Brochez was convicted of fraud and drug-related offences in March 2017 and was deported from Singapore after completing his sentence.
Those affected are 5,400 Singaporeans diagnosed with HIV up to January 2013, and 8,800 foreigners diagnosed with HIV up to December 2011. He released 14,200 patient names, HIV test results, phone numbers, addresses, ID numbers, and other medical information - including the names and contact information of 2,400 of their sex partners, the Singaporean Ministry of Health said in a statement.
"Our priority is the well-being of the affected individuals", it added, saying that it has been contacting affected individuals to inform and provide assistance to them since Saturday.
Outraged Singaporeans have blasted the Ministry of Health (MOH) for keeping silent about the leak of confidential information from the HIV registry for years, until the information was disclosed online by a foreigner. I have also had my fill of people who say we don't have to know everything, and that we should let the G handle everything. Ler has since been convicted of abetting Farrera-Brochez to commit cheating and of providing false information to the police and MOH.
The Health Ministry said it had begun contacting affected individuals about the incident and was working to "disable access to the information".
MOH said earlier on Monday that since 2016, new safeguards against the mishandling of information by authorised staff have been put in place.
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The ministry revealed that in May 2016, it was made aware of Brochez' possession of confidential information that appeared to be from the HIV Registry, and upon making a police report, both Ler and Brochez' property was searched by the police, and all relevant material found was seized and secured.
"We are working with relevant parties to scan the Internet for signs of further disclosure of the information", it said, adding that Brochez now was under police investigation and local authorities were seeking assistance from their foreign counterparts.
"Police will not hesitate to take stern action, including prosecution, against those who have breached the OSA".
Ler has been charged under the Official Secrets Act for failing to take reasonable care of confidential information regarding HIV-positive patients, and is out on bail. He was sentenced to 24 months' jail.
To successfully apply for an employment pass to stay here with his boyfriend, Farrera-Brochez submitted a HIV-negative test result to MOM in March 2008 using Ler's blood for the test.
However, officials have warned that the culprit is still in possession of the information and could disclose it publicly in the future. But he does not have a practising certificate or access to MOH and public healthcare IT systems with patient records.
The Ministry of Health says the leak source has been "disabled", but if Brochez retained personal copies of the data it may surface again.