The COVID-19 pandemic caused millions of children to miss out on routine vaccines and countries must act now to restore essential immunization coverage to a least 2019 levels, the World Health Organization (WHO), a Geneva-based UN agency, has said. Under the banner of ‘The Big Catch-Up’, WHO and partners will support countries to get back on track so that more children and adults are protected from preventable diseases, as the agency marked the start of World Immunization Week. The ultimate goal is to ensure more children, adults, and their communities, are protected from vaccine-preventable diseases, allowing them to live happier, healthier lives. Particular focus will be given to the 20 countries that are home to three-quarters of the children who missed vaccinations in 2021, which include Pakistan, India, Afghanistan, North Korea, the Democratic Republic of the Congo, Ethiopia, Somalia and Myanmar. The coronavirus pandemic saw essential immunization levels decrease in over 100 countries, leading to rising outbreaks of measles, diphtheria, polio and yellow fever. An estimated 25 million children missed out on vaccination in 2021 alone. The decline in immunization was fueled by factors that included overburdened health services, closed clinics, and disruptions in the import and export of medical supplies such as syringes and vials. At the same time, communities and families experienced lockdowns, which restricted travel and access to health services, while financial and human resources were limited as governments responded to the emergency. WHO also pointed to other contributing factors such as conflicts, climate crises and vaccine hesitancy. World Immunization Week runs through Sunday, and WHO has joined forces with the UN Children’s Fund (UNICEF); Gavi, the Vaccine Alliance, the Bill & Melinda Gates Foundation, and many other global and international partners. WHO said it is also using the week to highlight successful vaccination campaigns underway in several countries, as well as some “bright spots of resilience”, such as India, which saw a strong recovery in essential immunization last year, according to initial reports. Uganda also maintained high coverage levels during the pandemic. Catherine Russell, the UNICEF Executive Director, underscored the need to unite to strengthen services, build trust and save lives. “Routine vaccines are typically a child’s first entry into their health system and so children who miss out on their early vaccines are at added risk of being cut out of health care in the long run,” she said. “The longer we wait to reach and vaccinate these children, the more vulnerable they become and the greater the risk of more deadly disease outbreaks.”