Merck on Monday announced that it will stop developing its two Covid-19 vaccine candidates, citing data indicating that the experimental vaccines prompted inadequate immune responses.

Related: The U.S. Covid-19 vaccination scenario planning guide

The announcement came as President Biden looks to accelerate America's Covid-19 vaccine rollout, stating on Monday that his administration could accelerate the country's vaccination effort by administering 1.5 million shots daily.

Merck discontinues development of Covid-19 vaccine candidates

According to STAT News, Merck had been developing two single-dose Covid-19 vaccine candidates. Both of the experimental vaccines contained viruses intended to replicate once they entered the body, which made them unique from other experimental Covid-19 vaccines.

Specifically, one of the vaccine candidates, called V590, contained the vesicular stomatitis virus (VSV) that's also used in Merck's effective Ebola vaccine, according to STAT News. Merck's other vaccine candidate, called V591, contained a measles virus and was similar to a type of vaccine that Merck has now made for decades, STAT News reports.

But Merck on Monday said data from Phase 1 clinical trials on the experimental Covid-19 vaccines showed that, although the vaccine candidates were "well tolerated," they produced "immune responses … inferior to those seen following natural infection and those reported for other … vaccines." In light of that data, Merck decided to pause its own research and development into the vaccine candidates, the company said.

However, Merck said it plans to continue its joint research with the International AIDS Vaccine Initiative (IAVI) on V591 to determine whether using a different route of administration might improve the vaccine candidate's effectiveness, according to STAT News. IAVI President Mark Feinberg told STAT News that one reason V591 may have underperformed in the Merck's Phase 1 clinical trial is because it had been administered by intramuscular injection. Feinberg said an oral or intranasal administration route may work better for the vaccine candidate.

Overall, Nick Kartsonis, SVP for infectious disease and vaccines at Merck Research Laboratories, during an interview with STAT News said, "We're disappointed by this result. But it also allows us to continue to focus on our therapeutic candidates and move those forward. And, you know, we are open to continue the work to see if we can address the pandemic in any way, we can add value."

Merck said it will now focus its research on developing two experimental Covid-19 treatments: MK-7110, which is a potential antiviral treatment against the novel coronavirus, and MK-4482, which is designed to reduce the immune system's over-response to the virus.

Biden aims for 1.5M Covid-19 vaccinations a day

Merck's announcement came as the United States continues to grapple with a slower-than-expected rollout of America's two authorized Covid-19 vaccines, which are manufactured by Pfizer and BioNTech, as well as Moderna. CDC data shows that, as of Monday morning, the federal government had distributed about 41.4 million doses of the country's authorized Covid-19 vaccines, and about 22.7 million Americans had received their first dose of the two-dose vaccines.

President Biden last week announced that his administration set a goal of administering 100 million Covid-19 vaccine doses within his first 100 days in office, which would require the country to administer an average of one million doses per day. Last week, the United States administered an average of 1.16 million vaccine doses per day, according to The Hill.

Biden on Monday said he's "confident that we will be in a position within the next three weeks or so to be vaccinating people at the range of a million a day or in excess of that." Further, he added that he thinks the United States "may be able to get that to ... 1.5 million a day."

Paul Offit, a vaccine expert at Children's Hospital of Philadelphia, suggested that Biden's goal may fall short of what's need to quickly reach heard immunity in the United States, however. Offit told The Hill that the United States would need nearly 250 million Americans to be vaccinated against Covid-19 for the country to reach herd immunity, which would dramatically slow down Covid-19's spread.

"For a two-dose vaccine, that's 500 million doses, so when you talk about giving a million doses a day, we're probably better off with about three million doses a day if we can get there," Offit said. "A million doses a day is not enough to get there quickly" (Herper/Branswell, STAT News, 1/25; Zimmer, New York Times, 1/25; Chappell, NPR, 1/25; Merck release, 1/25; Linskey, Washington Post, 1/25; Niedzwiadek, Politico, 1/25; Hellmann, The Hill, 1/25; CDC data, updated 1/25).