By MIKE STO­BBE-As­so­ci­at­ed Press

NEW YORK (AP) — Three out of every four U.S. chil­dren have been in­fect­ed with the coro­n­avirus and more than half of all Amer­i­cans had signs of pre­vi­ous in­fec­tions, Cen­ters for Dis­ease Con­trol and Pre­ven­tion re­searchers es­ti­mat­ed in a re­port Tues­day.

The re­searchers ex­am­ined blood sam­ples from more than 200,000 Amer­i­cans and looked for virus-fight­ing an­ti­bod­ies made from in­fec­tions, not vac­cines. They found that signs of past in­fec­tion rose dra­mat­i­cal­ly be­tween De­cem­ber and Feb­ru­ary, when the more con­ta­gious omi­cron vari­ant surged through the U.S.

For Amer­i­cans of all ages, about 34% had signs of pri­or in­fec­tion in De­cem­ber. Just two months lat­er, 58% did.

“I did ex­pect it to in­crease. I did not ex­pect it to in­crease quite this much,” said Dr. Kristie Clarke, co-leader of a CDC team that tracks the ex­tent of coro­n­avirus in­fec­tions.

The news came as Pfiz­er sought per­mis­sion to of­fer a boost­er dose to kids ages 5 to 11, just like peo­ple 12 and old­er can get.

In the CDC re­port, the most strik­ing in­crease was in chil­dren. The per­cent­age of those 17 and un­der with an­ti­bod­ies rose from about 45% in De­cem­ber to about 75% in Feb­ru­ary.

The old­er peo­ple were, the less like­ly they had ev­i­dence of past in­fec­tions. That may be be­cause old­er adults have high­er vac­ci­na­tion rates and they may be more like­ly to take oth­er COVID-19 pre­cau­tions, such as wear­ing masks and avoid­ing crowds, Clarke said.

Re­port­ed COVID-19 cas­es had a huge surge in De­cem­ber and Jan­u­ary, then fell al­most as dra­mat­i­cal­ly as they had risen. But dai­ly case counts have been trend­ing up again in re­cent weeks.

The case num­bers are be­lieved to be an un­der­count, but of­fi­cials do think re­cent in­creas­es re­flect a true rise in in­fec­tions. Many COVID-19 in­fec­tions are mild enough that pa­tients do not seek care or con­fir­ma­to­ry lab tests. CDC of­fi­cials say they plan to re­lease a study soon that es­ti­mates that in re­cent months there were three in­fec­tions for every re­port­ed case.

An­oth­er re­cent trend: U.S. health of­fi­cials say they have seen two weeks of in­creas­es in COVID-19 hos­pi­tal­iza­tions, though the num­bers re­main rel­a­tive­ly low. Hos­pi­tal ad­mis­sions num­ber about 1,600 per day, a 9% in­crease in the pri­or week, the CDC re­port­ed.

Avail­able ev­i­dence nev­er­the­less of­fers rea­son to be hope­ful about how the pan­dem­ic is go­ing, of­fi­cials sug­gest­ed.

“We are not an­tic­i­pat­ing more se­vere dis­ease from some of these sub­vari­ants, but we are ac­tive­ly study­ing them,” CDC Di­rec­tor Dr. Rochelle Walen­sky said Tues­day.

The tests that showed how many peo­ple had pre­vi­ous in­fec­tions can de­tect an­ti­bod­ies for one to two years af­ter in­fec­tion, and pos­si­bly longer. Stud­ies have shown pre­vi­ous in­fec­tion can pro­tect some peo­ple against se­vere dis­ease and hos­pi­tal­iza­tion, but CDC of­fi­cials stressed that the pre­vi­ous­ly in­fect­ed should still get COVID-19 vac­cines.

The study looked for any de­tectable lev­el of an­ti­bod­ies; it did not dis­tin­guish how many peo­ple had an­ti­body lev­els that might be pro­tec­tive. Sci­en­tists are still try­ing to un­der­stand what role these kinds of an­ti­bod­ies play in pro­tec­tion from fu­ture virus ex­po­sures.

Of­fi­cials con­tin­ue to urge Amer­i­cans to get vac­cines and boost­ers, which of­fer ad­di­tion­al pro­tec­tion against COVID-19 for all, in­clud­ing those who were pre­vi­ous­ly in­fect­ed.

Cur­rent­ly the U.S. of­fers a boost­er dose start­ing at age 12 but Pfiz­er and BioN­Tech on Tues­day asked the Food and Drug Ad­min­is­tra­tion to al­low healthy el­e­men­tary-age kids to al­so get one -- about six months af­ter their last shot. The com­pa­nies cit­ed a small study show­ing the ex­tra shot for kids ages 5 to 11 revved up an­ti­bod­ies ca­pa­ble of fight­ing the su­per-con­ta­gious omi­cron vari­ant. Pfiz­er’s kid-size shots are a third of the dose giv­en to any­one 12 or old­er.

AP Med­ical Writer Lau­ran Neer­gaard in Wash­ing­ton con­tributed to this re­port.