The figure is the highest in the world with DTP-3 coverage falling from 91 per cent to 85 per cent. In 2019, about 1.4 million children had missed their first dose.
While disruptions in immunisation services were widespread in 2020, the middle-income countries in the WHO Southeast Asian and Eastern Mediterranean Regions were the most affected.
Compared with 2019, 3.5 million more children missed their first dose of diphtheria, tetanus and pertussis vaccine (DTP-1) while three million more children missed their first measles dose.
The study, based on data from 160 countries, suggests that Covid-19 may likely lead to the resurgence of measles, polio and other killers as the pandemic unravels years of progress in routine immunisation and exposing millions of children to deadly, preventable diseases.
"This evidence should be a clear warning -- the Covid-19 pandemic and related disruptions cost us valuable ground we cannot afford to lose -- and the consequences will be paid in the lives and wellbeing of the most vulnerable," said Henrietta Fore, UNICEF Executive Director, in a statement.
"Even before the pandemic, there were worrying signs that we were beginning to lose ground in the fight to immunise children against preventable child illness, including with the widespread measles outbreaks two years ago. The pandemic has made a bad situation worse. 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," Fore added.
Globally, 23 million children missed out on basic vaccines through routine immunisation services in 2020 -- 3.7 million more than in 2019, the data showed.
Up to 17 million of these likely did not receive a single vaccine during the year, widening already immense inequities in vaccine access.
Most of these children live in communities affected by conflict, in under-served remote places, or in informal or slum settings where they face multiple deprivations including limited access to basic health and key social services.
"Even as countries clamour 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," said Dr Tedros Adhanom Ghebreyesus, WHO Director-General.
"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," he added.
Further, vaccinations against human papillomavirus (HPV) -- which protect girls against cervical cancer later in life -- have also been highly affected by school closures.
As a result, across countries that have introduced HPV vaccine to date, approximately 1.6 million more girls missed out in 2020. Globally only 13 per cent girls were vaccinated against HPV, falling from 15 per cent in 2019.
In addition to routine immunization disruptions, there are currently 57 postponed mass vaccination campaigns in 66 countries, for measles, polio, yellow fever and other diseases, affecting millions more people.