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Sanofi & GSK Covid-19 vaccine posts strong clinical data, paves way for Phase 3

An experimental vaccine that pairs influenza technology from Sanofi and GlaxoSmithKline is showing efficacy against Covid-19, clinical trial results suggesting the shots could offer yet another preventative measure for infection and support its use as a booster for those who have already been vaccinated. The two pharmaceutical giants now plan to advance their vaccine candidate to a global clinical test in coming weeks.

According to early Phase 2 results announced Monday, the vaccine from Sanofi and GSK triggered a strong immune response among all adults. Seroconversion rates, a measure of the development of detectable antibodies in the blood against a pathogen, ranged from 95% to 100%. The levels of neutralizing antibodies, which keep pathogens from infecting cells, were comparable to levels generated in people who developed Covid-19 from natural infection.

The results announced Monday are preliminary. The companies said they will publish full results in a peer-reviewed scientific journal.

The Sanofi-GSK vaccine consists of two shots given 21 days apart. The vaccine candidate is protein-based, using antigen engineered by Sanofi. The company makes that antigen with technology that is the basis for its FDA-approved Flublok influenza vaccine. That antigen is paired with adjuvant, an ingredient that boosts the immune response, supplied by GSK. This adjuvant has already been used in pandemic influenza vaccine.

By developing a vaccine based on established influenza technologies, the Sanofi and GSK vaccine could offer some advantages. Unlike the ultra-freezing temperatures required for transporting and storing the messenger RNA vaccines from Moderna and partners Pfizer and BioNTech, the Sanofi and GSK vaccine can be kept at standard refrigerator temperatures, making it easier to store and distribute globally. The basis of this vaccine on technologies that have already been reviewed and approved for influenza means that Sanofi and GSK should have a smoother and faster review compared to mRNA vaccines, a newer technology that regulators had not previously evaluated. And finally, GSK and Sanofi already have manufacturing in place for producing their respective components of the vaccine, which should help them ramp up production quickly.

The encouraging preliminary Phase 2 data follow a setback the two companies encountered late last year. In December, Sanofi and GSK said that the immune response to the vaccine in a Phase 1/2 test was insufficient in older adults. Though the response in those 18 to 49 was strong, the failure to produce those results in older adults who are more vulnerable to infection meant that the companies needed to refine the concentration of the antigen in the vaccine. The change extended the development timeline by about two months.

The double-blind Phase 2 study began in February, enrolling patients in the U.S. and Honduras. The 722 volunteers were evenly split between those 18 and 59 and volunteers 60 and older. The dose-ranging test randomly assigned patients to receive one of three antigen dose levels.

The Phase 3 study will test the 10 microgram dose, the middle of the three doses evaluated in the mid-stage study. Sanofi and GSK expect that this pivotal clinical trial will enroll more than 35,000 adults from across the world. The study will also test the efficacy of two different vaccine formulations against two variants of the novel coronavirus, the one first detected in Wuhan, China, and the variant first identified in South Africa.

The companies did not release any additional details about the clinical trial design. A placebo-controlled study presents challenges now that several Covid vaccines are authorized for emergency use. It may be considered unethical to offer clinical trial participants a placebo. Furthermore, enrollment may be difficult if people simply choose to vaccinate with one of the authorized vaccines rather than participate in the clinical trial.

Sanofi and GSK believe there’s still a place for their vaccine, potentially as a booster shot. They note that in the Phase 2 study, a single injection elicited high neutralizing antibody levels in study participants who showed signs of previous SARS-CoV-2 infection. The companies say those results suggest strong potential for their vaccine as a booster shot.

“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 booster vaccines, which can be stored at normal temperatures, increases,” Thomas Triomphe, executive vice president and global head of Sanofi Pasteur, said in a prepared statement.

If the pivotal clinical trial posts positive results and the regulatory review goes smoothly, Sanofi and GSK say they expect regulatory approval in the fourth quarter of this year. In parallel with the Phase 3 study, Sanofi and GSK said they plan to conduct booster studies with different formulations addressing variants of the novel coronavirus. These studies will assess the ability of a lower dose of the vaccine to produce a booster response regardless of the initial vaccine that a patient received.

Photo: Nathan Laine/Bloomberg, via Getty Images

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