An Overview of the Merck COVID-19 Vaccine

Merck Discontinues Vaccine Candidates

Jan. 25, 2021: Merck announced that it will stop developing its two COVID-19 vaccine candidates, V591 and V590. The company stated in a press release that "the immune responses were inferior to those seen following natural infection and those reported for other SARS-CoV-2/COVID-19 vaccines."

Merck, one of the largest pharmaceutical companies in the world, focused its early efforts in the fight against the coronavirus disease 2019 (COVID-19) on looking for ways to treat people who were sick.

The U.S. company also worked on two vaccine candidates for COVID-19 but announced on Jan. 25, 2021, that it will stop developing them due to a lack of immune responses in trials.

Merck continues to focus on its drug candidates for treating COVID-19. Its antiviral medication, molnupiravir, received emergency use authorization in December 2021.

The discontinued vaccine candidates were viral vector vaccines that use modified versions of other viruses to deliver instructions to cells. The company began enrolling participants in an early-stage clinical trial for COVID-19 vaccine candidate V591 in late August 2020 and an early-stage clinical trial for candidate V590 in late October 2020.

Merck was expected to share some preliminary results from the vaccine trials in early 2021 but instead announced the end of their development. Merck plans to still submit its studies of these candidates for publication in a peer-reviewed journal.

Merck KGaA / Creative Commons

How It Works

Merck's vaccine candidate V591 was a recombinant vaccine that used a live virus (measles) vaccine as the delivery method for transporting a small piece of the coronavirus' genetic code to create an immune response. Recombinant viruses are genetically engineered viruses, which can be designed to act as vaccines when antigen genes are added.

V591 was launched by Merck with the purchase of Themis, a private company focused on immunity and vaccines that Merck acquired in May 2020.

Merck collaborated with the International AIDS Vaccine Initiative (IAVI) on the development of its other vaccine candidate, V590. 

V590 used recombinant vesicular stomatitis virus (rVSV) technology—the same technique Merck used to develop a vaccine to fight the Ebola virus. For this particular vaccine, Merck was using a common animal virus that had been changed to use certain proteins on the virus that help launch an immune response to SARS-CoV-2, the virus that causes COVID-19.

How Effective Is It?

There is no data released yet on how the Merck vaccines performed, but the company announced in a press release that "both V590 and V591 were generally well tolerated, but the immune responses were inferior to those seen following natural infection and those reported for other SARS-CoV-2/COVID-19 vaccines."

When Will It Be Available?

Merck stopped its COVID-19 vaccine development, so its vaccine candidates will not be made available.

Who Can Get the Merck Vaccine?

Merck is no longer developing vaccine candidates. In its initial study for the V591 vaccine, Merck enrolled 260 participants 18 and older, grouping them by those 18 to 55 and those older than 55. Merck's enrollment for the V590 trial was also 18 and older with similar age groupings.

Side Effects and Adverse Events

The Merck studies were not far enough along to provide information on adverse events or side effects associated with the vaccines.

Funding and Development

Merck has received more than $38 million in funding from the U.S. government for its COVID-19 vaccine program as part of Operation Warp Speed, a public-private partnership initiated by the U.S. government to facilitate and accelerate the development of a COVID-19 vaccine, and from the Biomedical Advanced Research and Development Authority (BARDA).

BARDA is a program that falls under the U.S. Department of Health and Human Services. That program and Operation Warp Speed have sought to speed up how quickly vaccines to fight COVID-19 can be developed and approved for use.

Merck now uses its manufacturing facilities to support the production of other COVID-19 vaccines and treatments, including the Johnson & Johnson vaccine.

The information in this article is current as of the date listed, which means newer information may be available when you read this. For the most recent updates on COVID-19, visit our coronavirus news page.

5 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Merck. Merck discontinues development of SARS-CoV-2/COVID-19 vaccine candidates; continues development of two investigational therapeutic candidates.

  2. Merck. IAVI and Merck collaborate to develop vaccine against SARS-CoV-2.

  3. National Institutes of Health. A study to assess safety, tolerability, and immunogenicity of V591 (COVID-19 vaccine) in healthy participants (V591-001).

  4. U.S. Department of Health and Human Services. BARDA’s rapidly expanding COVID-19 medical countermeasure portfolio.

  5. Our COVID-19 response. Merck.

By Rachael Zimlich, BSN, RN
Rachael is a freelance healthcare writer and critical care nurse based near Cleveland, Ohio.