Herd immunity

Imagine a community where most people have an invisible shield that keeps a contagious disease from spreading. That’s essentially what herd immunity is — when enough people are immune to a disease (either through vaccination or past infection), it becomes very hard for the illness to travel from person to person. Even those who aren’t immune get some protection because the disease has fewer opportunities to spread. Think of it like a forest where most trees are fireproof; even if a spark lands, the flames have trouble moving from one tree to another.

The concept of herd immunity became widely discussed during the COVID-19 pandemic, but it’s actually been around for over a century. One of the earliest researchers to study it was A.W. Hedrich, an American epidemiologist, who in the 1930s analyzed measles outbreaks in Baltimore. He noticed that when a large percentage of children in the city were immune, the spread of measles slowed down dramatically — even among those who hadn’t been infected before. His observations laid the foundation for using herd immunity as a public health strategy.

Herd immunity doesn’t require everyone to be immune, but the exact percentage needed depends on the disease. For something as contagious as measles, about 95% of the population needs immunity to stop it from spreading. For less contagious diseases, the threshold is lower. Vaccines are the safest way to achieve this because they provide immunity without the risks of getting sick. Without enough immune people, outbreaks can happen quickly — which is why vaccination programs are so crucial.

Today, herd immunity remains a cornerstone of disease prevention. It’s the reason diseases like smallpox have been eradicated and why others, like polio, are on the verge of disappearing. Public health experts continue to build on Hedrich’s work, using mathematical models to figure out how to protect populations. It’s a reminder that sometimes, protecting yourself also protects everyone around you — a kind of quiet teamwork against the invisible threats in the air.