Illinois, suburban Cook County and Chicago officials say they’re pausing use of the Johnson & Johnson COVID-19 vaccine while federal regulators review reports that it has led to blood clots in six patients in the U.S.

Officials say anyone who has received J&J who develops a severe headache, abdominal pain, leg pain, or shortness of breath within three weeks after being vaccinated should contact their doctor.

The chances of experiencing this adverse reaction are extremely low: less than 1 in a million. And this recommendation does not apply to anyone who’s received a Pfizer or Moderna vaccine.

Here’s what we know so far about how this news will impact vaccination efforts in the Chicago area.

Impact on local supply

The J&J shot hasn’t been widely used across Illinois, as other vaccines arrived first. So far, Illinois has received more than 330,000 doses of the J&J vaccine and Chicago has gotten nearly 89,000 doses.

That’s about 5% of the more than 7 million doses of COVID-19 vaccines that have been shipped to Chicago and the state. Most doses are two other vaccines made by Pfizer and Moderna.

More than 47,000 Chicago residents have received a Johnson & Johnson shot, and the city planned to administer about 13,000 shots in the coming week, according to Chicago Public Health Commissioner Dr. Allison Arwady.

At a news conference Tuesday, Arwady said the impact of the J&J pause won’t be hugely detrimental to Chicago’s vaccination efforts.

“It will have an impact, probably not as large an impact as it might have [had], simply because we had already seen a slowdown in the amount of Johnson and Johnson … because of … manufacturing problems,” Arwady said.

“So just to give you some perspective on this, we had received a one-time 40,000 Johnson and Johnson allocation, which we had then seen drop to fewer than 5,000 in this current week … of the three vaccines, it is the one that has been by far the one we’ve administered the least of.”

But, Arwady says, it is an important tool in the fight against COVID-19, as it’s a single dose and doesn’t require special ultra-cold storage, unlike the Pfizer vaccine. So it’s easier to take on the go, and doesn’t require a follow-up appointment.

Impact on vaccine sites that carry Johnson & Johnson

Chicago’s Department of Public Health said officials are working closely with federal and local partners “to determine how this impacts the city’s vaccine operations.”

One major concern was how this news would affect vaccination efforts at Chicago State University, a site specifically designed to increase vaccine access among communities of color and which was set to use the J&J shot. The Chicago Department of Public Health says it will immediately switch to Pfizer vaccinations at that site, and no appointments will be affected.

But the city is pausing vaccinations in several other efforts, including its homebound program designed to vaccinate elderly Chicagoans who can’t travel. That program will be paused until at least April 19. The city’s also pressing pause on its newly-opened mass vaccination site for union workers in Chinatown.

It’s also postponing vaccinations planned for South Shore and West Englewood through the city’s traveling vaccine bus, for which the J&J vaccine has been useful.

“[We’re] really trying to bring vaccine to people in a way that …may not always require an appointment, doesn’t require that second dose … that kind of approach has been important for some of our predominantly Black neighborhoods,” Arwady said.

Last week, the department announced it would use Johnson & Johnson’s vaccine for walk-up appointments at the United Center starting April 20. Those appointments are currently paused as well, as Chicago officials wait to hear from FEMA, the federal agency running the site.

In terms of other providers, such as hospitals and clinics, city officials said 78 had received J&J shipments in the past two weeks, but that those shipments were relatively small, around 100 doses or so. Those providers have been advised to pause administration of the vaccine.

Chicagoans who haven’t been able to find a coveted vaccine have found success crossing over into the suburbs. On Tuesday, Cook County officials announced their sixth mass vaccination site in south suburban Matteson, where as many as 4,000 people a day could be vaccinated.

Leaders said of the more than 500,000 doses of vaccine administered, only about 5% have been a Johnson & Johnson shot. Only the mass vaccination site in south suburban Tinley Park has been administering J&J.

In light of putting J&J on hold, the county on Tuesday reduced the number of first dose appointments they planned to open from about 35,000 to 27,000 “to ensure that we would be able to make it through the week,” said Israel Rocha Jr., who is CEO of the county’s public health system, known as Cook County Health.

“We have enough to be able to meet all of our doses and all of the appointments that we have given for this week. So we are good until Saturday,” Rocha said. “We are working with the state and local officials to make sure that we have enough vaccine supply.”

Anyone who has booked a J&J appointment will not lose their slot, and instead will receive a Pfizer or Moderna vaccine, Rocha said.

What Chicago and Illinois doctors are saying

Some doctors are concerned this news could worsen the rise in COVID-19 cases currently hitting Chicago and Illinois.

“Johnson & Johnson gets you immune faster than any of the other vaccines, and so it’s the best one to use if you’re trying to stave off a wave that’s headed your direction,” said Dr. Emily Landon, an infectious disease expert at the University of Chicago Medical Center.

“So my first thought was, ‘Oh, god, this is going to potentially increase the damage from this surge that we’re just beginning,”

Experts have been warning of increased cases and hospitalizations. Chicago has seen double-digit percent increases in weekly case rates. In suburban Cook County, though the increase in the number of cases and hospitalizations has slowed, the county’s main public hospital, John H. Stroger Jr., has been beefing up staff and supplies to prepare for an influx of patients, Rocha said.

“All these steps are being taken out of abundance of caution,” Rocha said.

Despite the J&J news, he and other government and public health leaders stressed that people still get vaccinated.

At a local pharmacy in Logan Square, co-owner Noura Hamoui said her staff made rounds of calls Tuesday to offer those scheduled for the J&J shot to reschedule a Moderna or Pfizer shot. But, she said most of the patients they reached declined.

“I think they are hesitant now. They’re afraid maybe also with the other vaccines [that] something will go wrong with them [also]. Now with the pause, I don’t know how people will trust the vaccine again,” Hamoui said.

Experts are reiterating that the Pfizer and Moderna vaccines are safe, and that side effects potentially linked to the Johnson & Johnson vaccine are still under investigation, and have been rare.

What’s next

If you are an everyday Illinoisan searching for a vaccine, keep doing so, experts say. And if you’ve gotten the Johnson & Johnson shot, there’s no need to worry.

“You have a much better chance of being hit by a bus [than developing these symptoms],” said U of C’s Landon. “If you end up with symptoms of anything, call your doctor. And don’t hesitate to ask for help. If you and if you have a lot of anxiety about this, then set up an appointment to just do a checkup with your doctor.”

“But to be honest with you, your risk is still extremely low. It’s less than one in a million .. And [these blood clots] may not be associated at all, it may just be that they’re doing this out of an abundance of caution.”

The CDC’s Advisory Committee on Immunization Practices will meet Wednesday. Federal officials said that they will review all available data.

Dr. Peter Marks, director of the FDA’s Center for Biologics Evaluation and Research, said Tuesday morning the agency wants to get information to health care providers quickly so they know how to handle someone who might develop these rare side effects.

Mariah Woelfel, Kristen Schorsch and Becky Vevea are WBEZ reporters covering COVID-19. You can follow them on Twitter at @MariahWoelfel, @kschorsch, and @beckyvevea.