The U.S. government may be forced to ration COVID-19 shots this coming fall in the absence of new funding from Congress. As a result, free shots may be limited to only those who are at the highest risk of serious infection.

On Monday, a senior administration official told NBC News that the government has been hamstrung in its ability to conduct negotiations with drugmakers Pfizer and Moderna without the additional funds. This comes at a time when both companies are preparing a new version of their COVID-19 vaccine that would improve resistance against the sub-variants that have made up the bulk of cases nationwide.

The fall and winter seasons are usually the heaviest in terms of infections in part because it forces people into closer contact indoors while also coinciding with the onset of flu season. In November 2021, the Omicron variant of COVID-19 led to a surge in cases that lasted through early 2022.

Throughout the COVID-19 pandemic, vaccines have been freely available to those who want to receive the shot. But this model has been challenged by the expiration of money available to the administration despite its repeated pleas to Congress for support.

Initially, the Biden administration requested $22.5 billion as part of a March spending bill, but it failed to make it into the Senate’s final apportionment after skepticism from Republican senators, who demanded to know how previous spending rounds were distributed. A bipartisan compromise bill of $10 billion in spending was hammered out in April by Senate Majority Leader Sen. Chuck Schumer, D-N.Y., and Sen. Mitt Romney, R-Utah, but failed to advance.

As the administration now shifts to cost-cutting in other areas to keep the shots free for patients, one official emphasized that time is of the essence.

“It’s going to be a pretty tough fall and winter if Congress abdicates its responsibilities and does not show up with funding for the American people,” the official told CNBC. “We’re going to do what we can but at the end of the day, our hands are going to be tied.”