The youngest U.S. children may not be able to get vaccinated at the local pharmacy. Here’s why.

U.S. children as young as 6 months are now eligible for coronavirus vaccines, and many parents are breathing a sigh of relief. But the pharmacy that offered easy access to immunization to you or an older child might not accommodate the under-5 group.

That is not necessarily a surprise. Many pharmacies do not normally give shots to very young children. But parents should know that age requirements may vary among chains.

Under the federal Public Readiness and Emergency Preparedness (PREP) Act, pharmacists can give Covid vaccines to all children ages 3 and older during the public health emergency. This effectively gets around the patchwork of widely varying state-level laws around vaccination against diseases, some of which limit pharmacists to administering vaccines to adults and others that set minimum ages that can range from 3 to 17.

While the majority of big pharmacy chains are giving Covid vaccines starting at age 3, some are not. Publix and Kroger, for example, are offering shots starting at age 5.

CVS is offering Covid shots to children as young as 18 months at stores that house its MinuteClinic locations, but set the age minimum at 5 for other stores.

It was not clear on Thursday why some pharmacies set their age limits where they had. Erin Rolfes, a spokeswoman for Kroger, declined to comment on reasons its pharmacies had 5 set as the lower limit. Publix did not respond to messages seeking comment. A CVS spokeswoman, Amy Thibault, said only that the chain was using pharmacists, pharmacy interns and pharmacy technicians to administer shots to children 5 and older.

Mitchel Rothholz, who leads immunization advocacy at the American Pharmacists Association, said all pharmacists are trained to administer vaccines to those ages 3 and older, but some may prefer to go through additional training before working with children younger than that.

“Whether the practice does it or not is based on their comfort level and resource availabilities,” he said.

Even if your pharmacy of choice is not offering shots to your eligible child, the pharmacist there can point you in the right direction. “I always encourage parents and caregivers to have a discussions with the pharmacy or reach out to their pediatrician,” Mr. Rothholz said.

Asked about many pharmacies’ policies to vaccinate children 3 and older on Thursday, Dr. Rochelle Walensky, the director of the Centers for Disease Control and Prevention, said “much of this may be related to the PREP Act, but also ability of pharmacies to handle younger kids.”

She also pointed to what she described as “the willingness of parents to potentially want to take their younger kids to where they routinely get vaccinated, whether it’s their local community health center or their local pediatrician.”

Some parents have already encountered roadblocks.

Kelly Jensen, of Woodstock, Ill., said she placed at least 10 phone calls early this week to locate a vaccine for her 14-month-old daughter. Many larger pharmacies are not administering the shot to children that young, and Ms. Jensen said her pediatrician was not offering the vaccine to young children, either, citing waning interest.

“The thing that is so frustrating is that I could find no information,” Ms. Jensen, 37, said. “I felt like I was losing my mind trying to find anything.”

Other kinds of clinics could be a better option for many parents but some parents have turned to social media groups and grass-roots networks to find pharmacies that can accommodate their children.

A group of volunteers called Vaccinate Under 5 created a national database of pharmacies and providers administering the vaccine to young children. The map, populated by online submissions from parents and doctors, was up and running with appointments for children under 5, the group said.

“We didn’t want more hurdles to access when parents have already been through so much,” the group said in a statement. “As parents ourselves, we were waiting months for the choice to vaccinate and provide them with the same baseline immunity that other ages have been able to access for months.”

On Tuesday, Ms. Jensen drove to a doctor’s office 30 minutes from her home, where she was then told the vaccine was not available as she had been notified. A day later, the office called her to notify her that the vaccine was in stock. She rushed over.

“I had my child during a pandemic,” Ms. Jensen said. “She’s now over a year old in a pandemic, and to finally have some kind of help for her poor little system is just like, now it feels like maybe we can sort of have a normal, normal-ish childhood for her.”

Here are the policies for some major pharmacy chains: