Boston health officials Monday said security measures have been tightened aboard the city’s ambulances following the discovery of suspected drug tampering by a Boston EMS paramedic.

The alleged tampering, discovered Sept. 6, 2011, may have exposed as many as 64 patients to blood-borne infections during the summer of 2011, when they were treated with the compromised painkillers and sedatives, officials said.

Boston EMS Chief James Hooley said since the alleged incident, they have started a “very strict” inspection schedule and have improved packaging of the drugs.

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“We feel pretty confident we have put additional safeguards in place to prevent something like this from happening again,” Hooley said.

He said the system already had a “robust” security, which required double signatures on the medications used, and periodic inspections by supervisors, as well as an annul inspection by state officials.

The Boston Public Health Commission, which runs the city’s ambulance service, began notifying patients over the weekend and offering free medical tests to determine whether they were exposed to infectious diseases.

“We have had, for the most part, a very positive response,” Barbara Ferrer, executive director of the commission, said in news conference Monday afternoon at the agency’s Boston headquarters. “People have been glad that we did in fact notify them, and I think it’s mixed in terms of what people are worried about or whether or not people want to get tested.”

Officials do not believe the paramedic carried any infectious diseases, but acknowledged that health officials do not know for sure. Nor do they know how the individual may have tampered with the medications, which are in liquid form and are injected.

“The paramedic in question did not treat all 57 patients personally, and we also have no reason to believe that this individual had or was transmitting an infectious disease to any patients,” Ferrer said.

Letters and phone calls are going out to 57 of the 64 who may have been affected. Seven died shortly after they were transported to the hospital by Boston Emergency Medical Services, and officials believe their deaths were not related to the suspected medication tampering and instead were due to their “initial catastrophic injury or medical event.”

Dr. Anita Barry, director of the commission’s Infectious Disease Bureau, said the chances of any of the 57 being infected are quite small. She said that if the patients want to be tested for potential infections, they would suggest they be screened for blood-borne illnesses such as HIV and hepatitis C.

Someone who is directly stuck with a needle from someone who has HIV has a 0.3 percentchance of getting infected, Barry said. “We are miles away from that here. This has got to be much much, much lower risk,” she added.

Similarly, Barry said, someone who is directly stuck with a needle from a person who has hepatitis C, has a 1 to 2 percent risk of getting infected.

The paramedic, who officials declined to identify, is believed to have tampered with the powerful drugs during a six-week period in the summer of 2011, but officials said they could not be more specific about the exact dates because of an ongoing criminal investigation. No charges have been filed against the paramedic, who was relieved of duties when the problem was discovered in September 2011.