The governors of Texas, Florida and Arizona have announced executive orders to curtail the use of so-called vaccine passports, or documentation of Covid-19 vaccinations.
Those states joined several others that have pushed back against vaccination certification, arguing that it violates privacy and civil rights. The Biden administration has said it won't build a national vaccination app, but private companies are racing to create digital "passports" that show proof of immunization.
The government said that as of Sunday, half of all adults in the U.S. had had at least one vaccine shot. About 32.5 percent of the population has been fully vaccinated, the Centers for Disease Control and Prevention reported.
Lawrence Gostin, director of the O'Neill Institute for National and Global Health Law at Georgetown University, previously told NBC News that states' executive orders are unlikely to have much impact.
"Governors have no power to prohibit cities or counties from issuing passports or banning the private sector," he said. "But if the Florida or other state legislature passed a law, it could preempt local governments from issuing passports."
President Joe Biden's chief medical adviser, Dr. Anthony Fauci, told Politico that the federal government won't mandate vaccine passports for businesses or travelers.
White House press secretary Jen Psaki told reporters last week, "Development of a vaccine passport, or whatever you want to call it, will be driven by the private sector." She said the administration does plan to provide recommendations for digital vaccination certificates so there are applicable standards nationwide.
Such "passports," which have been under consideration by the private sector for months, could be scannable QR codes that people could pull up on their phones or simpler green check marks or red X's.