US biotech firm Novavax said Thursday it was initiating its final Phase 3 clinical trial for its experimental Covid-19 vaccine.

The trial will be carried out in the United Kingdom and aims to enroll 10,000 volunteers, aged 18-84, with and without underlying conditions, over the next four to six weeks.

"With a high level of SARS-CoV-2 transmission observed and expected to continue in the UK, we are optimistic that this pivotal Phase 3 clinical trial will enroll quickly and provide a near-term view of NVX-CoV2373's efficacy," said Gregory Glenn, the company's president of research and development, using the technical name for the formulation.

It is the eleventh Covid-19 vaccine candidate to reach the Phase 3 stage globally.

The company has been awarded $1.6 billion by the US government to develop and fund the drug, which is administered by two intramuscular injections.

The Maryland-based company uses insect cells to grow synthesized pieces of the spike protein of the virus, which it hopes will evoke a robust human immune response.

It also uses an "adjuvant," a compound that boosts the production of neutralizing antibodies.

The company says the drug, which is a liquid formulation, can be stored at two degrees celsius to eight degrees celsius, refrigerator temperature.

In the spring, the company said it had proven the efficacy of a seasonal flu vaccine it had developed using the same technology.