The C.D.C. urges parents to get childhood vaccinations up to date following a steep decline last year.

Pediatricians are urging U.S. parents to get their children caught up on routine vaccinations, following a decline in the number of inoculations for diseases like measles as the pandemic forced restrictions, including shelter-at-home orders, last year.

New data from 10 jurisdictions that closely monitor immunizations confirm that the number of administered vaccine doses plunged between March and May of last year, especially among older children, the Centers for Disease Control and Prevention reported on Thursday.

Though vaccinations rebounded between June 2020 and September 2020, approaching pre-pandemic levels, the increase was not enough to make up for the earlier drop, the study found.

Vaccinations are required for attendance at most schools, camps and day care centers, but the authors of the C.D.C. study warned that the lag nonetheless “might pose a serious public health threat that would result in vaccine-preventable disease outbreaks.”

They expressed concern that the transition to remote learning during the pandemic may have hobbled enforcement of vaccination requirements, noting that even temporary declines in immunization can compromise herd immunity.

In 2018-2019, a measles outbreak occurred in Rockland County, N.Y., and nearby counties after measles vaccination coverage in area schools dropped to 77 percent, below the 93 percent to 95 percent figure needed to sustain herd immunity. “Pediatric outbreaks of vaccine-preventable diseases have the potential to derail efforts to reopen schools” in the fall, the researchers added.

Parents should plan ahead and schedule appointments now so that their children can be protected, said Dr. Yvonne Maldonado, who chairs the committee on infectious diseases at the American Academy of Pediatrics.

“We should start thinking about it,” Dr. Maldonado said in a phone interview. “People forget. We have regular pertussis outbreaks every four or five years, and are just waiting to see another one.”

“We’re probably going to start seeing more infections, because kids are going to get back together and there’s going to be less masking and social distancing,” she added.

The C.D.C. analyzed data from nine states and New York City. In eight of the jurisdictions, some form of stay-at-home order was issued last spring.

The number of administered doses of diphtheria, tetanus and pertussis vaccines (DTaP) dropped 15.7 percent among children under age 2, and 60 percent among those aged 2 to 6 in the spring of last year, compared with the same period in 2018 and 2019.

Doses of measles, mumps and rubella vaccine (MMR) declined by 22.4 percent among 1-year-olds, and 63 percent among those aged 2 to 8.

HPV vaccine administration declined by more than 63 percent among youngsters aged 9 to 17, compared with the same period in 2018 and 2019; and doses of Tdap (tetanus, diphtheria and pertussis) decreased by over 60 percent.