Europe's drug regulator said on Thursday it had started a real-time review of AstraZeneca's antibody-based COVID-19 therapy, the first protective shot other than vaccines against coronavirus.

The decision by the human medicines committee of the European Medicines Agency (EMA) to begin the rolling review was based on early results from clinical studies, the regulator said. It did not say when a conclusion was expected.

The move to start a real-time review, so called because data is evaluated as it is made available, came roughly a week after the Anglo-Swedish drugmaker sought emergency approval from U.S. authorities.

While vaccines rely on an intact immune system to develop targeted antibodies and infection-fighting cells, AstraZeneca's biotech compound, to be branded as Evusheld, contains lab-made antibodies designed to linger in the body for months to contain the virus in case of an infection.

The medicine has been proven to work among people who are not infected and was also shown this week to save lives and prevent severe disease when given as treatment within a week of first symptoms.

"EMA will evaluate more data on the quality, safety and effectiveness of the medicine as they become available," the European Union's (EU) watchdog said, adding the review would continue until AstraZeneca can formally submit an application for approval.

AstraZeneca has said that talks regarding supply agreements for Evusheld, also known as AZD7442, are ongoing with the United States and other governments.

There are some COVID-19 therapies based on the same class of medicine as AstraZeneca, called monoclonal antibodies, that aim to treat rather than prevent the illness. They are sold by rivals Regeneron, Eli Lilly and GSK-Vir outside the EU to stop the disease from worsening during early, milder stages of the infection.

Other antibody-based COVID-19 treatments under a real-time review by the EMA are GSK-Vir's treatment, Sotrovimab, and Lilly's cocktail. Gilead's remdesivir is the only drug of any kind to be approved in the EU for COVID treatment.

The EMA is also considering whether to launch a rolling review of Merck's experimental COVID-19 pill.