A nurse who went missing after being infected with malaria during a medical trial might have travelled abroad.
Matthew Lloyd, 35, who works at Southampton General Hospital, needs life-saving treatment after taking part in anti-malarial vaccine tests.
He failed to attend a follow-up appointment in Oxford on 7 October.
Detectives are exploring the possibility Mr Lloyd went abroad after receiving no confirmed sightings of him in the UK, despite national coverage.
On Wednesday, police released CCTV film of Mr Lloyd in London's Kings Cross area on 5 October.
Officers said it was the last known sighting of Mr Lloyd, who was exposed to the malaria during an appointment in London on 1 October.
Passport and cash
They say he travelled to Milton Keynes and London, and possibly to Birmingham, between 1 and 7 October, but has not been seen since.
Det Insp Becky Riggs said he was "likely" to have his passport on him and was in possession of about £900.
"The fact that there are no leads relating to his whereabouts suggests to me he may have got the treatment he needs for the malaria virus or has potentially gone abroad.
"This is just one possible explanation and we have no evidence to suggest where he might have gone."
Police have released a new picture of Mr Lloyd with his sister, Claire Holland, in a bid to trace him.
Earlier this week, his family said his disappearance was out of character.
As well as having connections with Oxford for the clinical trial, Mr Lloyd is known to have links with Cornwall and the Avon and Somerset police force area.