Merck announced today that it is abandoning two Covid-19 candidates that it had been working on. The Phase I trials showed poor stimulation of antibodies, both neutralizing and non-neutralizing, in their trial subjects. The company said it will focus efforts on two anti-SARS-CoV-2 therapeutic agents it is developing, an injected and an oral antiviral.

Merck’s V590 and V591 vaccine candidates were built on a somewhat different technology than the other ones we’ve been hearing about. Both were based on “replicating viral vectors.” Like the adenovirus-based vaccines such as Janssen (Johnson & Johnson) and AstraZeneca, they splice the spike gene from the SARS-CoV-2 virus into another virus. Unlike those vaccines, the V590 and V591 viruses are intended to reproduce in your body.

V590 was based on VSV (vesicular stomatitis virus), a virus that mainly affects horses and cattle. V591 was based on the measles virus. Both VSV and measles are common choices for replicating viral vectors.

V590 received a small amount of research funding from the US government, but was not part of Operation Warp Speed. V591 was being funded by CEPI, the Coalition for Epidemic Preparedness.

Merck will continue work on V590 in order to test it with a different route of administration. Like most other vaccines in the first wave, V590 had been injected into muscle; the company speculates that it might work better if delivered nasally or orally.

Derek Lowe’s latest vaccine roundup has some more details on Merck’s exit.