Getting a vaccine has been a huge undertaking. How all 50 states scramble to dole them out is the next massive challenge.

The largest mass vaccination campaign ever attempted in the United States could begin as soon as this week, with the federal government turning over millions of doses to the states and territories.

Everything depends on them.

But days before the first COVID-19 vaccine is cleared for use, an exclusive USA TODAY Network survey of health officials in all 50 states revealed a patchwork of preparations and different distribution plans that may mean wide variations in what the rollout looks like as it expands across the nation.

Many states are struggling to prepare because information about what, when and how much vaccine is coming constantly changes, and extra funding to make the undertaking possible depends on Congress. Preparedness varies widely depending on how well a state's health department is funded, how hard the pandemic has hit and how robust its immunization system was pre-pandemic.

Overall, state officials express confidence for this month's launch. They're meeting two and three times a week to prepare for the onslaught, hiring more people, beefing up computer systems, requisitioning or buying everything from all-terrain delivery vehicles for remote places to dry ice machines for keeping the precious vaccine at its required sub-zero temperature.

Most expect to be able to immunize the relatively small first round of health care workers and nursing home residents. The complexity, however, ratchets up considerably when the numbers go from uniform groups of tens of thousands per state to millions of essential workers, people 65 and older and those with multiple underlying illnesses.

As Hawaii's planning document says, quoting boxer Mike Tyson, "Everyone has a plan until they get punched in the mouth."  ...

 ALSO SEE: Trading COVID-19 vaccines. Regular deliveries. Tracking doses. What we know about Operation Warp Speed distribution process.