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Merck Discontinues Its COVID-19 Vaccine Candidates

Scientist adding vaccine dose to a syringe.

Meyer & Meyer / Getty Images

Key Takeaways

  • Merck announced it will halt the development of its two COVID-19 vaccine candidates.
  • The vaccines did not produce a strong enough immune response in early clinical trials to protect people against COVID-19.
  • The company will instead focus on developing two therapies for treating severe cases of the virus.

Merck announced on Monday that it plans to discontinue the development of its SARS-CoV-2, the virus that causes COVID-19, vaccine candidates, V591 and V590. The pharmaceutical company reported disappointing results in phase 1 clinical trials of its two vaccines. 

In an announcement, the pharmaceutical company said that the vaccine candidates were well tolerated by subjects, but didn’t produce an immune response stronger than what has been reported for people naturally infected with COVID-19 and other vaccines.

“We are grateful to our collaborators who worked with us on these vaccine candidates and to the volunteers in the trials,” Dean Y. Li, MD, PhD, president of Merck Research Laboratories, said in the company’s press statement. “We are resolute in our commitment to contribute to the global effort to relieve the burden of this pandemic on patients, healthcare systems, and communities.”

The company plans to focus its COVID-19 efforts on researching and producing two therapeutic candidates. MK-7110 works to reduce the immune system’s over-response to the virus in hospitalized patients and appears to be effective in clinical studies. The other, MK-4482, being developed in partnership with Ridgeback Biotherapeutics, is an antiviral medication that is still being tested

“Merck is committed to deploying its expertise and resources to have the greatest possible impact on the pandemic,” the company told Verywell in an email. “We will also continue to work closely with governments, public health agencies, and other stakeholders to ensure we remain focused on the areas of pandemic response where we can best contribute.”

What This Means For You

The announcement to scrap the vaccine candidates is a setback for one of the largest pharmaceutical companies in the world. However, dozens of companies including Johnson & Johnson, Oxford/AstraZeneca, and NovaVax are in the final round of clinical trials for their COVID-19 vaccines, so more options may be available to the public soon.

A Saturated Vaccine Development Environment

At the outset, the Merck vaccine candidates appeared to be promising because they would have created long-lasting immunity with a single dose. In December, the company signed a deal with the U.S. government to supply up to 100,000 doses of one of the vaccines for roughly $356 million. The approved Pfizer-BioNTech and Moderna vaccines require two doses, which can make distribution more challenging.

In the phase 1 trial, however, both vaccines produced lower levels of binding antibodies and neutralizing antibodies against SARS-CoV-2 than either of the approved vaccines or the AstraZeneca candidate.

Stanley Weiss, MD, an epidemiologist and professor at Rutgers New Jersey Medical School and Rutgers School of Public Health, tells Verywell clinical trials must be large enough to accurately gauge vaccine efficacy and can be very expensive.

With more than 200 Covid-19 vaccine candidates being developed worldwide, Weiss says pharmaceutical companies must be optimistic that their product is effective enough to continue investing in it. They may consider factors like efficacy rates, the ease of production and storage, cost of production, and the number of doses to reach maximum efficacy.

“Unless you think you have a product that's going to be more effective, or much cheaper, or easier to use than these various alternatives, you'd want to reconsider going forward, given the enormous expense,” Weiss says. “It's a business decision by Merck, and a scientific one based upon the evolving clinical data and evolving immunologic data, and our evolving understanding of immunology.”

“It's not a surprise to me that a big company like Merck may say, ‘let's put our resources elsewhere," Weiss adds.

With the introduction of new variants—such as those from the U.K., Brazil, and South Africa—the level of vaccine efficacy will likely become more important in the efforts to reach herd immunity. Moderna and Pfizer report 94% and 95% efficacy, respectively, for their vaccines. These are both mRNA vaccines, meaning they may easily be edited to recognize and neutralize new variants of the SARS-CoV-2 virus. Pfizer expects its vaccine to be effective against new strains, and Moderna announced it will develop booster shots to cover these variants.

“The game has changed in terms of what we need to achieve to reach herd immunity," Weiss says. "So, if you're going to have a vaccine that's less effective, that's a problem."

Looking Ahead 

One of the expected advantages of the Merck vaccines was the ability to sufficiently vaccinate people with only one shot. Minimizing the number of doses that people need to receive could increase vaccination rates.

Johnson and Johnson announced this Friday that its one-dose COVID-19 vaccine provided strong protection against the virus in clinical trials. It was found to be 72% effective in the US, 66% in Latin America, and 57% in South Africa and will be submitted to the U.S. Food and Drug Administration for authorization as soon as next week.

Merck says it will continue researching the potential of at least one of the vaccine candidates, to see if a different method of administering the vaccine will improve its efficacy. Delivering the vaccine via the nasal passage, for instance, may allow it to bind the cells which trigger antibody production.

Stanley Weiss, MD

It's not a surprise to me that a big company like Merck may say, ‘let's put our resources elsewhere.'

— Stanley Weiss, MD

For the most part, however, the company will focus efforts on therapeutical medications. Weiss says that therapy for people with serious COVID-19 illness may help prevent death, and effective therapy at the beginning of infection could prevent it from developing into serious disease.

“Even if we were fortunate and convince everyone that they should be vaccinated, there are still people who are going to become ill,” Weiss says. “So therapy remains very important.”

Multiple biopharmaceutical companies, including Oxford-AstraZeneca and NovaVax, are in the last stage of clinical trials for their vaccine candidates. As scientists continue to develop vaccines, therapeutic medicines, and other means to combat COVID-19, Weiss says he is hopeful for what’s to come.

“The speed at which things are occurring is amazing,” Weiss says. “Sometime in the next hours, days, weeks, months, we'll have some other breakthroughs.”

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  1. Merck. Merck discontinues development of SARS-CoV-2/COVID-19 vaccine candidates; continues development of two investigational therapeutic candidates. Updated January 25, 2021.

  2. Merck. Merck announces supply agreement with U.S. government for initial doses of investigational biological therapy for the treatment of patients with severe and critical COVID-19. Updated December 23, 2020.

  3. Johnson & Johnson. Johnson & Johnson announces single-shot Janssen COVID-19 vaccine candidate met primary endpoints in interim analysis of its phase 3 ENSEMBLE trial. Updated January 29, 2021.