­it­ed in­for­ma­tion cur­rent­ly avail­able from case re­ports sug­gests that per­sons treat­ed with Paxlovid who ex­pe­ri­ence COVID-19 re­bound have had mild ill­ness; there are no re­ports of se­vere dis­ease,” the agency said at the time.

When Biden was ini­tial­ly re­leased from iso­la­tion on Wednes­day, O’Con­nor said the pres­i­dent would “in­crease his test­ing ca­dence” to catch any po­ten­tial re­bound of the virus.

Pa­tients should re-iso­late for at least 5 days. Per CDC guid­ance, they can end their re-iso­la­tion pe­ri­od af­ter 5 full days if fever has re­solved for 24 hours (with­out the use of fever-re­duc­ing med­ica­tion) and symp­toms are im­prov­ing. The pa­tient should wear a mask for a to­tal of 10 days af­ter re­bound symp­toms start­ed.”

Paxlovid has been proven to sig­nif­i­cant­ly re­duce se­vere dis­ease and death among those most vul­ner­a­ble to COVID-19. U.S. health of­fi­cials have en­cour­aged those who test pos­i­tive to con­sult their doc­tors or phar­ma­cists to see if they should be pre­scribed the treat­ment, de­spite the re­bound risk.

Biden is ful­ly vac­ci­nat­ed, af­ter get­ting two dos­es of the Pfiz­er coro­n­avirus vac­cine short­ly be­fore tak­ing of­fice, a first boost­er shot in Sep­tem­ber and an ad­di­tion­al dose March 30.

While pa­tients who have re­cov­ered from ear­li­er vari­ants of COVID-19 have tend­ed to have high lev­els of im­mu­ni­ty to fu­ture re­in­fec­tion for 90 days, Jha said that the BA.5 sub­vari­ant that in­fect­ed Biden has proven to be more “im­mune-eva­sive.”

“We have seen lots of peo­ple get re­in­fect­ed with­in 90 days,” he said, adding that of­fi­cials don’t yet have da­ta on how long those who have re­cov­ered from the BA.5 strain have pro­tec­tion from re­in­fec­tion.