Police departments around the U.S. that are requiring officers to get vaccinated against COVID-19 are running up against pockets of resistance that some fear could leave law enforcement shorthanded and undermine public safety.

Police unions and officers are pushing back by filing lawsuits to block the mandates. In Chicago, the head of the police union called on members to defy the city’s Friday deadline for reporting their COVID-19 vaccination status.

Seattle’s police department sent detectives and non-patrol officers to emergency calls this week because of a shortage of patrol officers that union leaders fear will become worse because of vaccine mandates.

The standoffs are playing out at a time when many police departments already are dealing with surging homicide rates and staff shortages unrelated to the vaccine. Cities and police leaders are now weighing the risk of losing more officers to resignations, firings or suspensions over their refusal to get vaccinated.

Chicago’s mayor on Friday filed a complaint in court against the leader of the local Fraternal Order of Police, accusing him of “engaging in, supporting and encouraging work stoppage or strike” by saying the city’s more than 12,000 uniformed officers should ignore the order to report their vaccination status.

On Thursday, Mayor Lori Lightfoot said officers would not be sent home if they showed up to work Friday and refused to provide their information. Instead, she said, they would be put on unpaid leave after the weekend, because confirming compliance would take a few days.

Refusing to provide the information, Lightfoot said, would constitute an act of insubordination.

John Catanzara, president of the FOP local in Chicago, said about half of his members have not been vaccinated and called a mandate requiring inoculation “absolutely wrong.”

“They were willing to go into a no-pay status at midnight tonight and get sent home,” he said, suggesting during an appearance on Fox News that the city could not afford to lose police officers.

“You know, the reality is we have a profession nobody else wants to do right now. They cannot get anybody to go into this police academy,” he said.

In Los Angeles County, Sheriff Alex Villanueva said he won’t force his 18,000 employees to be vaccinated despite a county mandate. “I don’t want to be in a position to lose 5, 10% of my workforce overnight,” he said last week.

Hundreds of police officers in San Diego said they would consider quitting instead of complying with a vaccination mandate.

Resistance is bubbling up even though first responders have been hit hard by COVID-19. More than 460 law enforcement officers have died from the virus, according to the Officer Down Memorial Page, which tracks deaths in the line of duty.

Disputes over government and business vaccine requirements have spilled over into a variety of workplaces, including one of the nation’s premier nuclear weapons laboratories and the NBA.

Workers at Los Alamos National Laboratory — the birthplace of the atomic bomb — faced a deadline Friday to get vaccinated or risk being fired. A New Mexico judge denied a last-minute request by dozens of scientists and others to block the mandate.

In the NBA, the Brooklyn Nets are not allowing star Kyrie Irving to practice or play until he is vaccinated.

The number of Americans getting vaccines has steadily increased over the past three months as boosters have become available and mandates have taken effect. The number of shots administered per day has topped 840,000 on average.

No national statistics show the vaccination rate for America’s first responders, but individual police and fire departments across the country have reported figures far below the national rate of 77% for adults who have had at least one dose.

Police departments in Los Angeles, San Francisco and Denver are also among those under vaccine mandates or facing one. New York’s mayor has said he is looking at the idea despite opposition from the city’s largest police union.

The union representing some 1,000 Seattle police personnel argues that the matter should be the subject of labor negotiations. Union President Mike Solan has suggested that the mandate could worsen staffing shortages, which in turn could risk public safety.

The department has lost more than 300 officers in the past year and a half and anticipates another “mass exodus” in the coming weeks, Solan said.

Nearly 300 officers in Seattle had either not turned in paperwork showing they were vaccinated or were seeking an exemption, the mayor’s office said earlier this week.

In recent weeks, judges have rejected attempts by a group of Oregon State Police troopers and Denver police officers to block vaccine mandates.