Commentary

Understanding messenger RNA and other SARS-CoV-2 vaccines

Publish date: December 14, 2020

Timelines and testing other vaccines

Strong efficacy data from the two leading SARS-CoV-2 vaccines and emergency-use authorization Food and Drug Administration approval suggest the window for testing additional vaccine candidates in the United States could soon start to close. Of the more than 200 vaccines in development for SARS-CoV-2, at least 7 have a chance of gathering pivotal data before the front-runners become broadly available.

Testing diverse vaccine candidates, based on different technologies, is important for ensuring sufficient supply and could lead to products with tolerability and safety profiles that make them better suited, or more attractive, to subsets of the population. Different vaccine antigens and technologies also may yield different durations of protection, a question that will not be answered until long after the first products are on the market.

AstraZeneca enrolled about 23,000 subjects into its two phase 3 trials of AZD1222 (ChAdOx1 nCoV-19): a 40,000-subject U.S. trial and a 10,000-subject study in Brazil. AstraZeneca’s AZD1222, developed with the University of Oxford (England), uses a replication defective simian adenovirus vector called ChAdOx1.AZD1222 which encodes the SARS-CoV-2 spike protein. After injection, the viral vector delivers recombinant DNA that is decoded to mRNA, followed by mRNA decoding to become a protein. A serendipitous manufacturing error for the first 3,000 doses resulted in a half dose for those subjects before the error was discovered. Full doses were given to those subjects on second injections and those subjects showed 90% efficacy. Subjects who received 2 full doses showed 62% efficacy. A vaccine cannot be licensed based on 3,000 subjects so AstraZeneca has started a new phase 3 trial involving many more subjects to receive the combination lower dose followed by the full dose.

Johnson and Johnson (J&J) started its phase 3 trial evaluating a single dose of JNJ-78436735 in September. Phase 3 data may be reported by the end of2020. In November, J&J announced it was starting a second phase 3 trial to test two doses of the candidate. J&J’s JNJ-78436735 encodes the SARS-CoV-2 spike protein in an adenovirus serotype 26 (Ad26) vector, which is one of the two adenovirus vectors used in Sputnik V, the Russian vaccine reported to have 90% efficacy at an early interim analysis.

Sanofi and Novavax are both developing protein-based vaccines, a proven modality. Sanofi, in partnership with GlaxoSmithKline started a phase 1/2 clinical trial in the Fall 2020 with plans to commence a phase 3 trial in late December. Sanofi developed the protein ingredients and GlaxoSmithKline added one of their novel adjuvants. Novavax expects data from a U.K. phase 3 trial of NVX-CoV2373 in early 2021 and began a U.S. phase 3 study in late November. NVX-CoV2373 was created using Novavax’ recombinant nanoparticle technology to generate antigen derived from the coronavirus spike protein and contains Novavax’s patented saponin-based Matrix-M adjuvant.

Inovio Pharmaceuticals was gearing up to start a U.S. phase 2/3 trial of DNA vaccine INO-4800 by the end of 2020.

After Moderna and Pfizer-BioNTech, CureVac has the next most advanced mRNA vaccine. It was planned that a phase 2b/3 trial of CVnCoV would be conducted in Europe, Latin America, Africa, and Asia. Sanofi is also developing a mRNA vaccine as a second product in addition to its protein vaccine.

Vaxxinity planned to begin phase 3 testing of UB-612, a multitope peptide–based vaccine, in Brazil by the end of 2020.

However, emergency-use authorizations for the Pfizer and Moderna vaccines could hinder trial recruitment in at least two ways. Given the gravity of the pandemic, some stakeholders believe it would be ethical to unblind ongoing trials to give subjects the opportunity to switch to a vaccine proven to be effective. Even if unblinding doesn’t occur, as the two authorized vaccines start to become widely available, volunteering for clinical trials may become less attractive.

Dr. Pichichero is a specialist in pediatric infectious diseases, and director of the Research Institute at Rochester (N.Y.) General Hospital. He said he has no relevant financial disclosures. Email Dr. Pichichero at pdnews@mdedge.com.