Police and GPs were on the alert today after reports of a bogus nurse giving injections to elderly people at rest homes in Hove.
At least two homes received visits from the woman. Two people were given vaccinations.
Health chiefs are monitoring the pensioners in an attempt to identify the substance with which they have been injected. So far, neither has suffered any ill effects.
Inquiries have failed to find any GP or clinic authorising the visits but inquiries by the Brighton and Hove City Primary Care Trust were continuing.
A spokesman admitted they were worried and this week the trust sent warning letters to GPs, district nurses and nursing and sheltered homes in the city.
A doctor said: "I've heard this lady arrives at the homes dressed smartly in a red and grey suit and appears to be completely genuine.
"There are very strict procedures before anyone is given access to patients. This is very, very worrying."
Police have also been informed.
Inspector Mark Powles said: "We are waiting for more information from the trust to clarify whether this woman is bona fide.
"If she proves to be bogus then we will be investigating serious offences of assault."
He appealed for anyone with information to come forward.
Brighton GP Xavier Nalletamby is one of dozens of doctors to receive the warning.
He said: "It seems completely bizarre anyone would want to do such a thing.
"I just thought this was a case where someone had not been properly identified. After all, this is the time of year when we go into homes to give flu jabs."
Dr Nalletamby said the letter asked whether they knew or could vouch for the woman and to report anything suspicious.
The PCT spokesman said: "This woman may be bogus but we need to be 100 per cent sure we are not making a mistake - she could be an agency nurse no one recognises."
He said so far no one had come forward to identify her.
He added: "Meanwhile, it is essential GPs, district nurses and staff at care, nursing and sheltered homes are made aware of the concern.
"We are working with the police on this matter. We need to make sure everything possible is being done to protect our patients in the city. We need to be vigilant."
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