Why a vaccinated child can be more at risk than an unvaccinated one

Picture a mother whose daughter has had every vaccine on schedule, on time, no exceptions. You'd expect that daughter to be about as safe from measles as a child can be. But if that family lives in a neighborhood where many other parents have opted out of vaccination, the fully vaccinated girl can actually be at higher risk of catching measles than an unvaccinated child living in a highly vaccinated town across the state.

That sounds backwards until you understand what vaccines are actually protecting you from. No vaccine works perfectly in every body. Some children who receive a shot never develop full immunity, and immunity from certain vaccines fades over time. What actually stops a disease from spreading is not any single vaccinated body, but the wall of immune bodies surrounding it. When enough people in a community carry immunity, a virus runs out of places to go. It cannot jump from host to host fast enough to sustain an outbreak, so even the small number of people whose vaccines failed, along with those who cannot be vaccinated at all, end up sheltered by the immunity circulating around them. Eula Biss came to think of this as a kind of banking system: every vaccinated body is a deposit that others can draw on, whether or not they ever contributed themselves.

This idea, now called herd immunity, was first noticed by a doctor in 1840, who observed that vaccinating only part of a population against smallpox could stop an epidemic in its tracks. It sounds abstract until a disease actually moves through a population, and then it becomes visible as a threshold. Below a certain proportion of immune people, outbreaks flare and die down in waves. Above it, disease can barely get a foothold.

The uncomfortable part of this idea is that it makes immunity a shared resource rather than a private possession. You are not just protecting your own child when you vaccinate. You are protecting the newborn too young for her first shot, the child with a compromised immune system, the person whose vaccine simply didn't take. And you, in turn, depend on everyone around you doing the same. Refusing to participate doesn't just risk your own household. It draws down a fund that other people, often more vulnerable ones, are relying on.

Start by treating your own vaccination status as part of a shared resource rather than a purely private decision. Look up the vaccination rate in your community or your child's school, since that number tells you more about your actual risk than your own vaccine record alone. When you're deciding whether a shot is worth it for someone healthy and low-risk, remember to weigh the newborns, the immune-compromised, and the people whose vaccines simply failed, who are all depending on the immunity circulating around them. If you ever advocate for vaccine exemptions or policy changes, ask who exactly would be drawing down the shared protection you'd be removing, and whether you're comfortable with that trade.