GlaxoSmithKline and Sanofi's Covid-19 vaccine saw strong results from its phase 2 trial and will now move on to a larger phase 3 trial as the drug firms try to catch up with the competition.
Interim results from the study show a 95% to 100% seroconversion following a second injection in patients from ages 18 to 95, with no safety concerns raised.
There was a high immune response after a single dose, as the companies hope the vaccine could be used as part of any booster programme for later in the year.
World Health Organization officials have called on wealthier nations to ensure poorer countries have access to initial vaccination programmes before administering their own booster initiatives.
Sanofi and GSK added there are also trials and research ongoing to see whether vaccines made by different companies can be mixed in patients.
Those who received the pharmaceutical brands' vaccine showed strong neutralising antibody levels compared with those who received a placebo in the study across 722 volunteers.
The phase 3 study will kick off "in the coming weeks", the companies said, and they pointed out their vaccine could be stored at normal temperatures.
Thomas Triomphe, executive vice president at Sanofi, said: "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."
Roger Connor, president of GSK Vaccines, said: "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."
The phase 3 trial will involve 35,000 volunteers and look at the effectiveness against the Wuhan and South African variants.
Bosses added that they intend to conduct booster studies with various variant formulations to assess the ability of a lower dose of the vaccine to generate a strong booster response regardless of the initial vaccine platform received.