There are currently no vaccines to prevent the coronavirus that has infected more than 9 million people and killed over 469,000 globally, and only a couple of medicines that have demonstrated benefit in hospitalised Covid-19 patients in clinical trials.
Many drugmakers are racing to come up with a safe and effective vaccine that can be produced at large scale.
Moderna Inc, the University of Oxford in collaboration with AstraZeneca Plc, and an alliance of BioNTech and Pfizer Inc grabbed headlines by moving to human trials as early as March.
Sanofi Chief Executive Paul Hudson said the firsts in the race now were not assured of securing victory.
"There are companies moving faster, but let us be brutally clear, speed has three downsides," he said of competition.
"They are using existing work, in many cases done for SARS; it is likely not to be as efficacious; and there is no guarantee on supply in large volumes," Hudson said.
The probability of success for Sanofi is "higher than anybody else," the CEO said.
The comments echoed those of GSK, whose chief medical officer for vaccines told Reuters on Friday the company was aiming at quality before speed.
Sanofi, whose Pasteur vaccines division has a long established reputation, notably in flu, is currently working on two vaccine projects.
One uses an adjuvant made by GSK to potentially boost its efficacy. It has received financial support from the U.S. Biomedical Advanced Research and Development Authority (BARDA).
The other, being developed with U.S. company Translate Bio Inc, relies on a different technology known as mRNA, similar to the Moderna approach.
Clinical trials of the vaccine developed with GSK, described as a recombinant vaccine because of the use of GSK's boosting adjuvant, are to start in September. Trials of the mRNA vaccine candidate should begin around the end of the year, the company said.
Sanofi said it had capacity to produce up to 1 billion doses a year of its recombinant vaccine, and that it would be able to supply up to 360 million doses of its mRNA vaccine annually.
In April, Sanofi said it had manufacturing capacity for 600 million doses for its recombinant vaccine, with the ambition to double production by mid-2021.
The company also said it would expand its collaboration with Translate Bio on developing vaccines, in a deal which will give the U.S. group $425 million in upfront payments.
The companies said they would expand their existing 2018 collaboration to develop 'mRNA' vaccines for infectious diseases.
Translate Bio will get an upfront payment of $425 million, consisting of $300 million in cash and a private placement common stock investment of $125 million at $25.59 per share, marking a premium of 57.6 per cent to Translate Bio's closing share price of $16.24 on June 22.
Translate Bio will also be eligible for potential future milestones and other payments of up to $1.9 billion, including $450 million of milestones under the 2018 agreement.