The pill molnupiravir showed only a 30 percent reduction in hospitalization and death based on data from 1,400 patients. Figures from a study released on October 1 also showed an "approximate" 50 percent efficacy rate.
The company submitted the results of the study to the Food and Drug Administration (FDA), which will hold a meeting to discuss the pill at the end of November.
Data from October showed that 7.3 percent of patients given the pill twice a day for five days were hospitalized, although none died after 29 days from treatment. Meanwhile, 14.1 percent of placebo patients were also hospitalized.
Other data showed that 6.8 percent of patients who took the drug were hospitalized, and one died. In the placebo group, 9.7 percent were admitted to the hospital.
Merck announced in September that it would seek authorization for the pill, which targets the enzyme that allows the COVID-19 virus to replicate itself by introducing errors in the genetic code of the virus.
The release of the full analysis came before the FDA published a set of documents intended to brief a panel of experts who are due to meet to discuss whether or not to recommend authorizing the pill.
The FDA asked the committee to weigh in on concerns over whether or not the drug could encourage the virus to mutate, and how these concerns should be mitigated.
Meanwhile, Pfizer introduced its own COVID pill. Data from the company showed that in the 1,200 patient study, the drug reduced COVID-19 related hospitalizations and deaths by 89 percent compared to the placebo group.
Merck's shares dropped nearly 30 percent to $80 in premarket trading amid the overall fall in the stock market due to the news of a new coronavirus variant.
Pills such as those developed by Merck and Pfizer have been touted as potential game-changers and can be taken as early as home treatments to help prevent hospitalizations and deaths. (Related: US government pays Merck $712 for 5-day molnupiravir course that only costs $17.74 to manufacture.)
The two drugs have different mechanisms of action: Merck's is designed to introduce errors in the genetic code of the virus, while Pfizer's belongs to a class of drugs known as protease inhibitors and is designed to block an enzyme that the virus uses for critical functions.
The European Union's medicines watchdog advised its member states last week that they can use Merck's COVID-19 pill in emergency situations that are triggered by the rising infection rates ahead of the formal approval of the pill across the bloc. The United Kingdom, meanwhile, approved molnupiravir on conditional grounds earlier in the month.
Merck's COVID-19 pill will still likely get U.S. authorization despite the disappointing efficacy data and the concerns raised by regulators. "Given the unmet need, we continue to believe molnupiravir will be authorized," analysts Matthew Harrison and Charlie Yang wrote in a note to clients.
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