(CNN) The coronavirus pandemic may have caused the "most widespread" disruption of routine childhood vaccinations in recent history, a new modeling study suggests.
At least 17 million children worldwide missed routine vaccinations because of the pandemic, according to the study's estimates, published Wednesday in the journal The Lancet.
Unvaccinated or under-vaccinated children are vulnerable to outbreaks of disease, and public health officials around the world are worried.
"Routine immunization services faced stark challenges in 2020, with the COVID-19 pandemic causing the most widespread and largest global disruption in recent history," wrote the researchers, from the University of Washington in Seattle, the World Health Organization in Geneva and the Pan American Health Organization in Washington, DC.
"This analysis offers, to our knowledge, the first modeled assessments of global and regional disruptions to vaccination coverage, by month, during the COVID pandemic in 2020," the researchers wrote.
The researchers analyzed immunization data from 1980 to 2019 to estimate how many routine vaccinations would have been expected for 2020 if the pandemic never happened -- and compared their models with actual data on vaccinations in 2020. The study included data on global mobility and routine immunization reports from last year.
The researchers found that, from January to December 2020, an estimated 30 million children who were eligible to receive the third dose of diphtheria-tetanus-pertussis vaccine and 27.2 million children eligible to receive the first dose of measles vaccine had missed those doses. The estimates represent about 8.5 million more children without DTP third doses and about 8.9 million more children without a dose of measles vaccine than were estimated to miss doses if the pandemic hadn't happened.
The researchers wrote that the reductions in vaccinations represented a 7.7% drop for DTP third doses and 7.9% drop for the first dose of measles vaccine. These disruptions in vaccinations impacted both high-income and low-income nations alike.
The findings suggest that, among all regions, south Asia experienced the largest acute declines in vaccinations, with DTP third doses administered falling by 58% and measles first doses falling by 43% in April of last year. Since then, improvements occurred, with vaccinations in south Asia nearing what would have been expected if the pandemic hadn't happened by the end of 2020, the researchers found.
In the United States, and other high-income regions, estimated DTP third doses administered fell by 22% and the number of measles first doses fell by 20.5% in April of last year. Then vaccination rates appeared to partially recover through to December, with annual reductions of 6% for DTP third doses and 7.9% for measles first doses.
Disruptions were most severe in April of last year across all regions, with the global number of doses administered falling by 31.3% for DTP third doses and by 30% for measles first doses, the researchers wrote. "The second half of 2020 showed signs of recovery, as global monthly doses administered began nearing expected estimates by December, 2020. Nevertheless, recovery efforts were far from complete," they wrote.
The "expected estimates" for 2020 in the study are still only estimates that the researchers modeled after data from previous years -- as no one really knows how many vaccinations would have occurred last year if the pandemic hadn't happened.
In a commentary that published alongside the study, Kaja Abbas of the London School of Hygiene and Tropical Medicine and Vittal Mogasale of the International Vaccine Instituted commended the new study's findings.
"These modeled estimates are of importance to national immunization programs for planning and implementing catch-up vaccination services to close the immunity gaps and prevent vaccine-preventable disease outbreaks," Abbas and Mogasale wrote. "The urgent need for catch-up vaccination efforts is conspicuous for most contagious childhood diseases, including measles and pertussis."
In separate data, the World Health Organization said Wednesday that 23 million children missed out on basic routine immunizations last year overall. That's 3.7 million more than in 2019, according to data released by WHO and UNICEF.
"Even as countries clamor to get their hands on COVID-19 vaccines, we have gone backwards on other vaccinations, leaving children at risk from devastating but preventable diseases like measles, polio or meningitis," WHO Director-General Tedros Adhanom Ghebreyesus said in the announcement.
"Multiple disease outbreaks would be catastrophic for communities and health systems already battling COVID-19, making it more urgent than ever to invest in childhood vaccination and ensure every child is reached."
Disruptions in immunization services were widespread last year, the announcement noted. WHO said the 10 nations with the greatest increase in children not receiving a first dose of diphtheria-tetanus-pertussis vaccine last year were: India, Pakistan, Indonesia, Philippines, Mexico, Mozambique, Angola, Tanzania, Argentina and Venezuela.
Globally, as compared with 2019, 3.5 million more children last year missed their first doses of DTP vaccine and 3 million more children missed their first measles doses, according to WHO.
"Even before the pandemic, there were worrying signs that we were beginning to lose ground in the fight to immunize children against preventable child illness, including with the widespread measles outbreaks two years ago," Henrietta Fore, UNICEF's executive director, said.
"The pandemic has made a bad situation worse," Fore said. "With the equitable distribution of COVID-19 vaccines at the forefront of everyone's minds, we must remember that vaccine distribution has always been inequitable, but it does not have to be."
The US Centers for Disease Control and Prevention has raised alarm that children in the United States have yet to catch up on their routine immunizations.
A reported published by the CDC in June found that administered vaccine doses of routine childhood immunizations dropped substantially from March through May last year, and although administered doses later increased from June through September, that increase "was not sufficient to achieve catch-up coverage."
The report warns that the "lag in catch-up vaccination might pose a serious public health threat that would result in vaccine-preventable disease outbreaks, especially in schools that have reopened for in-person learning."