Vaccines and the Economics of Herd Immunity

Why vaccination has effects on other people beyond the person vaccinated, and what that means for how it should be priced and encouraged.

Most healthcare decisions primarily affect the person making them. Getting vaccinated is different: it protects you, but it also protects the people around you, in a way that changes how economists think this decision should be encouraged, priced, and even mandated.

A positive externality

A positive externality occurs when a transaction or decision creates a benefit for people who weren’t party to it. When you get vaccinated against a contagious disease, you reduce your own risk of getting sick, but you also reduce the chance you’ll pass the disease on to someone else - a benefit that lands on people who had no say in your decision at all. This is the mirror image of the negative externalities covered elsewhere in this curriculum, like pollution imposing costs on people who didn’t create it.

Herd immunity: safety in numbers

Herd immunity is the protective effect that occurs when enough of a population is immune to a contagious disease, whether through vaccination or prior infection, that the disease struggles to spread even to people who aren’t immune themselves. Once enough of a population is protected, an infected person is statistically unlikely to encounter enough susceptible people to keep transmission chains going, which indirectly shields even those who can’t be vaccinated, such as newborns or people with certain medical conditions.

The unvaccinated infant in a vaccinated classroom

Imagine a baby too young to receive a particular vaccine, attending daycare alongside older children who have all been vaccinated against that disease. The baby has no direct protection from the vaccine itself, yet is still substantially safer than they would be in a daycare full of unvaccinated children, simply because the disease has so few susceptible people left to spread through. That protection exists purely because of other people's choices.

Why markets tend to under-provide vaccines

Because much of vaccination’s benefit flows to other people rather than the person vaccinated, an individual weighing whether to get vaccinated - purely based on their own personal cost and benefit - will tend to undervalue it relative to its true value to society as a whole. This creates a version of the free rider problem: if enough of your neighbors are vaccinated, you personally get much of the herd immunity benefit even without vaccinating yourself, which can weaken individual incentive even as it strains the system if too many people reason the same way.

Treating vaccination as a purely personal medical choice

It's easy to frame vaccination the same way as, say, choosing to exercise or eat well - a decision that mostly affects the person making it. Economically, that framing misses the core feature that makes vaccines different: a substantial share of the benefit is external, protecting other people, not just the individual deciding. That's precisely why vaccination is treated differently from most healthcare choices in policy discussions, including subsidies, public health campaigns, and school-entry requirements in many countries.

What this means for policy

Because of this externality, many governments subsidize vaccines heavily or provide them free at the point of use, reasoning that the price a private market would charge doesn’t reflect the vaccine’s full social value. Public health campaigns and, in some places, school or workplace vaccination requirements are also economic responses to the free rider problem: they aim to keep vaccination rates high enough to sustain herd immunity, since a population that’s vaccinated just below the threshold needed can still experience serious outbreaks.

Key takeaways

Vaccination creates a positive externality: benefits that extend to people beyond the person vaccinated.
Herd immunity protects even unvaccinated people once enough of a population is immune to a disease.
Because benefits are partly external, individuals may undervalue vaccination relative to its full social value.
This creates a free rider problem, where people can benefit from others' vaccination without vaccinating themselves.
Governments often subsidize or mandate vaccines specifically to counter this underprovision problem.