For decades, drug companies have depended on a component in the blood of the horseshoe crab to test injectable medicines, including vaccines, for dangerous bacterial contaminants called endotoxins.

Conservationists and some businesses have pushed for wide acceptance of an alternative test, to protect the horseshoe crabs and birds that feed on their eggs. They seemed to be on the brink of success, until the nongovernmental group that issues quality standards for such tests, the U.S. Pharmacopeia, announced that the alternative test — rFC, recombinant factor C — requires more study. It said it had 30 years of data on the current test and only two years on the new test.

The debate has been widely monitored as demand has grown for testing new vaccines against the coronavirus. Billions of vaccine doses would require testing for endotoxins, molecules in many bacteria.

Pharmaceutical companies must make sure the toxins are not present in any injectable drugs they make. Ingredients, like water, must be tested at each step.

“It is crazy-making that we are going to rely on a wild animal extract during a global pandemic,” said Ryan Phelan, head of the nonprofit Revive and Restore. Her group supports technological solutions to conservation problems, including replacing the test that uses a component of horseshoe crab blood, LAL, with a test that uses rFC, the synthetic equivalent.

But companies that produce LAL — Lonza, Charles River and Associates of Cape Cod — say the current production could handle the vaccine surge.

Phelan said this calculation “boggles the mind” because, “for every dose going out the door, each manufacturer will use 10 times the amount of LAL to test every step along the way in the process.”

Rob Anderson, a vice president at the U.S. Pharmacopeia, said more evidence was needed on rFC for safety tests. “We can’t put a time on that.”