Pharmaceutical giant AstraZeneca on Thursday announced that clinical trials of an antibody-based COVID-19 drug had shown robust efficacy and long-term prevention.

The Anglo-Swedish laboratory follows in the footsteps of U.S. firms Merck and Pfizer, which announced in October and early November respectively that they had developed drugs that could prevent severe forms of the disease, and that could be taken at the first sign of symptoms.

The AstraZeneca drug AZD7442, made from a combination of two antibodies, has been undergoing final stage clinical trials to assess its safety and efficacy.

A six-month follow-up trial "showed robust efficacy from a one-time intramuscular (IM) dose of the long-acting antibody (LAAB) combination," it said in a statement.

One 300mg IM dose reduced the risk of developing symptomatic COVID-19 compared to placebo by 83 percent, it added.

More than 75 percent of participants had comorbidities that put them at high risk for severe COVID-19 if they were to become infected, but no severe cases were recorded.

Another trial to test its efficacy in treating those already suffering from "mild to moderate" COVID-19 symptoms showed that one 600mg IM dose reduced the risk of developing severe illness or death by 88 percent.

"These compelling results give me confidence that this long-acting antibody combination can provide my vulnerable patients with the long-lasting protection they urgently need to finally return to their everyday lives," said Hugh Montgomery, professor of intensive care medicine at University College London.

"Importantly, six months of protection was maintained despite the surge of the Delta variant among these high-risk participants who may not respond adequately to vaccination."

AstraZeneca said the full results of both trials would be submitted for publication to a peer-reviewed medical journal.

It has already submitted an application to U.S. regulators for approval of the drug to treat COVID-19.