Police were called when it was ruled by doctors that the disease was sexually transmitted and she lacked the mental capability to consent to sex.
A report by Dr Margaret Flynn, found that Cassie, or 'Adult B', was 'failed by services'.
Cassie's mother believes the rape was carried out by a worker at the home during a night shift.
But because the suspected rape could have happened anytime during a 10-year period, police were unable to take forensic evidence and so the culprit will not be brought to justice.
A Met Police spokesman said the investigation was closed in 2017 as there were no new lines of enquiry.
He added: 'Due to the 10-year period the offence was alleged to have occurred between, no clear suspects could be identified and there were no forensic opportunities available.
'There are no lines of inquiry that require further examination at this time.'
The home has now been closed and all the five other residents are at new facilities.
Brent Council said the authority no longer places residents at homes run by the private company.
A spokesman said: 'All of the partners on the Safeguarding Adults Board, including the council, have expressed our deep and sincere regret to both Cassie and her family.
'We can confirm that Cassie is no longer living in a care home managed by the previous provider and the other five residents who lived there with her have also been moved to other homes with the co-operation of their families.
Cassie, and all the former residents are now safe and happy, and Cassie is having all her health and care needs met.
'We continue to monitor her progress regularly as well as maintaining contact with her family. We no longer place any residents in care homes run by this provider.
'The entire independent safeguarding board worked closely with the police and received regular updates regarding their investigations.
'A full investigation was conducted by the police who concluded that unfortunately and despite best efforts, there was insufficient evidence to bring a criminal prosecution against anyone.
'The council works closely with partner agencies, the CQC and the Safeguarding Adults Board in order to protect Cassie and all of our vulnerable residents. The Board carried out a full and independent multi-agency investigation after the police investigation was completed to ensure lessons were learnt.
'The Council takes its responsibilities to vulnerable adults very seriously and is continuously working to improve the support we provide to vulnerable adults in Brent, including setting up a team that specifically focuses on reviewing the quality of care and support for individuals in residential placements, and integrating the health and social care learning disability teams into a single team providing holistic support to adults with a learning disability.'
The case came to light as it emerged also emerged a woman in a vegetative state in Arizona gave birth.
Police have said the Native American, who has been incapacitated since age three, was sexually assaulted and delivered a baby last month at Hacienda HealthCare in Phoenix.
Male nurse Nathan Sutherland, 36, is facing one charge of sexual assault and one count of vulnerable adult abuse after a DNA test identified him as the baby's father.
