Routine vaccination rates for children have significantly dropped since the start of the coronavirus pandemic, leading doctors to worry that there could be outbreaks of vaccine-preventable diseases, including ones that carry more risk to children than COVID-19.

The number of routine vaccines distributed through the Centers for Disease Control and Prevention’s Vaccines for Children program — which provides immunizations for 16 diseases to roughly 50% of American children — has dropped by 13.1 million in the period covering 2020 and 2021.

It’s “a worrisome drop,” a CDC spokesperson told HuffPost, suggesting that. children are missing their regular checkups and falling behind on their immunization schedules, making them increasingly vulnerable to otherwise preventable diseases and infections, like measles, meningitis and diphtheria.

“As far as we know, it’s not because people are refusing vaccines. It’s simply because they were not visiting the doctor to the same degree,” Dr. Sean O’Leary, who serves as vice chair of the American Academy of Pediatric’s Committee on Infectious Diseases and is a pediatric infectious diseases professor at the University of Colorado School of Medicine, told HuffPost.

O’Leary, who last year co-authored an article on similar vaccination drops in Colorado, said that in some cases, parents have been too afraid to leave the house to see a doctor because of the coronavirus, or they figured their doctor’s office was closed. Because sports activities stopped during the pandemic, as did in-person learning in many places, physicals and vaccinations also weren’t required for kids as they usually are. 

“So a lot of kids just didn’t go in for well-care in the prior school year and that’s often where kids get vaccinated,” he said.

Lower-than-usual vaccination rates have been reported across the world. In July, the World Health Organization and UNICEF warned that about 23 million children did not receive their basic childhood vaccines in 2020 — the highest amount since 2009. A similar modeling study published in The Lancet, also in July, found that routine vaccinations for at least 17 million children worldwide were interrupted in 2020 because of the pandemic.

There has been some catch up in vaccinations since the start of these declines, but “we’re not back to where we need to be,” O’Leary said.

“It’s got a lot of us, quietly behind the scenes, very worried about outbreaks of some of these other diseases,” he added. “Frankly, a lot of the diseases we vaccinate kids for are actually more severe in kids than COVID.”

These vaccine-preventable diseases include polio, which can cause paralysis and death. There’s also the human papillomavirus (HPV), some strains of which can cause cancer. HPV — which is normally contracted in teens, according to the National Foundation for Infectious Diseases (NFID) — is annually responsible for around 35,000 cases of cancer in Americans. 

Meningococcal disease and whooping cough can also cause serious health complications and death, with 21% of meningococcal cases in the U.S. occurring in those aged 11 to 24. Of those cases, 10 to 15% will die, according to the NFID.

Then there’s measles. The highly contagious virus was declared eliminated in the U.S. in 2000 thanks to high vaccination rates, but it sprang back within unvaccinated communities in 2018 and 2019.

O’Leary called these recent outbreaks a “perfect example” of how high vaccination rates can successfully isolate a virus, preventing statewide or nationwide outbreaks and then be brought back by unvaccinated populations.

The measles outbreaks were “almost exclusively in pockets with low vaccination coverage,” he said. “Because we had higher vaccination coverage in the surrounding communities, it didn’t turn into a statewide or nationwide outbreak. The concern now is if one of those outbreaks were to happen, because we’ve seen this global dip in measles vaccination coverage, we could see those outbreaks extend beyond and more into the general population.”

There have been just six confirmed measles cases in the U.S. so far this year, according to CDC data.

Dr. Jonathan Mosser, who co-authored the vaccine modeling study in The Lancet and leads the University of Washington’s Vaccine Coverage and Vaccine-Preventable Disease team, repeated the need for children to be caught up on their vaccinations now that schools are reopening and virus control measures, like mask mandates, are being relaxed.

“The last thing that we need is a dual surge of COVID and influenza — or measles, or pertussis, or other vaccine-preventable diseases — this winter,” he told HuffPost in an email. “We need everyone who is eligible to get both the flu and COVID vaccines, make sure that children are up-to-date on their routine immunizations, and use other measures when needed — like masking and physical distancing — to make it through this winter together.”