Why are measles cases increasing? Over the past several years, this situation has been developing. Outbreaks commonly arise when certain groups, often small communities, experience vaccination rates below approximately 92–93%. When coverage falls beneath this threshold, community immunity—previously referred to as herd immunity—is compromised, diminishing the collective protection provided by widespread vaccination. As a result, outbreaks are more likely to occur within these populations.
Franciscan Health infectious diseases expert Erica Kaufman West, MD, explains.