AstraZeneca and Regeneron on Thursday reported contrasting data on the effectiveness of their Covid-19 antibody therapies against the Omicron variant of the coronavirus, underscoring the major challenges ahead for drugmakers.

US-based Regeneron said its REGEN-COV therapy, also called Ronapreve, is less effective against Omicron, though it is still active against the Delta variant, confirming indications from lab tests and computer modelling late last month.

Anglo-Swedish rival AstraZeneca, however, said a lab study found that its antibody cocktail Evusheld retained neutralising activity against Omicron, the first such data for the treatment.

The new variant has been detected in 77 countries since it was first identified three weeks ago, fuelling concerns that its large number of mutations will help it spread faster and evade protection provided by Covid-19 vaccines and therapeutics.

Earlier this week, German researchers found that Covid-19 therapies developed by Eli Lilly and Regeneron lose most of their effectiveness when exposed in laboratory tests to Omicron.

But lab studies this week showed GSK-Vir's antibody treatment retains neutralizing activity against all tested coronavirus variants, including Omicron.

The study on Evusheld was done by independent investigators of the US Food and Drug Administration using so-called pseudoviruses that feature major coronavirus mutations across suspicious variants that have emerged so far.

Road ahead

The researchers found the potency of Evusheld was within the ranges seen in those previously infected with Covid-19, showing promise for wide use of the therapy, which is the first preventative shot other than vaccines against the coronavirus.

AstraZeneca said more analyses of Evusheld against Omicron are being conducted by the company and third parties, with data expected "very soon."

Regeneron said REGEN-COV is still active against Delta, which currently is the most prevalent variant in the United States.

The European Union's public health body, however, warned on Wednesday that there was a "very high" risk Omicron would become dominant in Europe by early next year.

REGEN-COV is currently authorized for use in the United States to treat people with mild-to-moderate Covid-19 and for prevention of infection in those exposed to infected individuals, and others at high risk of exposure.

Evusheld has been granted authorisation to prevent Covid-19 infections in individuals with weak immune systems or a history of severe side-effects from coronavirus vaccines.

Both treatments belong to a class of medicines called monoclonal antibodies which mimic natural antibodies in fighting off infections.

Unlike vaccines, they do not rely on the body to create an immune response, and can therefore help individuals with weak or compromised immune systems.