A US health insurance company accidentally disclosed some of its customers’ HIV statuses to around 12,000 people last month.

Insurer Aetna sent out letters to clients with instructions for filing HIV medication prescriptions.

But for some members, a plastic window on the envelope stated the patient’s name and address as well as the information about medication, which inadvertently revealed their HIV status.

The letters, sent out to thousands of the insurer's members on 28 July, marked a breach of customer privacy and may expose people to discrimination, according to the Legal Action Center and the AIDS Law Project of Pennsylvania.

According to the groups, the letters were sent to customers currently taking medications for HIV treatment as well as for pre-exposure prophylaxis (PrEP), a course of drugs which help prevent a person from acquiring HIV.

It is not known how many people’s HIV statuses were exposed, but the groups have received numerous reports that the letters were seen by family members, roommates and neighbours.

In a letter to Aetna, the two groups asked the insurer to develop a plan to correct its procedures so mistakes are avoided in the future.

"Aetna’s privacy violation devastated people whose neighbours and family learned their intimate health information," Sally Friedman, legal director of the Legal Action Center in New York City, said in a statement.

"They also were shocked that their health insurer would utterly disregard their privacy rights."

Ronda Goldfein, executive director of the AIDS Law Project of Pennsylvania, added the casual disclosure of a person’s HIV status or use of HIV medication creates a "tangible risk of violence, discrimination and other trauma".