The Chicago Police Department has made it into a fifth year without an officer dying in the line of duty — the longest stretch since the 19th century, a WBEZ analysis has found.

“We’re very fortunate,” said former police Supt. Phil Cline, executive director of the Chicago Police Memorial Foundation. “Let’s hope we can keep that going.”

The most recent duty-related death took place Dec. 29, 2011, when masked men robbed a West Side convenience store and shot down Officer Clifton P. Lewis, an eight-year veteran who was working as a security guard there.

Since then, Chicago officers have gone more than 49 months without a fatality. They haven’t had a run that safe for at least 145 years. After the city’s first two known police fatalities, both in the 1850s, there is no record of a third until February 18, 1871, when one officer unintentionally shot another as they talked on the corner of Noble and Chicago.

WBEZ charted 566 fatalities based on records of the police memorial foundation and the Officer Down Memorial Page. Two-thirds of the deaths were caused by hostile gunfire. Other leading causes included vehicle collisions, heart attacks and accidental gunfire.

Cline credits today’s relative safety to body armor that officers have been wearing in recent decades. “We’ve had 40 Chicago police officers whose lives have been saved since 1980 by a bulletproof vest,” he said. “That’s huge.”

The foundation raises money to help officers buy vests, which start around $500 each and need replacing every five years.

Researchers say another factor is the level of violent crime. The bloodiest era for Chicago officers was Prohibition, particularly a 1928-1931 span, when 67 cops perished in the line of duty.

Beginning in the mid-1930s, however, Chicago officers did not have more than six duty-related deaths in any year until a crime wave that started in the late 1960s. In both 1969 and 1970, 10 officers died in the line of duty. Ever since, there’s been a general trend downward again.

The safety of recent years could have to do with the average age of Chicago officers. “It’s an old department and getting older,” said Wesley G. Skogan, a Northwestern University political scientist who studies policing. “That means wiser, less aggressive, more cautious and perhaps more judicious.”

Skogan also points to nonlethal weapons, such as Tasers and pepper spray, that help cops keep their distance from dangerous people.

Still another factor helping officers stay alive is the improvement of emergency-medical care. On Saturday, a 59-year-old cop working in the Harrison District’s lockup collapsed and lost consciousness. Two officers revived him using CPR and a defibrillator machine, according to police department statements.

The decline in Chicago officer deaths since the 1970s is part of a national trend. Firearms-related police fatalities have dropped especially fast, according to numbers from the National Law Enforcement Officers Memorial Fund. More cops now die in traffic incidents.

“When it comes to driving, our worst enemy is probably the cell phone,” said Rev. Dan Brandt, a Catholic priest who directs the Chicago Police Department’s Chaplains Ministry.

No matter the absence of duty-linked fatalities since 2011, Brandt says policing remains dangerous. When officers leave for work every day, he said, “they don’t know if they’re going to return home.”

Chip Mitchell is WBEZ’s West Side bureau reporter. Follow him on Twitter @ChipMitchell1 and @WBEZoutloud, and connect with him through Facebook, Google+ and LinkedIn.