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September 29, 2021

Why you can't get a delta-specific booster—yet

Daily Briefing

    mRNA vaccines tailored to specific variants, including the highly contagious delta variant, are currently being tested in clinical trials—but some health experts suggest these tailored vaccines may not be necessary.

    Your top resources on the Covid-19 vaccines

      Companies test variant-specific vaccines in clinical trials

      According to Vox, the mRNA technology used for Pfizer-BioNTech's and Moderna's Covid-19 vaccines allow scientists to reformulate the vaccine as needed—meaning they can develop Covid-19 vaccines tailored to specific variants of the coronavirus, including the delta variant.

      Currently, Pfizer-BioNTech is working on two reformulated vaccine candidates, one targeting the beta variant and one targeting the delta variant. According to Vox, results from the clinical trial testing the delta-specific vaccine are expected in the fourth quarter of this year.

      A spokesperson for both companies said, "The trials… shall enable the companies to rapidly react if an escape variant should require an updated vaccine. The companies are exploring the development, manufacturing, and regulatory pathway that BioNTech and Pfizer would pursue in this case to be able to adapt the vaccine within 100 days if necessary."

      Similarly, Moderna earlier this month announced it is developing four vaccine candidates that target different coronavirus variants. Two candidates target the beta and delta variants separately, one targets both the beta and delta variants, and one targets the beta variant and a wild-type strain, MedPage Today reports.

      According to Moderna, three of its four targeted vaccine candidates are currently in Phase 2/3 trials that will determine their efficacy. Testing of the fourth candidate, which targets both beta and delta, is scheduled to begin in the next few weeks, Vox reports.

      "The company's strategy is to develop booster vaccines against current variants of concern and against potential future variants of concern," Moderna said in a press release.

      Are variant-specific vaccines needed?

      Although the development of variant-specific vaccines is progressing, some health experts say these tailored vaccines may not yet be necessary.

      "Unless there is clear evidence of loss of protection, updating the vaccines every time a new variant takes over the population might not be the best strategy," Ramon Lorenzo Redondo, a molecular virologist at Northwestern University, said.

      Both Pfizer-BioNTech and Moderna have indicated their original Covid-19 vaccines remain effective against the coronavirus, including the delta variant, MedPage Today reports.

      In addition, both companies found that antibodies against the delta variant increased in clinical trials of booster doses of their already authorized vaccines. Specifically, a third dose of the Pfizer-BioNTech vaccine resulted in a five- to 11-fold increase in antibodies against the delta variant, and a third dose of the Moderna vaccine led to a 42-fold increase against the delta variant, Vox reports.

      Florian Krammer, a virologist at Mount Sinai, said it is "not clear whether there is even a benefit in changing the vaccine strain," since trial results showing additional doses of the current vaccines provide "as much protection ... against beta, but also against delta" as specifically reformulated vaccines.

      According to Dan Barouch, an immunologist at Beth Israel Deaconess Medical Center, delta-specific vaccines likely won't be much more effective than the original vaccines because the delta variant's spike protein looks and is shaped similarly to that of the original coronavirus. This means that antibodies created by the original vaccine formulas, if given third doses, should work effectively against the delta variant, Vox reports.

      Separately, Benjamin Linas, an epidemiologist at Boston University, said another reason development of delta-specific vaccines has been slow is because public health officials and drugmakers were concerned unused doses of the original vaccines would be wasted if people only wanted "new and improved" versions of the vaccines.

      "I don't know what message it would give if [Pfizer or Moderna] said, 'Oh we have a specially engineered vaccine for delta,'" Linas said. "Would that generate concern that the original vaccine didn't work? Which is absolutely not true."

      Separately, Angela Rasmussen, a virologist at the University of Saskatchewan, said while this variant-specific vaccine development could be useful for viruses such as the seasonal flu, "where there's already a process for rapidly evaluating reformulated vaccines," it doesn't make sense to do it for the coronavirus right now. "[I]n the urgent situation we face now with Covid, the regulatory process still takes time that right now we don't have," she said.

      Similarly, Celine Gounder, an infectious disease specialist and epidemiologist at NYU and Bellevue Hospital, said while developing variant-specific vaccines may be a good "practice run" to test the approval and manufacturing of such vaccines for more dangerous variants that may emerge in the future, it could at this time just end up averting some manufacturing plants away from the original vaccines.

      "Then the question is, is that really the best use of your manufacturing capacity when it hasn't been proven that you really need to do that to protect against delta?" Gounder said. "You're going to have, one, the delay of manufacturing and having to allocate specific manufacturing plants for that purpose, and two, the FDA review that would go into it, so that creates a delay. I don't think that trade-off makes sense for the delta variant." (Samuel, Vox, 9/27; Fiore, MedPage Today, 9/17)

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