Vaccination Rates and Outbreaks: The 95% Tipping Point

The 2025 measles outbreaks across the U.S. have brought the concept of herd immunity into sharp focus. Public health experts emphasize that a community needs at least 95% vaccination coverage with the MMR (measles, mumps, and rubella) vaccine to prevent the spread of measles. When rates dip below this critical threshold, the virus can circulate freely, leading to significant outbreaks. This tipping point is not just a theoretical number; it represents the fragile line between community protection and widespread vulnerability.

Spartanburg County, South Carolina, serves as a sobering real-world example of this principle. For the 2023-2024 school year, the county's kindergarten MMR vaccination rate had fallen to 92.1%, down from 95% in the 2019-2020 school year. This seemingly small drop created a critical gap in immunity. By December 2025, this vulnerability was fully exposed as the county became the epicenter of an outbreak with 135 confirmed cases, forcing hundreds of unvaccinated students into quarantine.

Dr. Linda Bell, South Carolina’s state epidemiologist, described the situation as “accelerating” due to “lower-than-hoped-for vaccination coverage.” The situation in Spartanburg and other similar areas underscores a nationwide challenge. While statewide vaccination rates may appear high, localized pockets of unvaccinated individuals can undermine collective protection and become hotspots for infectious diseases, threatening the nation's overall public health security.