US drugmaker Merck & Co is to stop development of its two Covid-19 vaccines after trial results proved disappointing and will focus pandemic research on treatments instead.
The company cited poor immune responses as its reason for halting the research.
It had been working with France's Pasteur Institute developing a vaccine based on an existing measles vaccine but it failed to prove its efficacy in Phase-1 clinical trials, the Paris institute said.
The partners announced a tie-up last May. Merck & Co operates outside the US and Canada as MSD.
The company was late to join the race to develop a vaccine to protect against the coronavirus, which has killed more than two million people and continues to surge in many parts of the world.
The vaccine candidates were V591, which it acquired with the purchase of Austrian vaccine maker Themis Bioscience, and V590, developed with nonprofit research organisation IAVI.
In early trials, both vaccines generated immune responses that were inferior to those seen in people who had recovered from Covid-19 as well as those reported for other Covid-19 vaccines, the company said.
Trial results were “disappointing, and a bit of a surprise,” said Nick Kartsonis, Merck’s senior vice president of clinical research for infectious diseases and vaccine.
“We didn’t have what we needed to be able to move forward,” Mr Kartsonis said. After evaluating the data, senior leadership decided to discontinue the programmes and divert resources to the company’s efforts to develop Covid-19 treatments.
Both shots generated fewer neutralising antibodies to halt infection than other Covid-19 vaccines, and produced inferior immune responses compared with people who had naturally contracted the coronavirus.
The announcement is a setback in the fight against the pandemic and comes a month after Sanofi and GlaxoSmithKline delayed launch of their shot to late 2021, underscoring the challenges of developing vaccines at record speed.
Tens of millions of doses of vaccines from rivals Pfizer and German partner BioNTech, as well as from Moderna, have so far been administered globally.
Johnson & Johnson, AstraZeneca and others are also racing to develop safe and effective vaccines to protect against the virus.
Merck & Co said it would focus Covid-19 research and manufacturing efforts on two investigational medicines: MK-7110 and MK-4482, which it now calls molnupiravir.
Molnupiravir, which is being developed in collaboration with Ridgeback Bio, is an oral antiviral being studied in both hospital and outpatient settings.
A phase 2/3 trial of the drug was set to finish in May, but initial efficacy results were due in the first quarter and would be made public if clinically meaningful.
Results from a Phase-3 study of MK-7110, an immune modulator being studied as a treatment for patients admitted to hospital with severe Covid-19, were expected in the first quarter.
Shares of Merck fell 1 per cent to $80.12 in trading before the bell.
Meanwhile, US biotechnology firm Moderna on Monday said lab studies showed its Covid-19 vaccine would remain protective against variants of the coronavirus first identified in the United Kingdom and South Africa.
But out of caution, the company will test adding a second booster of its vaccine - to make three shots in total - and has begun preclinical studies on a booster specifically for the South African variant.
"We are encouraged by these new data, which reinforce our confidence that the Moderna Covid-19 Vaccine should be protective against these newly detected variants," said Stephane Bancel, Moderna's CEO.
"Out of an abundance of caution and leveraging the flexibility of our mRNA platform, we are advancing an emerging variant booster candidate against the variant first identified in the Republic of South Africa into the clinic."