WASHINGTON — Johnson & Johnson said Thursday that US regulators extended the expiration date on millions of doses of its COVID-19 vaccine by six weeks.

The company said a Food and Drug Administration review concluded the shots remain safe and effective for at least 4 1/2 months. In February, the FDA originally authorized J&J’s vaccine for up to three months when stored at normal refrigeration levels.

Thursday's announcement comes after state officials warned that many doses in storage would expire before the end of the month. Vaccine expiration dates are based on information from drugmakers on how long the shots stay at the right strength. J&J said the FDA added six weeks based on data from ongoing studies assessing the vaccine’s stability. The FDA has been reviewing expiration dates on all three US authorized vaccines as companies have continued to test batches in the months since the shots first rolled out. The vaccines from Pfizer and Moderna, authorized in December, have a six-month shelf life.

The J&J extension will help maintain vaccine supplies even as the number of Americans getting shots has slipped. The country averaged about 800,000 new injections per day last week. That's down from a high of nearly 2 million daily shots two months ago. Government officials and companies have turned to incentives to encourages shots, including paid time off to $1 million lottery prizes.

As vaccinations have slowed it's become clear the US is unlikely to meet President Joe Biden's goal to have 70% of 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. 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, which started shipping shots months earlier, have already supplied more than enough doses to meet US demand. More than 129 million Americans have been fully vaccinated with the companies’ two-dose shots. By comparison, just 11 million Americans have been vaccinated with the J&J shot. About 10 million more J&J doses have been sent to states, according to the CDC.

Use of J&J’s vaccine has been hurt by links to a rare blood clot disorder. That issue led US health officials to “pause” use of the shot for an 11-day review. Officials lifted the hold in late April after concluding that the vaccine’s benefits outweighed its risks J&J's rollout has also been slowed by contamination problems at a Baltimore factory that helps make the shots. The facility was shuttered after an FDA inspection uncovered unsanitary conditions and other problems in April. None of the vaccines made there have been distributed.

AP Health Writer Linda A. Johnson contributed to this report.

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