Use it or lose it.

The Biden administration will begin reallocating COVID-19 vaccine doses from states with lower demand to those with greater interest — as part of a new push to get at least 70 percent of Americans inoculated by July 4.

The shift comes as the overall demand for vaccines has dropped nationwide, with some states turning down their full or part of their weekly allocation for doses.

The feds will now start giving the declined doses to other states in an effort to speed up vaccination efforts in those areas, a senior administration official said Tuesday.

Any state could turn down doses one week but still apply for a surplus the following week, the official said.

This week, Iowa turned down nearly three-quarters of the shots available it in its upcoming shipment due to low interest in vaccines.

The White House notified governors about the change Tuesday morning.

On Tuesday afternoon, President Biden announced his new goal of getting at least one shot into the arms of 70 percent of Americans by July 4 — and reaching 160 million fully vaccinated by the same date.

“Now that we have the vaccine supply, we’re focused on convincing even more Americans to show up and get the vaccine that is available to them,” Biden said. 

“If we succeed in this effort, as we did with the last then Americans will have taken a serious step towards a return to normal.”

In addition to the shifting allocations, the president has directed federally-partnered pharmacies and state sites to allow walk-in appointments.

“You’ll soon be able to get vaccinated without an appointment at the vast majority of our 40,000 pharmacy locations across the country,” Biden said.

“We’re also encouraging our state and local partners to have a walk-in ability as well at the sites that they run.”

Currently, 44.4 percent of people in the US — and 56.3 percent of adults — have received at least one dose of the vaccine. Meanwhile, about 105 million Americans are fully vaccinated against the virus.

To get vaccines to more hard-to-reach regions, FEMA will also launch pop-up and smaller sites as well as increase the number of mobile units.

Experts have said that between 70 percent and 90 percent of the US will likely need to be vaccinated or infected to reach herd immunity.

But scientists are increasingly expressing skepticism that the US will reach that status — as more contagious COVID-19 variants spread and many Americans remain hesitant to get the jab.

Biden on Tuesday acknowledged that his latest goal may be “harder” in some ways than his push to get 200 million shots administered in his first 100 days.

“We know we’re going to get to a place where the doubters exist or the people who… just not sure how to get to where they want to go [for the vaccines]. It’s going to be hard,” he said.

“We’re going to keep at it, and I think the end of the day, most people will be convinced by the fact that their failure to get the vaccine may cause other people to get sick, and maybe die.”

With Post wires