The first Americans could receive a COVID-19 vaccine as soon as 24 hours after the FDA grants approval, which would kick off the largest inoculation campaign in U.S. history starting in mid-December.

“Within 24 hours from the approval, the vaccine will be moving and located in the areas where each state will have told us where they want the vaccine doses,” Dr. Moncef Slaoui, the chief scientific adviser for the government’s “Operation Warp Speed” vaccine program, told NBC’s “Meet the Press.”

The effort to roll out vaccines across the country of 330 million people comes as President Trump has blocked the normal transition of government before the inauguration of President-elect Joe Biden on Jan. 20.

Slaoui said he hoped for a smooth transition and did not expect the vaccination effort to be derailed.

The U.S. Food and Drug Administration’s outside advisers will meet on Dec. 10 to discuss whether to authorize the COVID-19 vaccine developed by Pfizer Inc. and German partner BioNTech for emergency use.

Moderna Inc. is expected to seek approval later in December for its COVID-19 vaccine.

Vaccines will be distributed based on each state’s population, Slaoui said. Each state will decide who gets the vaccine first with the recommendation that priority be given to health care workers, front-line workers and the elderly who face the highest risks of dying from the virus.

About 70 percent of the population needs to be immunized to achieve herd immunity, a goal the country could reach by May, he said.

As new COVID-19 cases continue to surge, millions of Americans are ignoring federal and state warnings to stay home for Thanksgiving to prevent overwhelming already strained hospitals.

Many people are trying to get tested before the holiday on Thursday, leading to long lines in New York City and elsewhere.

Testing shortages still plague many parts of the country, with most pharmacies offering COVID-19 tests in suburban Chicago fully booked ahead of Thanksgiving and long lines at state drive-through testing facilities.

“We’re clearly involved now in a very, very difficult surge here throughout the United States and even globally,” Dr. Anthony Fauci, the nation’s top infectious disease expert, told NBC.

Last week Biden called the vaccination program a “massive undertaking” and “one of the greatest challenges we will face as a nation.”

The United States must distribute tens of millions of vaccines while also combating misinformation about vaccines spread on social media. A recent Gallup poll showed only 58 percent of Americans would get the vaccine, up from 50 percent in September.

Incoming White House Chief of Staff Ron Klain said it was crucial to have a seamless flow of information between Trump’s coronavirus experts and Biden’s transition team to avoid delays in distribution after Biden takes office on Jan. 20.

Biden warned last week that “more people will die if we don’t coordinate.”

The number of U.S. coronavirus cases has surpassed 12 million and rose by more than 1 million cases in less than a week for the first time.

Deaths have topped 255,000 with many health experts warning deaths will rise to more than 2,000 a day in the coming weeks.