By LAU­RAN NEER­GAARD | AS­SO­CI­AT­ED PRESS

(AP) — Par­ents anx­ious to fi­nal­ly vac­ci­nate their youngest chil­dren against COVID-19, strap in: A lot is set to hap­pen over the next week.

On Wednes­day, both Mod­er­na and Pfiz­er will have to con­vince what’s es­sen­tial­ly a sci­ence court — ad­vis­ers to the Food and Drug Ad­min­is­tra­tion — that their shots work well in ba­bies, tod­dlers and pre-school­ers.

Kids un­der 5 are the on­ly group not yet el­i­gi­ble for COVID-19 vac­ci­na­tion in the U.S. If the agency’s ad­vis­ers en­dorse one or both shots for them -- and the FDA agrees -- there’s still an­oth­er hur­dle. The Cen­ters for Dis­ease Con­trol and Pre­ven­tion must rec­om­mend whether all tots need im­mu­niza­tion or just those at high risk from the virus.

Adding to the com­plex­i­ty, each com­pa­ny is of­fer­ing dif­fer­ent dose sizes and num­ber of shots. And the week won’t even start with the lit­tlest kid de­bate: Mod­er­na first will ask FDA’s ad­vis­ers to sup­port its vac­cine for old­er chil­dren.

On­ly a hand­ful of coun­tries, in­clud­ing Chi­na and Cu­ba, have of­fered dif­fer­ent types of COVID-19 vac­ci­na­tions to chil­dren younger than 5.

Here’s a primer to help keep all the de­vel­op­ments straight.

PFIZ­ER’S PLAN FOR THE LIT­TLEST

Pfiz­er has a pe­di­atric track record -- its COVID-19 vac­cine is the on­ly type the FDA al­lows for chil­dren of any age. Two dos­es plus a boost­er are cleared for every­one 5 and old­er. Shots for the 5- to 11-year-olds con­tain a third of the dose giv­en to teens and adults.

For kids younger than 5, Pfiz­er and its part­ner BioN­Tech low­ered the dose even more, to a tenth of the adult dose. The trade-off is a need for three shots, the first two giv­en three weeks apart and the last at least two months lat­er.

MOD­ER­NA’S PLAN FOR THE LIT­TLEST

Mod­er­na is seek­ing FDA clear­ance for two shots, each a quar­ter of its adult dose, giv­en about four weeks apart for kids younger than 6. (Mod­er­na test­ed slight­ly dif­fer­ent age groups than Pfiz­er.)

The FDA cur­rent­ly al­lows Mod­er­na’s vac­cine to be used on­ly in adults. But some coun­tries al­low full-size dos­es for teens and half-size shots for kids ages 6 to 11.

MAK­ING THEIR CASE

Pfiz­er dis­ap­point­ed par­ents back in De­cem­ber when a study found two shots weren’t quite strong enough. So re­searchers test­ed a third shot in young­sters age 6 months through 4 years dur­ing the win­ter surge of the omi­cron vari­ant.

Pfiz­er’s pre­lim­i­nary da­ta showed af­ter three shots, chil­dren de­vel­oped high lev­els of virus-fight­ing an­ti­bod­ies with no safe­ty prob­lems. In ad­di­tion, the vac­cine ap­peared 80% ef­fec­tive in pre­vent­ing symp­to­matic COVID-19. But that cal­cu­la­tion was based on just 10 cas­es di­ag­nosed among study par­tic­i­pants by the end of April, and it could change if more cas­es have oc­curred since.

Mod­er­na’s study found tots ages 6 months through 5 years de­vel­oped high an­ti­body lev­els af­ter two of its shots. But while there were no se­vere cas­es of COVID-19 dur­ing the tri­al, the vac­cine was on­ly about 40% to 50% ef­fec­tive at pre­vent­ing milder in­fec­tions. Mod­er­na has be­gun test­ing a boost­er for tots.

WHEN COULD SHOTS START?

If the FDA au­tho­rizes one or both shots — a de­ci­sion ex­pect­ed short­ly af­ter its ad­vi­so­ry pan­el’s meet­ing — all eyes move to the CDC. That agency rec­om­mends how to use vac­cines. Which tots should get COVID-19 vac­ci­na­tion will be an im­por­tant de­bate as the coro­n­avirus doesn’t tend to make chil­dren as sick as adults yet near­ly 500 deaths in U.S. chil­dren un­der 5 have been re­port­ed.

The CDC’s own vac­cine ad­vis­ers are sched­uled to meet next Fri­day and Sat­ur­day, and a fi­nal de­ci­sion by the CDC’s di­rec­tor, Dr. Rochelle Walen­sky, is ex­pect­ed short­ly af­ter they’re done.

If all those steps fall in­to place, vac­ci­na­tions are ex­pect­ed to be­gin in most ar­eas June 21.

VAC­CI­NA­TION SITES

Pe­di­a­tri­cians, oth­er pri­ma­ry care physi­cians and chil­dren’s hos­pi­tals are ex­pect­ed to vac­ci­nate most of the youngest kids. Lim­it­ed drug­stores are ex­pect­ed to of­fer them for the lit­tlest kids — par­ents should check lo­cal avail­abil­i­ty. The Biden ad­min­is­tra­tion says it al­so is work­ing with a va­ri­ety of oth­er groups, such as chil­dren’s mu­se­ums, to of­fer pop-up clin­ics and reach even more young­sters.

WHAT IF MY CHILD RE­CENT­LY HAD COVID-19?

About three-quar­ters of chil­dren of all ages are es­ti­mat­ed to have been in­fect­ed at some point dur­ing the pan­dem­ic. It’s a ques­tion sure to come up as CDC’s ad­vis­ers make rec­om­men­da­tions for the lit­tlest kids, so stay tuned.

For old­er ages, the CDC has rec­om­mend­ed vac­ci­na­tion any­way to low­er the chances of re­in­fec­tion. There’s no firm guid­ance on how long to wait; the CDC has said peo­ple may wait three months af­ter an in­fec­tion.

DE­MAND IS UN­CLEAR

There are rough­ly 18 mil­lion chil­dren younger than 5, and many par­ents are ea­ger to get their tots vac­ci­nat­ed. But it’s un­clear how many ul­ti­mate­ly will, giv­en dis­ap­point­ing vac­cine up­take by old­er chil­dren.

Ac­cord­ing to the CDC, just 29% of kids ages 5 to 11 have got­ten two dos­es, and about 60% of 12- to 17-year-olds.

WHAT ABOUT MOD­ER­NA AND OLD­ER KIDS?

On Tues­day, the FDA’s ad­vis­ers will con­sid­er whether to clear Mod­er­na shots for old­er kids, those 6 to 17 -- a de­ci­sion that might al­le­vi­ate some par­ent con­fu­sion.

The agency has held up the teen vac­cine for months while it in­ves­ti­gat­ed a rare side ef­fect, heart in­flam­ma­tion. That’s most­ly a risk for teen boys and young men, and al­so can oc­cur with the Pfiz­er vac­cine.

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The As­so­ci­at­ed Press Health and Sci­ence De­part­ment re­ceives sup­port from the Howard Hugh­es Med­ical In­sti­tute’s De­part­ment of Sci­ence Ed­u­ca­tion. The AP is sole­ly re­spon­si­ble for all con­tent.