Phase 2 Trial of COVID-19 Vaccine Candidate Shows Strong Immune Responses in Adults

High antibody levels were also generated in participants with evidence of a prior SARS-CoV-2 infection, suggesting that this could be developed as a booster.

The phase 2 trial of a COVID-19 vaccine candidate from GlaxoSmithKline and Sanofi has found strong neutralizing antibody responses in all adult age groups, according to a statement.

The candidate is an adjuvanted recombinant COVID-19 vaccine with a global pivotal phase 3 trial expected to begin in the coming weeks. Phase 2 interim results showed 95% to 100% seroconversion following a second injection in all age groups between the ages of 18 and 95 and across all doses.

The trial was conducted in healthy adults aged 18 years and older, including those with high-risk medical conditions. Two injections were administered 21 days apart, with 3 antigen dose levels of 5, 10, and 15 µg. The trial enrolled 722 volunteers in Honduras and the United States beginning in February 2021 and included equivalent numbers of adults between the ages of 18 and 59 years and those 60 years and above.

“Our phase 2 data confirm the potential of this vaccine to play a role in addressing this ongoing global public health crisis, as we know multiple vaccines will be needed, especially as variants continue to emerge and the need for effective and boost vaccines, which can be stored at normal temperatures, increases,” Thomas Troimphe, MSc, MBA, executive vice president and global head of Sanofi Pasteur, said in the statement. “With these favorable results, we are set to progress to a global phase 3 efficacy study.”

Overall, the antibody levels were comparable to those generated by natural infection, with higher levels observed in younger adults between the ages of 18 and 59 years, according to the statement.

After a single injection, high antibody levels were also generated in participants with evidence of a prior SARS-CoV-2 infection, suggesting that the vaccine could be developed as a booster vaccine.

“These positive data show the potential of this protein-based adjuvanted vaccine candidate in the broader context of the pandemic, including the need to address variants and to provide for booster doses,” Roger Connor, president of GSK Vaccines, said in the statement. “We believe that this vaccine candidate can make a significant contribution to the ongoing fight against COVID-19 and will move to phase 3 as soon as possible to meet our goal of making it available before the end of the year.”

In addition to this vaccine development with GSK, Sanofi is working on a messenger RNA vaccine in partnership with Translate Bio, according to the statement.

In March 2021, the companies initiated a phase 1/2 clinical trial of this candidate to assess safety, immune response, and reactogenicity. First results are expected in the third quarter of 2021.


Sanofi and GSK COVID-19 vaccine candidate demonstrates strong immune response across all adult age groups in phase 2 trial. News release. Sanofi. May 17, 2021. Accessed May 21, 2021.