First Participants Dosed in Study Evaluating COVID-19 Booster Candidates for Moderna Vaccine


An ongoing phase 2 clinical study has begun to vaccinate patients with a modified version of Moderna’s COVID-19 vaccine.

In order to address the potential need for booster vaccine candidates in light of the emergence of SARS-CoV-2 variants, an ongoing phase 2 clinical study has begun to vaccinate patients with a modified version of Moderna’s COVID-19 vaccine called mRNA-1273.351.

The company’s modified vaccine was developed to target the SARS-CoV-2 variant B.1.351, which was first identified in South Africa. However, mRNA-1273.351 is also being assessed as a potential booster vaccine for other emerging variants with key-receptor-binding domain mutations.

The modified vaccine is a multivalent candidate that combines Moderna’s previously authorized vaccines, mRNA-1273 and mRNA-1273.351, into a single vaccine. The combination of both candidates is designed to trigger a broad immune response that can work as both a primary series for initial vaccinations of patients and as a boost to those who have previously received mRNA-1273.

Prior research has demonstrated that vaccination with mRNA-1273 produces neutralizing titers against all key variants tested, including the variant B.1.1.7 that was first identified in the UK and the variant B.1.351. The study had found that there was an additional 6-fold reduction in neutralizing titers against B.1.351 in particular.

However, despite the efficacy of mRNA-1273 against these emerging variants, Moderna has decided to establish a clinical development strategy that amends their ongoing phase 2 study to specifically address the emerging variants. The trial is enrolling 60 participants who previously were vaccinated with mRNA-1273 to receive a single booster shot of either: 20 µg of the variant-specific booster candidate mRNA-1273.351 targeting the B.1.351 variant, 50 µg of mRNA-1273.351, or 50 µg of the multivalent booster candidate mRNA-1273.211, which combines mRNA-1273 and mRNA-1273.351 into a single vaccine.

In a prior amendment to the ongoing phase 2 clinical study, participants who had been vaccinated with mRNA-1273 received a single 50 µg booster dose of mRNA-1273. However, the findings of this protocol extension have not been published as the dosing is ongoing.

In addition to the trial assessing the multivalent modified vaccine mRNA-1273.351, the National Institute of Allergy and Infectious Diseases (NIAID) is also conducting a phase 1 clinical trial designed to investigate the use of monovalent and multivalent modified mRNA-1273 vaccines as a primary series in vaccine-naïve individuals and as a booster vaccine for those previously vaccinated with mRNA-1273. However, this study has not yet begun as the NIAID is waiting to receive a safe-to-proceed authorization from the FDA.


Moderna Announces First Participants Dosed in Study Evaluating COVID-19 Booster Vaccine Candidates. Cambridge, MA: Moderna; March 10, 2021. Accessed March 11, 2021.

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