COVID-19 vaccine candidate from GSK and Sanofi to move to phase 3 placebo controlled study

Simon Collins, HIV i-Base

On 17 May 2021, GSK reported selected top-line results from a phase 2 study of a candidate COVID-19 vaccine being developed with Sanofi. This also included plans for a large international phase 3 study. [1]

The press release reported that the two-dose adjuvanted recombinant vaccine generated “95% -100% seroconversion rates” after the second dose. Also that “neutralising antibodies that were “comparable to those generated after natural infection”.

The phase 2 study included over 700 participants in the US and Honduras, with half aged 18 to 59 and half aged over 60.

As context, this 95% to 100% is not an efficacy percentage and some currently authorised vaccines generate antibody responses that are much higher than from natural infection.

The upcoming phase 3 study plans to enroll 35,000 volunteers, in a randomised placebo controlled study. This is controversial because all participants in research studies should be offered the current standard of care. Rather than comparing the new vaccine to a placebo, it is more ethical to compare it to one or more vaccines that are already authorised. 

It might also be a challenge that many countries will also have already vaccinated people at highest risk, based on age or other comorbidities. This might make enrolment using the current design difficult.

Other studies include whether a lower dose can be used as a booster.

In this partnership, Sanofi provides the recombinant antigen and GSK provides the pandemic adjuvant. 

GSK is also working with CureVac on an mRNA COVID vaccine. [2]


  1. GSK press statement. Sanofi and GSK COVID-19 vaccine candidate demonstrates strong immune responses across all adult age groups in Phase 2 trial. (17 May 2021).
  2. Joint press statement. GSK and CureVac to develop next generation mRNA COVID-19 vaccines. (3 February 2021).

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