A British doctor pleaded guilty Monday for the attempted murder of his mother’s partner by disguising himself as a nurse to inject the victim with poison but telling the victim it was a Covid-19 vaccine.

Thomas Kwan, 53, sported a wig, facial hair and medical mask when he administered the fake Covid vaccine to Patrick O’Hara on Jan. 22 in Newcastle, located in northern England, more than three hours north of London.

Northumbria police said Kwan arranged a fake medical appointment with O’Hara by sending him bogus letters stating that he needed a Covid vaccination, according to Reuters. 

The police said Kwan wore a disguise so that neither his mother nor O’Hara, who are both in their 70s, would recognize him.  They believed that he was merely a nurse. 

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O’Hara developed a skin condition around the injection area and became “seriously unwell” just hours after it was injected, police said. O’Hara visited his general practitioner and was immediately admitted to the hospital. Upon his arrival, he showed hospital staff letters of his injection, which they deemed fake.

He underwent skin grafts to repair “extensive damage the poisoning caused to his body” and was left with “life-changing injuries,” according to police. The skin condition is a fatal flesh-eating bacterial infection called necrotizing fasciitis. The British Crown Prosecution Service said O’Hara was injected with “an as-yet unconfirmed toxin.”

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Police say that Kwan installed fake license plates on his vehicle before driving to O’Hara’s home, where the poisonous injection took place. Prosecutor Peter Makepeace said that Kwan’s motive likely regarded his mother’s will, which states that her partner would receive her home should she die and that O’Hara was still alive.

When police searched Kwan’s home, they found files on his computer that contained downloads of a “poisoner’s handbook” and other files related to using poisons to kill a person.

“Mr. Kwan used his encyclopedic knowledge of, and research into, poisons to carry out his plan,” Makepeace told jurors on the first day of the trial.

Kwan was charged with attempted murder and causing grievous bodily harm with intent. Although he initially pleaded not guilty, he pleaded guilty Monday to the attempted murder charge.

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His sentencing will be held at a later date, and he remains in custody.

“The weight of evidence faced by Kwan was overwhelming and he has now admitted (to) attempted murder,” Detective Chief Inspector Jason Henry, of Northumbria Police, said. “While nothing can change the impact of Kwan’s actions on his victim, we do hope the fact he has been brought to justice will help them move on with their life.”