Q&A: Equity in Immunization

With Stephanie Shendale, Scientist, WHO's Department of Immunization, Vaccines & Biologicals.

1. Who are zero-dose children, and why are they a key indicator of inequity in immunization?

Zero-dose children are programmatically defined as infants who have not received their first dose of the diphtheria, tetanus, and pertussis (DTP)-containing vaccine by their first birthday.

Zero-dose children are a key indicator of inequity in immunization because their status rarely stems from a simple, isolated oversight. Zero-dose children are usually concentrated in marginalized and deprived environments, including remote rural regions, dense urban slums, and areas impacted by conflict or humanitarian crises. Because immunization coverage is higher than that of other health interventions, a child who misses vaccinations is often also affected by broader systemic issues, including weak local health infrastructure, social or ethnic discrimination, and limited maternal education. Consequently, mapping zero-dose children not only tracks gaps in immunizations; it uncovers deeper, structural inequalities where entire communities are routinely excluded from the most fundamental components of primary healthcare. 

2. What are the main reasons children miss vaccinations?

Children miss life-saving vaccinations due to overlapping structural and socio-economic barriers, including poverty, limited access to quality healthcare and basic services such as clean water and sanitation, illiteracy, and gender inequality. Conflict and instability can also displace health workers, disrupt supply chains, damage roads and electricity grids, and make it dangerous for families to travel to health facilities for immunization services.

Beyond structural barriers, demand for vaccination also plays an important role. Gender norms and social dynamics affect whether caregivers – particularly mothers – can seek or consent to vaccination. Misinformation, mistrust of health systems, and logistical barriers — including distance, cost of travel, and health worker shortages — all compound one another, meaning that the children most at risk are often the hardest to reach through conventional immunization delivery models. 

3. What made the “Big Catch-Up” initiative unique in its approach to reaching zero-dose children?

Launched during World Immunization Week 2023, the Big Catch-Up (BCU) was a multi-year, multi-country effort to address vaccination declines driven, in part, by disruptions to services during the COVID-19 pandemic. What set the BCU apart was the deliberate focus to catch up children over the age of one that were previously missed by routine immunization systems. Although WHO recommends catch-up vaccination policies for children who miss routine doses, many countries do not have systems in place to reach and vaccinate zero-dose or under-immunized children after their first or second birthdays. The BCU focused on the accumulated cohorts of older children up to 5 years old — children who should have received critical routine vaccines before the age of 1 — while strengthening long-lasting systems to identify, screen, vaccinate, and monitor coverage rates in these older children. 

4. Why is strengthening routine immunization systems essential to closing gaps in vaccine equity?

Periodic, campaign-based catch-up efforts, however well designed, can only ever be a gap-filling measure. Timely vaccination, according to national immunization schedules and embedded in primary healthcare, remains the most effective and sustainable way to protect children and communities. Routine immunization programmes provide a sustainable, continuous safety net that protects children right from birth, preventing immunity gaps from forming in the first place.

Strengthening these systems means investing in resilient primary healthcare infrastructure, including securing stable long-term financing, ensuring robust cold chain and logistics systems, and maintaining a well-trained health workforce. When routine systems are strong, immunization becomes an integrated, predictable service rather than an intermittent event. This ensures that everyone, everywhere, has access to reliable, life-saving care on an ongoing and equal basis. 

5. What lessons learned from the “Big Catch-Up” should be taken forward to meet global targets of reducing the number of un- and under-immunized children?

First, the BCU shows that targeted political commitment and dedicated financing can make a measurable difference. When countries have the resources, tools and political will, children routinely missed by immunization programmes can be reached.

Second, reaching zero-dose children becomes harder the older they get as they have fewer contacts with health services after their first year. Additionally, the perceived importance of vaccination for older children among health workers and caregivers in some communities is often lower than for infants. This also impacts routine vaccination coverage in the second year of life (2YL) which remains lower than infant immunization in many settings. More efforts are needed to promote the importance of vaccination across the life course and ensure health workers and caregivers understand that it is better to vaccinate late than never.

Looking ahead, however, the most important lesson may be the BCU's own warning about its limitations: large-scale catch-up efforts are resource-intensive and should serve only as a gap-filling measure that is complementary to routine immunization. Timely vaccination – that is, reaching children on time with all vaccines recommended through the routine immunization schedule – must remain the goal. 

Global targets for immunization, including reducing zero-dose children, are based on coverage achieved within the first year of life, underscoring the importance of reaching children with life-saving vaccines as early as possible, and strengthening systems to deliver so that no future generations of children require another Big Catch-Up.