Moderna has begun giving booster doses of its COVID-19 vaccine to trial participants as part of an amendment to a phase 2 clinical study.
The phase 2 trial amendment has enrolled 60 participants who have already received Moderna’s original mRNA-1273 vaccine. They will receive a single booster dose of a modified candidate addressing the B.1.351 variant or a multivalent booster candidate.
The variant-specific booster candidate – mRNA-1273.351 – encodes for the spike protein of the SARS-CoV-2 variant B.1.351, first discovered in South Africa.
Moderna will evaluate two doses of mRNA-1273.351 – 20 µg and 50 µg – to determine if this candidate can act as a booster to address and improve protection against the B.1.351 variant.
The study will also evaluate a 50 µg dose of Moderna’s multivalent booster candidate – mRNA-1273.211 – which combines Moderna’s original vaccine with mRNA-1273.351 in a single vaccine.
In a previous protocol amendment, phase 2 study participants who had already been vaccinated with mRNA-1273 were given a single 50 µg booster dose of the authorised vaccine.
Moderna has previously conducted in vitro studies using blood samples from individuals who had been vaccinated with mRNA-1273 and exposed to the UK and South Africa variants, to determine the level of protection that the jab offers against the new strains.
Vaccination with Moderna’s original COVID-19 vaccine produced neutralising antibodies against both the UK and SA variants, known as B.1.1.7 and B.1.351 respectively.
In this early laboratory study, researchers observed no significant impact on antibodies with the UK variant, relative to prior variants.
However, Moderna noted that there was a six-fold reduction in neutralising antibodies observed with the South African B.1.351 strain, although the company added that the levels induced by its vaccine should still offer protection.
Pfizer/BioNTech also reported that the effectiveness of their COVID-19 vaccine could be reduced by the South African B.1.351 variant.
A laboratory study used an engineered version of the virus that contained all the same mutations on the spike protein that have been observed in the B.1.351 variant.
The results, published in The New England Journal of Medicine (NEJM), showed that neutralisation of the South African B.1.351-spike virus was reduced by around two-thirds in the BNT162b2-vaccinated blood samples.
This was in comparison to the level of neutralising antibodies observed against the most common version of the novel coronavirus in US trials of the vaccine.
However, the researchers noted that it is difficult to determine the effect this reduction in antibodies will have on the vaccine’s efficacy against the variant.
Similarly to Moderna, Pfizer/BioNTech have said that they are working on a booster dose for their COVID-19 vaccine, with the aim of offering protection against new variants.