Herd Immunity

Measles is so contagious (each infected person spreads it to 12-18 others) that you need 95% vaccination coverage to achieve herd immunity—the highest threshold of any common disease. This means just 5% of a population opting out can collapse protection for everyone, which is why previously eradicated measles has resurged in communities with even small pockets of vaccine hesitancy. The math is unforgiving: miss the threshold by a few percentage points, and vulnerable babies too young for vaccination suddenly face real danger.

Charles Darwin lost three children to infectious diseases, including his beloved 10-year-old daughter Annie to what was likely tuberculosis in 1851. This personal devastation occurred just decades before herd immunity principles would be formalized, and Darwin himself became a vocal supporter of vaccination efforts. His grief-stricken letters reveal how even the most privileged families were helpless against diseases that herd immunity would eventually help control—a reminder that population-level protection is a relatively recent human achievement.

Here's the uncomfortable truth: as herd immunity succeeds, rational self-interest argues against vaccination. When diseases become rare due to high vaccination rates, individuals perceive personal risk as minimal while vaccine side effects (however minor) become relatively more concerning—incentivizing free-riding on others' immunity. This creates a self-destructing system where success breeds the conditions for failure, a paradox that no amount of individual choice can resolve without some form of collective coordination.

The most powerful effect of vaccination isn't protecting the vaccinated—it's protecting those who can't be vaccinated. Studies of pneumococcal vaccine introduction showed that for every child directly protected, 2-4 additional people (elderly, immunocompromised, infants) were indirectly shielded from disease. This means the majority of lives saved by vaccines are people who never received them, making herd immunity arguably the most significant collective good in modern medicine.

COVID-19 exposed a critical assumption flaw: traditional herd immunity calculations presumed sterilizing immunity (vaccinated people can't transmit). But COVID vaccines primarily prevent severe disease while only reducing transmission, and the virus mutates rapidly enough to partially escape immunity. This meant the classical threshold calculations (based on smallpox and measles models) became unreliable in real-time, forcing public health to communicate shifting targets—a nightmare for public trust and a reminder that not all pathogens play by the same immunological rules.

Herd immunity is the ultimate invisible infrastructure—you only notice it when it fails. The reason you can safely take your newborn to a grocery store or sit in a crowded theater isn't individual immunity but the statistical improbability that enough infectious people are present to start an outbreak. Like clean water or stable electricity, it's a collective achievement that enables modern social life, yet remains unappreciated until someone flips the switch off by refusing to contribute to the grid.