(CNN) -- About 4,500 Chicago police officers didn't report their vaccination status by October 15 as mandated by the city, officials said Monday.
That means roughly 35 percent of the city's 12,770 police officers could be placed on no-pay status in the foreseeable future.
The Chicago Police Department had the lowest response rate of any department in the city, but of the about 64 percent who did report, the majority of officers say they are vaccinated, according to data released by the city.
Specifically, 6,894 indicated being vaccinated while 1,333 reported they have not.
Those who said they are not vaccinated are required to opt into twice weekly testing for Covid-19 until the end of the year to remain in compliance with the city policy.
The city required employees to be either vaccinated or test two times a week by October 15, and then report their status by that same deadline. Those who did not report their status risk being put on unpaid leave.
City officials said the disciplinary process will be ongoing and likely happen on a rolling basis, but that this also "starts the process" of trying to get a final number of employees into compliance. The vaccine reporting portal is still open.
One city official said, "It's not going to be an immediate 'we're firing you.' "
The first step is employees will be sent home in a no-pay disciplinary status as the city vaccine policy is a condition of employment. As of Monday afternoon, no one has been disciplined yet, but that process can now begin at any time.
"It'll hopefully wake them up quickly," that city official said.
Most Chicago departments had at or near a 100% response rate to the vaccine reporting requirement.
A memo obtained by CNN states any civilian or sworn employee who disobeys a direct order to comply with the city's vaccination policy "will become the subject of a disciplinary investigation that could result in a penalty up to and including separation from the Chicago Police Department. Furthermore, sworn members who retire while under disciplinary investigations may be denied retirement credentials."
A separate memo issued Saturday informed officers their elective time off would be restricted.
"Until further notice, the use of elective time by sworn CPD members is restricted. Furthermore, the use of elective time will require prior approval from the Deputy chief or above within the requesting member's chain of command," the memo obtained by CNN said.
The memos were issued as the police union and Mayor Lori Lightfoot clash over the city's requirement that city employees be vaccinated and disclose their vaccine status. The deadline to do so was Friday.
The union -- the Chicago Fraternal Order of Police -- filed a lawsuit Friday in Cook County Circuit Court last week alleging Lightfoot, Chicago Police Superintendent David Brown, the city and the police department have failed "to comply with the collective bargaining agreement's status quo."
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