You certainly could fly in the US without one. I don't recall any one being stop on the road and have the Gestapo ask for their immunization record. But I live in Texas.
CDC uses a
Do Not Board list to prevent travelers from boarding commercial airplanes if they are known or suspected to have a contagious disease that poses a threat to the public’s health.
The Do Not Board and Lookout lists have been used for people with suspected or confirmed infectious tuberculosis (TB), including multidrug-resistant tuberculosis (MDR-TB), and measles. During 2020-2022, CDC used these authorities to restrict travel of people with COVID-19 and close contacts who were recommended to quarantine. These authorities were also used for mpox during 2022. Travel restrictions can also be used for other suspected or confirmed contagious diseases that could pose a public health threat during travel, including viral hemorrhagic fevers such as Ebola.
CDC public health travel restrictions—the public health Do Not Board and Lookout lists—to help prevent travelers from spreading contagious diseases.
www.cdc.gov
The rabid right has gotten more rabid lately. Somethings bothering them these days.