Injections at Advocate Condell Medical Center were paused on Friday night.
"Following a thorough internal review—including infectious disease and clinical experts across our system," the vaccinations resumed Sunday, a spokesperson for Advocate Aurora Health, which runs the hospital, told The Epoch Times in an emailed statement.
One of the four cases is believed to have been a severe allergic reaction.
"That person was treated, observed in the hospital overnight, and received additional treatment, and has since been discharged home and is doing well," Dr. Robert Citronberg, executive medical director of Infectious Disease and Prevention for Advocate, told reporters in a virtual briefing on Saturday.
Officials declined to say whether the person who suffered anaphylaxis had a history of allergies.
The other three people who suffered adverse effects went home Friday and are doing well, the spokesperson said.
Advocate is increasing its post-vaccine evaluation period to 30 minutes for all individuals across all our sites as a precaution.
Approximately 10,000 people have been vaccinated across Advocate sites as of Monday, the spokesperson said.