After making a late entry into the race to develop a COVID-19 vaccine, Merck is dropping out due to disappointing test results, shelving two drug candidates that scientists hoped would be a major addition to the antivirus fight.

The pharma giant said Monday that in initial clinical studies, the vaccines were “generally well tolerated” but the immune responses were “inferior” to those in people who have survived COVID-19 and those reported for other vaccines.

“The announcement marks a shocking setback for one of the most storied vaccine makers,” STAT reported, noting that “The hope was that Merck’s vaccines, which were unique because they used viruses that could replicate once they were in the body, would be long-lasting, one-dose vaccines.”

Vaccines made by Moderna and Pfizer/BioNTech are now available in the U.S. but demand has so far vastly exceeded supply.

Merck’s “exit removes a major vaccine player from the constellation of companies seeking shots to curb the spread of the virus, as the few that have been able to bring shots to market struggle to meet heavy demand,” The Wall Street Journal said.

The company, a pioneer of vaccines for measles, rubella, and shingles, announced its COVID vaccine program in May, saying it had acquired Vienna-based Themis, which had been working with the Institut Pasteur on a drug modeled on a measles vaccine, and was partnering with the nonprofit IAVI to develop a drug related to Merck’s existing Ebola vaccine.

“Some scientists had hoped Merck’s vaccines might have meant the longest protection due to their proven platforms,” the Journal said.

But according to Nicholas Kartsonis, who oversees Merck’s vaccine research, the test data for antibody production, including the neutralizing antibodies that play a key role in preventing the virus from entering cells and replicating, did not meet expectations, falling below what is seen in the blood of COVID-19 survivors.

Merck said it would now focus on advancing two experimental COVID-19 drugs.

“The news is a reminder that developing new vaccines is difficult, and that the world is lucky that the first two — the Moderna and Pfizer/BioNTech shots — were so effective,” STAT said.

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