WASHINGTON (AP) — Johnson & Johnson said Thursday that U.S. regulators extended the expiration date on millions of doses of its COVID-19 vaccine by an extra six weeks.

The company said a Food and Drug Administration review concluded the shots remain safe and effective up to 4 1/2 months. The announcement comes after state officials warned that many unused doses in storage would expire before the end of the month.

Expiration dates for vaccines are determined based on data the manufacturer submits to regulators proving how long the shots stay at the right strength. FDA originally authorized J&J's vaccine for use for up to 3 months after manufacture when stored at normal refrigeration levels. But J&J said the FDA extended the period to to 4 1/2 months, based on data from ongoing studies assessing the vaccine’s stability.

The FDA has been reviewing expiration dates on the three authorized vaccines as companies have continued to test samples of batches in the months since the shots first rolled out.

The extension will help maintain vaccine supplies even as the rate of new injections continues to slip. The country averaged about 800,000 new injections per day last week. That's down from a high of nearly 2 million per day two months ago. Government officials have turned to a growing number of incentives to encourages shots -- from paid time off to $1 million lottery prizes.

As vaccinations have slowed it's become clear the U.S. is unlikely to meet President Joe Biden's goal target to have 70% of American adults partially vaccinated by July 4. Roughly 64% of Americans older than 18 have had at least one dose, according to the Centers for Disease Control and Prevention.

J&J’s vaccine was highly anticipated because of its one-and-done formulation and easy-to-ship refrigeration requirements. The shot was expected to play a key role in vaccination campaigns in rural areas and low-income countries with limited health care infrastructure.

But rival drugmakers Pfizer and Moderna have already supplied more than enough doses to meet U.S. demand, with more than 129 million Americans fully vaccinated with the companies’ two-dose shots. By comparison just 11 million Americans have been vaccinated with the J&J shot.

The U.S. is set to announce plans to donate 500 million Pfizer vaccine doses for global use over the next year, on top of 80 million doses already pledged by the end of the month.

Uptake of J&J’s vaccine in the U.S. has also been hurt by contamination problems at a Baltimore factory that helps manufacture the shots. The facility was shuttered after an FDA inspection uncovered multiple problems in April and none of the vaccines made there have been distributed.

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AP Health Writer Linda A. Johnson contributed to this story

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The Associated Press Health and Science Department receives support from the Howard Hughes Medical Institute’s Department of Science Education. The AP is solely responsible for all content.