With the emergence of the rapidly spreading omicron variant, COVID-19 booster shots are now strongly recommended by not only Chicago and Illinois health officials, but by the Center for Disease Control and Prevention as well.
"Everyone ages 18 and older should get a booster shot ... when they are six months after their initial Pfizer or Moderna series," CDC Director Dr. Rochelle Walensky said in November.
"The recent emergence of the omicron variant (B.1.1.529) further emphasizes the importance of vaccination, boosters, and prevention efforts needed to protect against COVID-19," she added.
The World Health Organization echoes that sentiment, saying the omicron variant is highly contagious, and that "preliminary evidence suggests there may be an increased risk of reinfection."
Get Chicago local news, weather forecasts, sports and entertainment stories to your inbox. Sign up for NBC Chicago newsletters.
Here's what the CDC says about side effects of each booster shot currently available.
What are the most common side effects of the Pfizer or Moderna COVID booster shot?
The most common side effects reported after getting a third shot of an mRNA vaccine, the type made by Moderna and Pfizer, were pain at the injection site, fatigue, muscle pain, headache and fever, followed by chills and nausea, according to data from the CDC.
What are the most common side effects of the Johnson & Johnson booster shot?
The data available for Johnson & Johnson was more limited, but people reported fever, fatigue and headache after receiving a second dose of that vaccine, according to the agency.
Omicron Variant in the U.S.: Where it's Been Detected, And What we Know About Each Case
How strong are side effects from COVID booster shots?
Overall, the CDC says that so far, reactions reported after getting a booster shot were similar to those after the two-dose or single-dose primary series.
Fever, headache, fatigue and pain at the injection site were the most commonly reported side effects, and overall, most side effects were mild to moderate. However, as with the two-dose or single-dose primary series, serious side effects are rare, but may occur.
Which booster shot should you get, based on your first COVID vaccine dose?
Federal regulators have recommended getting the same shot as your first dose for booster doses, and Dr. Allison Arwady, Chicago's Commissioner of the Department of Public Health, said that applies particularly to those who got an mRNA vaccine.
"If you got Moderna or Pfizer, I would recommend sticking with the same one that you got initially," Arwady said.
Some advisers, however, said they would prefer that Johnson & Johnson recipients receive a competitor's booster, citing preliminary data from an ongoing government study that suggested a bigger boost in virus-fighting antibodies from that combination.
Where in Chicago Can You Get a COVID Booster Shot?
"If you got Johnson & Johnson, I have been advising folks if they got J&J, depending on the reason they chose J&J the first time, if they're going to get that booster I've been recommending getting likely one of the mRNA series - the Moderna or the Pfizer," Arwady said. "That's where we saw the biggest increase in antibodies in the studies."
What's the difference between the Moderna and Pfizer booster shot?
According to the CDC, Pfizer and Johnson & Johnson COVID boosters are the same dosage as the first round of shots.
Moderna, however, is half the dose of the vaccine used in the initial series.
The reason why Moderna is a half dose is because Moderna had a higher dose of the mRNA the only active part of the vaccine to start with. So it's part of why the side effects are sometimes a little higher and the folks who have Moderna.