Pauline OdhiamboThe day measles reached her village, Aisha Mwangi was carrying her four-year-old son, Kito, on her hip. She had carried him like that since he was a baby—through the lockdowns, through the empty clinics, through the lingering fear that Covid-19 had sewn into the very air they breathed.
Kito was born in 2021, deep in the pandemic’s grip.
The health post in her rural community in southeastern Kenya was understaffed and overwhelmed. The outreach vans stopped coming. By the time Aisha felt safe enough to bring him in, the window for his first vaccines had passed.
“They said, ‘He is too old now for the routine schedule,’” she tells TRT Afrika while sitting on a wooden stool outside her home, the afternoon sun casting long shadows. “So I waited. I hoped. I prayed.”
‘Zero-dose’ children
She didn’t know it then, but Kito was one of millions—a “zero-dose child,” in the language of global health. For two years, he remained invisible to the system, vulnerable to diseases that had been nearly forgotten. Then, in early 2024, a community health worker arrived with a message. There was a new effort, she said. It was called the Big Catch-Up. And it was for children exactly like Kito.
“They came with a cooler box, a smile, and a needle,” Aisha says, her voice softening. “Kito cried. But I cried more. Because for the first time, someone remembered him.”
That memory—of being remembered—is now shared by an estimated 18.3 million children across 36 countries, most of them in Africa and Asia.
Between 2023 and 2025, the Big Catch-Up (BCU) delivered more than 100 million doses of life-saving vaccines, according to a joint announcement from Gavi, the Vaccine Alliance, the World Health Organization (WHO), and UNICEF at the start of World Immunization Week on April 24-30.
Among those reached: 12.3 million “zero-dose” children who had never received a single vaccine, and 15 million who had never received the measles vaccine—a critical gap in a time of surging outbreaks.
For Aisha, the numbers became real one afternoon when Kito, now five and full of energy, ran through the compound without a cough, without a rash, without the fever that had swept through a neighboring village the previous season.
“He is protected,” she says. “That is worth more than gold.”
Reaching the unreachable
The Big Catch-Up was born out of a crisis.
When the COVID-19 pandemic disrupted health systems worldwide, routine immunization plunged to levels not seen in a generation. But unlike previous diphtheria-tetanus-pertussis (DTP) campaigns that focused on infants, BCU did something unprecedented: it systematically targeted older children—aged 1 to 5—the ones who had slipped through the cracks and grown older waiting for a second chance.
The results have been striking. In Ethiopia alone, more than 2.5 million previously zero-dose children received their first DTP1 vaccine. The country also delivered nearly 5 million doses of inactivated polio vaccine (IPV) and over 4 million doses of measles vaccine. Nigeria reached 2 million zero-dose children with DTP1 and administered 3.4 million IPV doses. Twelve countries, including Burkina Faso, Niger, Somalia, Tanzania, and Zambia, reported reaching more than 60% of their zero-dose children under five.
But behind those numbers are stories like Aisha’s—and like that of Mamadou Diallo, a senior nurse in a crowded clinic on the outskirts of Niamey, Niger. For him, the BCU was not a statistic but a daily, exhausting, hopeful grind.
“Before the Big Catch-Up, we would see a mother with a three-year-old and say, ‘Sorry, you are too late,’” Mamadou says, wiping sweat from his brow after a long morning of vaccinations. “The schedule said the first vaccine must be before one year. So we turned them away. Or they turned away themselves, ashamed. That was the old way.”
He pauses, gesturing to the line of mothers and children snaking outside his clinic door.
“This initiative changed our training. It changed the policy. We learned to screen every child who walks in—no matter their age. We learned to ask: ‘Has this child had anything? Anything at all?’ And then we caught them up. All of them.”
Cautious triumphs
Mamadou recalls one boy, four years old, who had never seen a needle. The mother was terrified that vaccines would harm him.
“We sat with her for an hour. We explained. We showed her the vials. She finally agreed. When her son received his first shot—just a simple polio drop—she wept. She said, ‘I thought you had forgotten us in this neighborhood.’ That is the heart of it. These are not ‘missed children.’ They are forgotten families. And the BCU helped us find them.”
Despite the triumph, the three agencies are sounding an urgent alarm. As of 2024, an estimated 14.3 million infants under the age of one still failed to receive a single vaccine through routine immunization programs. Measles cases surged to roughly 11 million globally in 2024, with the number of countries facing large outbreaks nearly tripling since 2021.
“The Big Catch-Up has shown what is possible when governments, partners, and communities work together,” says Dr. Sania Nishtar, CEO of Gavi. “But catch-up is a bridge. It is not the destination. Routine immunization remains the most sustainable way to protect every generation.”
Echoing that sentiment is Dr. Tedros Adhanom Ghebreyesus, Director-General of WHO.
“The success of the Big Catch-Up is a testament to health workers and national immunization programs,” he says. “But we cannot rest. Millions of infants born this year will also need protection.”
UNICEF Executive Director Catherine Russell adds a sobering note: “We have caught up with some of the children who missed routine vaccinations during the pandemic—but many more remain out of reach. The gains must be sustained through investment in strong, reliable immunization systems.”
For Aisha Mwangi, those global debates feel distant. What matters is what is happening in her own home.
“Kito just started school,” she says, smiling. “He plays with other children. He eats with his hands. He is not afraid of the nurse anymore. And I am not afraid of the next outbreak.”
She lifts her son onto her lap. “They say this catch-up is over now. But I hope they remember us again. Because there are still children out there. Waiting.”
