Population-Level Surveillance of Breakthrough Infections Post-Immunization

Abstract

Breakthrough infections -- documented infections occurring after completion of a vaccination course -- moved from a vaccinology footnote to a central object of population-level surveillance during the COVID-19 pandemic, as hundreds of millions of people were vaccinated within months of one another against a still-evolving pathogen under near-real-time case counting. This article synthesizes the methodological literature that grew out of that experience: the migration from randomized efficacy trials to test-negative case-control surveillance networks, the role of scheduled-testing cohorts such as SIREN in capturing infections that never reach a testing site, and the genomic and wastewater systems needed to distinguish waning immunity from variant-driven immune escape. We trace the well-replicated finding that vaccine effectiveness against infection declines substantially within months of vaccination while effectiveness against severe disease is better preserved, the immunological correlate-of-protection work that anticipated this pattern, and the emerging evidence that hybrid immunity from vaccination plus prior infection is more durable than either exposure alone. We close by examining unresolved surveillance problems -- denominator uncertainty, calendar-time confounding, ascertainment drift from home testing, and the gap between individual-level and population-level inference -- that remain open regardless of any single pathogen's biology.