Merck cans both its COVID-19 vaccines due to weak clinical data 

Merck
Merck’s COVID-19 efforts are now focused on two therapeutics that are nearing key readouts. (Merck)

Merck has stopped development of its two COVID-19 vaccine candidates after getting a look at phase 1 data. The vaccine powerhouse said neither candidate triggered immune responses comparable to those achieved by rival jabs, leading Merck to cut its losses and focus on COVID-19 therapeutics. 

Last year, Merck executives emphasized the proven nature of their vaccine platforms and cast doubts on the likelihood of rivals with more experimental technologies completing development in months. The narrative was that, while companies such as Moderna and Pfizer had taken early leads by using mRNA, Merck would come from behind and deliver the effective, scalable, one-dose vaccines needed to end the pandemic.

Things played out differently in reality. Moderna and Pfizer, working with BioNTech, smashed Merck’s record for the fastest vaccine R&D program and delivered stellar data that left little room for rivals to improve on short-term efficacy. Now, Merck has exited the COVID-19 vaccine race altogether. 

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Merck dropped out of the race after reviewing phase 1 data on V590 and V591. A full post mortem will take place when Merck publishes the results in a peer-reviewed journal, but the top-line findings explain the decision. The immune responses to V590 and V591 were inferior to the natural responses seen in patients infected with SARS-CoV-2 and the responses triggered by rival vaccines. Cross-trial immune comparisons can be unreliable, but clearly the data were weak enough to kill the vaccines. 

The decision marks the end for V590 and V591. Merck worked on recombinant vesicular stomatitis virus vaccine V590 with IAVI, building on its experience of busing the vector to protect against Ebola. V591 is a measles virus vector vaccine Merck acquired in its takeover of Themis. 

Merck has achieved success with other replicating virus vaccines. However, the travails of V590 and V591 could kill off the approach in COVID-19. Other companies, notably AstraZeneca and Johnson & Johnson, are working on non-replicating viral vector vaccines but Merck was the most prominent developer of replicating viral vaccines.

The failures of V590 and V591 end Merck’s status as a prominent developer of COVID-19 vaccines, although the Big Pharma is yet to completely step away from the space. IAVI said it is continuing to work with Merck to evaluate whether changing the route of administration will improve outcomes.

For now, Merck’s COVID-19 efforts are focused on two therapeutics that are nearing key readouts. Full results from a phase 3 trial of MK-7110, the fusion protein Merck acquired in its takeover of OncoImmune, are scheduled for the first quarter of 2021. Initial efficacy data from a phase 2/3 trial of the Ridgeback Bio-partnered molnupiravir are due around the same time. Merck said it will share the initial efficacy data publicly if the results are clinically meaningful.