Pfizer and Moderna have launched two 30,000-subject trials of Covid-19 vaccines that could clear the way for regulatory approval and widespread use by the end of this year.
The trials are the first late-stage studies supported by the US administration's effort to speed development of measures against the coronavirus, adding to hope that an effective vaccine will help end the pandemic.
Both vaccine candidates rely on a new technology that allows for faster development and manufacturing than traditional vaccine production methods but does not have an extensive track record.
So-called mRNA, or synthetic messenger RNA (mRNA), teaches the immune system to recognise and neutralise the coronavirus by mimicking its surface.
Moderna, which has never brought a vaccine to market, has received nearly $1 billion from the US government, which is helping bankroll several vaccine candidates under its Operation Warp Speed programme.
Pfizer has an agreement to sell vaccines for 50 million people to the US government for around $2 billion, if the vaccine is effective.
More than 150 coronavirus vaccine candidates are in various stages of development, with some two dozen prospects already conducting human testing.
Johnson and Johnson is launching clinical trials in the US this week and could start a larger, late-stage trial as early as September.
British drugmaker AstraZeneca said it will begin large-scale US trials this summer of its vaccine under development with Oxford University researchers.
Dublin-listed pharmaceutical company Open Orphan has also announced a new contract with Codagenix to conduct a first human trial evaluating safety and immunogenicity of a single-dose Covid-19 vaccine.
Manufacturers are ramping up production while testing is underway in order to respond as soon as possible to virus, which is still spreading rapidly around the world. Covid-19 has killed nearly 650,000 people worldwide and battered economies.
Moderna could have tens of millions of doses ready when and if the vaccine is deemed safe and effective.
Pfizer said that if the trial was successful, it could seek regulatory approval as soon as October and supply vaccines for 50 million patients, at two doses each, by the end of the year.
Pfizer aims for about 1.3 billion doses by the end of 2021, and Moderna aims for 500 million to a billion doses a year, beginning 2021, its chief executive Stephane Bancel said.
The late-stage trials are designed to evaluate the safety of vaccines and determine if they can prevent symptomatic Covid-19.