In 1963, only 20 U.S. states mandated that children receive several vaccines before starting school.
Notes on verification
Confirmed by four independent sources (KFF, Wikipedia, Britannica, WHYY citing a public health historian), all consistently citing 20 states with vaccine mandates by 1963. Minor phrasing variation ('at least one vaccine' vs 'several vaccines') does not affect the core numeric claim. [cascade flags: aggregator_dropped_primary_exists,wikipedia_dropped_other_sources_exist | tier=unverified indep_score=0.3 clusters=2 claim_tier=notable] [rescored 2026-09-15: curated origin-host map (PR #65); unclassified hosts no longer scored as aggregators]
Sources
- Vaccine requirements and vaccine research highlights - Mayo Clinic (seed:health_and_medicine)
- https://www.kff.org/other-health/state-and-local-policies-on-school-vaccine-requirements-overview-and-current-status/ (corroboration)
- https://whyy.org/segments/vaccines-and-schools-a-public-health-power-couple/ (corroboration)