Moderna has dosed the first subjects in a Phase II clinical study with its modified Covid-19 vaccines designed to address the potential need for booster vaccine candidates.

Enrolment is being carried out by amending the ongoing Phase II clinical study.

A variant-specific booster candidate, mRNA-1273.351, encodes for prefusion stabilised SARS-CoV-2 variant B.1.351’s spike protein. This variant was first seen in the Republic of South Africa.

It is being evaluated as a booster vaccine to increase the extent of the response to emerging variants with key-receptor-binding domains (RBD) mutations.

A multivalent booster candidate, mRNA-1273.211 combines Moderna’s authorised vaccine mRNA-1273 against ancestral strains and mRNA-1273.351 in a single vaccine.

It can potentially elicit a broad immune response as a primary series, as well as a booster to those who are already vaccinated with mRNA-1273.

According to the previously published data, vaccination with the Moderna Covid-19 vaccine produced neutralising titers against all key variants tested, including variants B.1.1.7 and B.1.351, with a six-times reduction in neutralising titers against B.1.351.

As a precautionary measure, the company is pursuing a clinical development strategy against the emerging variants.

An amendment to the Phase II study will have 60 subjects vaccinated with mRNA-1273 to receive a single booster dose of either 20µg of mRNA-1273.351 or 50µg of mRNA-1273.351 or 50µg of mRNA-1273.211.

In a previous protocol amendment, Phase II study subjects vaccinated with mRNA-1273 were given a single 50µg booster dose of mRNA-1273. Dosing in this protocol extension is progressing.

The National Institutes of Health (NIH) unit National Institute of Allergy and Infectious Diseases (NIAID) will also carry out a Phase I clinical trial at the same time.

This study will evaluate the monovalent and multivalent modified mRNA-1273 vaccines as a primary series in naïve people and as a booster vaccine in those vaccinated with mRNA-1273.

The institute will begin the study on obtaining safe-to-proceed authorisation from the US Food and Drug Administration.