A vaccine protects the child who receives it. Whether a disease keeps circulating in a population is a separate question, and it depends on how many people in that population are immune.
Why does a vaccinated child not settle the question?
An infection travels as a chain: each case has to find new susceptible people to infect. Every immune person is a dead end for the organism, because the organism arrives and has nowhere to go. One child’s immunity does not stop a chain by itself, but many immune children together can.
Herd immunity is the state in which most of a population has been immunised. The pathogen still exists, but it cannot spread for want of susceptible hosts, because each case meets too few of them.
This matters most for the people who cannot be protected directly, since they are protected by the immunity of those around them. They include:
- the infant too young for the first dose;
- the child with a contraindication;
- the child whose treatment suppresses the immune system.
The organism rarely reaches these children, and that is their protection.
Why do some diseases need more coverage than others?
If the immunity of the others protects the few, how much of it is enough? A schedule is judged by its coverage as much as by its products, because a vaccine that is not delivered protects no one. A modest fall in coverage can be followed by a large change in disease. As the susceptible share grows, each case finds more people to infect, and an organism that was held back can start spreading again.
The coverage required rises with how transmissible the pathogen is. Transmissibility is expressed as the basic reproduction number: the average number of people one case infects in a fully susceptible population. A more contagious organism infects more people per case. It can therefore keep transmission going even when many of its contacts are immune, and it can sustain transmission in a population with fewer susceptible people. That is why a larger immune share is needed to stop it.
The same reasoning holds for pathogens that spread less efficiently, and they need lower coverage.
What is the target for measles, and where can a good average hide a gap?
Measles has one of the highest basic reproduction numbers of any human pathogen, as the note on Pediatric Vaccine-Preventable Diseases: The Mandatory Schedule explains. So what does its target look like in practice? Eliminating measles requires coverage of 93% to 95% with two doses. Two doses are needed because the second dose closes the gap left by children who do not respond to the first, as the note on Pediatric Vaccines: Types and the Italian Schedule describes. The WHO target for measles elimination is at least 95% coverage with both doses.
The 95% target is to be sustained in every district, not reached only on average. A national figure is an average over places, and it can hide a cluster of unvaccinated children. Unvaccinated children tend to live and mix near one another, in a community or a school. The contacts of a susceptible child are then likely to be susceptible too, and the cluster can keep spread going even when the national figure meets the target.
Measles is the most sensitive indicator of a coverage gap. Its threshold is the highest, so it has the least slack. It spreads so efficiently that a small deficit is enough to sustain an outbreak.
What has happened where coverage fell?
The picture is clearest at the scale of the world and of Europe.
An estimated 207,500 people died worldwide of measles in 2019, although the measles vaccine is cheap and effective. The gap has not closed since 2019. In 2023 there were an estimated 10.3 million measles cases worldwide, 20% more than in 2022, and 107,500 measles deaths, mostly in children under 5 years.
In 2024 the WHO European Region reported 127,350 measles cases. This was double the number reported for 2023 and the highest the Region had seen since 1997. Children under 5 years accounted for 43% of these cases, and nearly 74,000 people were admitted to hospital.
None of this reflects a failure of the vaccine. Two doses of a measles-containing vaccine prevent about 97% of cases in the children who receive them. Because of this, a resurgence points at delivery rather than at the product.
Italy: what did the law change, and is it enough?
The point is clearest in one country where coverage fell and then rose. In Italy, reported polio coverage fell from about 96% in 2011 to about 93% in 2016. Law 119 of 31 July 2017 then extended the vaccinations mandatory for minors up to 16 years from four to ten. After the law, coverage rose.
For measles, first-dose coverage at 24 months was 87.3% in 2016 and 91.8% in 2017. It reached 94.1% at 30 months as at June 2018. These are figures at two different ages, so they are not one series.
The recovery was real, and the margin it leaves is narrow. Measles needs sustained two-dose coverage of 93% to 95%. Coverage that is improving is not yet coverage that is sufficient.
Polio: why does a disease near eradication still come back?
For the disease closest to eradication, what does falling short look like? Polio is the clearest example of a disease that vaccination has brought to the edge of eradication without finishing the job. Wild poliovirus type 1 remains endemic in Afghanistan and Pakistan. The global case count has fallen to the tens: 99 cases in 2024. WHO still treats international spread of poliovirus as a public health emergency of international concern. That emergency status is a statement about coverage rather than about the virus.
In 2022 two events showed what this means. Virus was detected in London sewage, and a paralytic case occurred in New York State in an under-vaccinated community. Why do a sewage finding and a single case matter so much? Poliovirus multiplies in the gut and is shed in faeces, so sewage can reveal virus circulating in a community. Most infections do not cause paralysis, so a paralytic case suggests that other infections have gone unseen. Both of these points are developed in the note on Pediatric Vaccine-Preventable Diseases: The Mandatory Schedule.
Both events involved circulating vaccine-derived poliovirus type 2, not the wild virus. This is virus descended from the live oral vaccine strain, spreading among under-vaccinated people. That strain replicates and can, rarely, revert, as the note on Pediatric Vaccines: Types and the Italian Schedule explains.
Type 2 virus was found repeatedly in sewage from north and east London. The paralytic case was a young adult in Rockland County, New York, who had never been vaccinated. The London sewage virus and the Rockland County virus were genetically linked to each other and to samples from Jerusalem. The oral vaccine that can seed such viruses is no longer used in Italy.
Re-emergence was driven by under-vaccination, not by a new strain. The outbreak was a coverage problem with a coverage remedy.
The same discipline at two scales
Coverage is the population-scale version of the discipline that antibiotic stewardship applies to one child, as the note on Pediatric GAS Pharyngitis: Treatment and Antibiotic Stewardship sets out. The discipline is to use the narrowest effective intervention, and to finish it. A ten-day antibiotic course stopped on day three and a two-dose measles schedule allowed to drift below its threshold are the same mistake at different scales. In both cases the cost is paid later, often by people who were not the ones being treated.
If preventing the spread of an infection and treating one child follow the same discipline, how should a clinician see a sore throat and a vaccine schedule as one skill?