Digital vaccine verification programs could make it easier to enforce safeguards and tamp down new outbreaks. “But that only works when you have mass adoption, and mass adoption requires trust and actual buy-in with what the state health department is doing, which is not necessarily present in all states,” said Alan Butler, executive director of the Electronic Privacy Information Center, a Washington-based nonprofit organization. Hawaii is the only state enforcing some version of a vaccine passport. It requires travelers to upload a photo or PDF of their Hawaii vaccination document or pass a pre-arrival COVID-19 test to avoid having to quarantine for 10 days. Earlier this month, California became just the third state — behind New York and Louisiana — to offer residents a way to voluntarily display digital proof of their COVID-19 shots. None of those states requires the use of their digital verification systems to access either public or private-sector places.