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Many of the leading vaccine candidates in development (eg Moderna, Pfizer, Sanofi) are based upon injection of DNA or RNA genetic material that is taken up into the body’s cells, programmed to produce SARS-CoV-2 viral proteins, and then generate a specific immune response. While this approach has shown promising early results in Phase I and II trials, the genetic vaccine model is completely new and there are no existing vaccines currently in use based on this technology.

So today’s report will first look back in order to look forward, by focusing on Merck’s existing viral vector technology that has been previously used in a vaccine to the Ebola virus. Merck currently has two SARS-CoV-2 vaccine candidates based on the same technology: the first uses a recombinant vesicular stomatitis virus platform to infect host cells to first produce and then release viral proteins leading to the production of protective antibodies. A second Merck vaccine utilizes a measles virus vector platform which was earlier developed by the renown Institut Pasteur (and exclusively licensed to Merck) to create the same immune response.

Together, these two vaccine candidates have given Merck a separate lane within the vaccine development pipeline with a few distinct advantages. First, both vaccine candidates are based upon single-dose administration and do not require a second booster dose. Secondly, the measles-based vaccine can potentially be given orally, which could be advantageous for very young or older populations. And finally the Merck vaccines have a slightly different mechanism of action compared to similar viral-vector models (eg Oxford/Astrazeneca, Johnson and Johnson, CanSino). While many of these are further along in clinical trials, all of the competitor vaccines are based on the common adenovirus (compared to Merck’s use of the measles or stomatitis virus). Recent data has raised concerns about a potential obstacle to widespread use of these vaccines as many individuals have pre-existing immunity to the adenovirus. With existing circulating antibodies to the adenovirus vector, this could block the body from developing an immune response.

Merck has announced the beginning of clinical trials for both vaccines set to begin later this month and should have initial results by later this year. While their late start to clinical trials may not be leading the pack, their unique model could parlay their other advantages to ultimately put them ahead compared to the other vaccine models in development.

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