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If it proves effective, it could protect vulnerable people who haven't yet been, or can't be, vaccinated.
And it could help to contain outbreaks.
The trial, run at University College London Hospitals (UCLH) NHS Trust, is looking at whether an injection of two different antibodies could prevent someone who has been exposed to Covid from developing the disease - or at least from becoming very ill.
Vaccines take weeks to offer full protection, meaning it's too late for them to be given once someone already has the virus brewing in their system.
But this monoclonal antibody treatment, developed by the drugs company AstraZeneca, should work to neutralise the virus immediately.