KENILWORTH, N.J. (NewsNation Now) — Merck is stopping development on two of its COVID-19 vaccines following inferior results in early-stage studies.

The drugmaker said Monday that it would discontinue development of its vaccine candidates, V590 and V591. The company said its potential vaccines were generally well tolerated by patients, but they generated an inferior immune system response compared with other vaccines.

Instead, it will focus on studying two possible therapeutic treatment candidates for the virus — MK-4482 and MK-7110 — neither of which has yet to be approved by regulators.

Molnupiravir (MK-4482) is an experimental oral antiviral agent being developed in collaboration with Ridgeback Bio. Initial data is expected by the end of March.

MK-7110 (formerly CD24Fc) is a “potentially first-in-class investigational recombinant fusion protein that modulates the inflammatory response” to coronavirus. Preliminary “Phase 3 studies showed a greater than 50 percent reduction in the risk of death or respiratory failure in patients hospitalized with moderate to severe COVID-19.” Full results— including initial efficacy data — are expected at the end of March.

Merck entered the race to fight COVID-19 later than other top drugmakers.

It said last fall that it had started early-stage research in volunteers on potential vaccines that require only one dose. Vaccines developed by Pfizer and Moderna were already in late-stage research at that point.

The Food and Drug Administration allowed emergency use of both the Pfizer and Moderna vaccines late last year. Each requires two shots.

Since inoculations began in December, nearly 22 million doses have been administered to people nationwide, according to the Centers for Disease Control and Prevention.

A total of 3.2 million people have received both doses required for those vaccines.

The government is paying Merck about $356 million to fast-track production of one of its potential treatments under Operation Warp Speed, a push to develop COVID-19 vaccines and treatments. The money will allow Kenilworth, New Jersey, company to deliver up to 100,000 doses by June 30, if the FDA clears the treatment for emergency use.

The treatment, known as MK-7110, has the potential to minimize the damaging effects of an overactive immune response to COVID-19. This immune response can complicate the life-saving efforts of doctors and nurses.

Five potential vaccines have reached late-stage testing in the United States, the final phase before a drugmaker seeks approval from regulators. Results from a single-dose candidate developed by Johnson & Johnson are expected soon.

The announcement is a setback to 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.