World Immunization Week, marked each year from April 24 to April 30, arrived this year with a set of numbers public health officials had been warning about since the previous autumn. The World Health Organization estimates that 10.3 million people contracted measles in 2023, a 20 percent jump from the year before. The WHO European Region recorded 127,350 confirmed cases in 2024, the highest total there in more than 25 years. Measles is back not because the virus changed, but because the vaccinated share of the world's children stopped rising.
The campaign has been promoted by WHO since 2012 as a moment for countries to close routine immunization gaps. Two doses of measles-containing vaccine, the measles, mumps, and rubella vaccine in most high-income countries and measles-rubella combinations elsewhere, are about 97 percent effective at preventing disease. No antiviral can cure measles once it takes hold. The prevention target is precise: at least 95 percent of children need two doses to stop sustained spread. In 2023, first-dose coverage held at 83 percent and second-dose at 74 percent, essentially unchanged from 2022. The distance between 83 and 95 is where outbreaks happen.
Coverage in the Eighties Is Not Enough
Measles has an R0, or basic reproduction number, commonly estimated between 12 and 18. A single case in a susceptible group will, on average, infect more than a dozen others; influenza's R0 sits closer to 1.3. The virus can remain airborne for up to two hours after an infected person has left a room, which means a clinic waiting room or a school hallway can transmit disease long after the index case has gone. That biology makes suboptimal coverage uniquely punishing. A room where 85 percent of people are immune may feel safe, but the remaining 15 percent is enough to sustain a chain.
Coverage also has to be uniform, not just high. Because measles spreads so efficiently, pockets of unvaccinated children can anchor an outbreak even when district-wide figures look respectable. Epidemiologists distinguish between average coverage and herd immunity. Herd immunity requires enough immunity distributed evenly to extinguish transmission chains before they grow. The global average masks exactly what measles exploits: clusters of missed children living near one another, often in the same schools or neighborhoods.
What 2023 and 2024 Numbers Show
WHO and the US Centers for Disease Control and Prevention jointly estimate that 10.3 million measles cases occurred in 2023, with 107,500 deaths. The death count fell 8 percent from 2022, an improvement in case management rather than prevention. Large or disruptive outbreaks hit 57 countries in 2023, up from 36 the year before. The map has no single centre: outbreaks struck in countries with weak routine immunization and in countries with well-funded health systems that had let coverage erode by a few percentage points.
The European Region's 2024 count sharpens the second half of that sentence. The region's 127,350 cases were more than double the 2023 figure and the highest since 1997. Romania reported 30,692 cases; Kazakhstan followed with 28,147. Children under five made up 40 percent of cases. That same region had once been held up as proof that measles elimination was achievable outside the Americas, which makes the reversal important: it shows how quickly ten years of progress can be undone when routine immunization is interrupted and catch-up campaigns are delayed.
Access Gaps Precede Hesitancy
It would be tempting to locate the entire explanation in parental refusal. That explanation is convenient because it places responsibility on individuals rather than systems. It does not follow, however, that hesitancy is the main driver of the 22 million children who missed a first measles dose in 2023. In many of the countries now reporting outbreaks, the binding constraint is that vaccine never reached the district refrigerator, the outreach session was cancelled because of conflict, or the family could not afford the transport and lost wages required to return when stock arrived.
Even in Europe, access rather than refusal explains much of the surge. WHO Europe and UNICEF pointed to missed routine doses during pandemic-era disruptions and uneven catch-up campaigns. Romania's 30,692 cases in 2024 reflected, in part, cohorts of children who had missed doses years earlier and were never caught up. Stock-outs are a mundane failure with large consequences. Measles-containing vaccine must be kept between 2°C and 8°C, and a power cut can spoil an entire month's supply without anyone knowing the vials have lost potency. When parents arrive at a clinic and find no vaccine, the cost falls on them. The second visit is the one that matters, and many families cannot make it. That is why coverage data often show a steep drop between first and second doses: first doses reach clinics during mass campaigns, while second doses depend on routine services that are harder to sustain.
A Texas Outbreak Shows High-Income Countries Are Not Exempt
The United States declared measles eliminated in 2000, meaning continuous domestic transmission had stopped for at least 12 months. Elimination is not the same as eradication: imported cases still arrive and can seed outbreaks where local coverage slips. The outbreak that began in West Texas early in 2025 was a case study. More than 400 cases were reported in the first months of the outbreak, and two people died. They were the first measles deaths in the United States since 2015. The outbreak spread through an undervaccinated community and into neighboring New Mexico, where a resident who had not been vaccinated also died.
The Texas outbreak did not happen because measles vaccine was unavailable. It happened because the political and social infrastructure of vaccination had frayed in one community. School immunization exemptions had been rising in parts of the state, and health workers had to rebuild trust one family at a time while the outbreak was already moving. The US response also exposed a contradiction: a country with ample vaccine supply spent months fighting a disease it had eliminated a quarter-century earlier because routine coverage in a single county had been allowed to fall below the threshold that matters.
The Infrastructure Response
WHO's Immunization Agenda 2030 frames the long-term task. Endorsed by member states, it calls for 90 percent coverage of essential vaccines and for halving the number of children who receive no vaccine at all. The agenda treats routine immunization as a health system function, not a campaign afterthought. That means paying for the cold chain, the trained vaccinators, the district-level data systems, and the outreach workers who can reach children that clinics never see.
Several specific changes would matter more than another global statement.
- Catch-up campaigns need to be timed around school entry and child health weeks, not only outbreak response.
- Cold-chain equipment and generator fuel deserve the same budget line as vaccine doses.
- Surveillance for fever and rash must be linked to routine health information systems so a rise in cases is visible before it becomes an outbreak.
- Local health workers need delegated authority to vaccinate outside fixed clinics, including markets and transit points where working families already gather.
What Precedent This Week Should Set
World Immunization Week usually produces pledges. The more difficult question is what those pledges are worth in the districts where coverage is failing. A global week of speeches will not matter to a family in a conflict zone if the cold-chain lorry cannot pass a checkpoint, or to a school nurse in West Texas trying to build trust with a parent who has heard one too many contradictory claims. The precedent set now will be measured not by the number of countries that endorsed a statement, but by whether the vaccine actually reaches the children who were counted as missed last year. If the same 22 million are missing from next year's coverage table, the world will know that access, not exhortation, was the gap all along.
