The suspect said he was from the NHS before jabbing the victim in the arm with a "dart-like implement".
Police are hunting for a man who injected a 92-year-old woman with a fake coronavirus vaccine and charged her £160.
Detectives have warned that the suspected conman "may endanger people's lives" unless he is caught.
The victim allowed him into her home in Surbiton, southwest London, on 30 December after he said he was from the NHS.
She said she was jabbed in the arm with a "dart-like implement" and was told to pay £160, which the fraudster said would be refunded.
It is not known what substance - if any - was injected, but the woman showed no ill effects after being checked over at her local hospital.
Detective Inspector Kevin Ives from the City of London Police said: "This is a disgusting and totally unacceptable assault on a member of the public which won't be tolerated.
"It is crucial we catch him as soon as possible as not only is he defrauding individuals of money, he may endanger people's lives."
Images released by police show the suspect dressed in a navy blue tracksuit with white stripes down the side.
The pictures are from a second visit he made to the woman's home, when he asked for another £100.
He is described as a white man in his early 30s, who is around 5ft 9in tall, of medium build, with light brown hair that is combed back and speaks with a London accent.