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Pharmaceutical company Merck announced Monday it is discontinuing development of its Covid-19 vaccine candidates after early studies showed immune responses were inferior to natural infection and other Covid-19 vaccines.

Merck said its vaccine candidates appeared to be safe, but the company did not provide results from early trials. It said in a news release it plans to submit results from Phase 1 trials of its vaccine candidates, known as V590 and V591, for publication in a peer-reviewed journal.

In June, Merck announced it had acquired the company Themis to focus on a measles vector-based coronavirus vaccine candidate. Another candidate, developed with the nonprofit International AIDS Vaccine Initiative, was based on technology used for Merck’s Ebola vaccine.

Demand for Covid-19 vaccines is high, especially as the more contagious variants of the virus begin to spread in some places. There are 64 vaccine candidates in trials worldwide, according to the World Health Organization.

Two Covid-19 vaccines, from Pfizer/BioNTech and Moderna, have already been authorized for emergency use in the United States by the US Food and Drug Administration; both are about 95% effective after two doses.

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Johnson & Johnson has said results from large-scale trials of its single-dose Covid-19 vaccine will be available by the end of the month. AstraZeneca’s vaccine is already in use in some countries, and may be considered for authorization in the United States in the coming months. Novavax’s vaccine is in Phase 3 trials and the company said results are expected as soon as early first quarter of this year.

Merck said on Monday it is continuing to study and scale-up manufacturing for two potential coronavirus treatments, MK-7110, which may help control the inflammatory response to the virus, and molnupiravir, known as MK-4482, an oral antiviral developed with Ridgeback Bio.