With both Pfizer and Moderna preparing for additional shots specifically targeting the omicron variant, what do we know so far about vaccines and their effectiveness against the strain?
With booster shot eligibility and guidance changing, the omicron variant has already caused a shift as experts rush to slow the surge in cases nationwide.
Here's what we know so far:
How are vaccines performing against the omicron variant? Is one better than the others?
Get Chicago local news, weather forecasts, sports and entertainment stories to your inbox. Sign up for NBC Chicago newsletters.
Health experts say being vaccinated can be the difference between getting COVID and being hospitalized or dying from COVID, but beyond that, symptoms can also vary depending on your vaccine.
Chicago Department of Public Health Commissioner Dr. Allison Arwady said last month that data showed both the Pfizer and Moderna vaccines were performing better than Johnson & Johnson's vaccine at preventing breakthrough infections, but all three protect against hospitalization and death.
"The CDC has changed its guidance to recommend as the first choice either Pfizer or Moderna," Arwady said. "I want to reassure people that the J&J vaccine continues to do a good job of protecting against those severe illnesses, but we are seeing more breakthroughs."
Last month, the CDC's Advisory Committee on Vaccine Practices on Thursday unanimously recommended Pfizer and BioNTech's or Moderna's COVID-19 vaccines, which use mRNA technology that hasn't been linked to the blood clots, over J&J's, which uses a more traditional virus-based technology. The CDC confirmed 54 cases of mostly younger women who developed blood clots with low blood platelet levels — a new condition called thrombosis with thrombocytopenia syndrome, or TTS — after receiving J&J's shots. Of those cases, 36 required treatment in intensive care.
The panel's presentation of the data showed the greatest rick of TTS is among females 30 to 49 years of age.
The panel recommended the unusual move of giving preference to the Pfizer and Moderna vaccines, and the CDC's director, Dr. Rochelle Walensky, accepted the panel's advice.
"Given the current state of the pandemic, both here and around the world, any vaccination is better than no vaccination," Walensky said during a press briefing by the White House COVID-19 response team. "Individuals who are unable or unwilling to receive an mRNA vaccine will continue to have access to Johnson & Johnson's Covid-19 vaccines."
Several countries, including Canada, already have policies that give preference to the Pfizer and Moderna vaccines. But J&J told the committee its vaccine still offers strong protection and is a critical option especially in parts of the world without plentiful vaccine supplies or for people who don't want a two-dose shot.
While blood clots are rare, “unfortunately cases of COVID-19 are not,” J&J’s Dr. Penny Heaton said.
More recently, real-world data from the United Kingdom has shown that Pfizer's and Moderna's vaccines are only about 10% effective at preventing symptomatic infection from omicron 20 weeks after the second dose, according to study from the U.K. Health Security Agency. However, the original two doses still provide good protection against severe illness, the study found.
Booster shots are up to 75% effective at preventing symptomatic infection, according to the study.
While breakthrough infections are seemingly on the rise across all vaccines, cases in fully vaccinated and boosted individuals appear to be milder compared to unvaccinated patients, experts said.
"If [fully vaccinated people] get COVID, as opposed to getting seriously ill and having fevers for days and difficulty breathing, etc., they may only experience it as a mild illness," Arwady said. "They may only feel like they have a cold. That's good because they're not getting seriously sick. They're not threatening the healthcare system, but it's certainly of some concern because they do have the potential to transmit to others."
The unvaccinated, however, are experiencing similar symptoms to early on in the pandemic, Arwady said.
"People who are unvaccinated present in the same way: fevers, cough, chills, shortness of breath," she said.
Arwady's comments echo those of other medical experts who are watching omicron cases.
In New York, where cases continue to surge, an ER doctor who became known on social media during the pandemic for his documentation of the battle against COVID, reported breakthrough cases he has seen in those with booster shots experienced "mild" symptoms.
"By mild I mean mostly sore throat. Lots of sore throat," Craig Spencer wrote on Twitter. "Also some fatigue, maybe some muscle pain. No difficulty breathing. No shortness of breath. All a little uncomfortable, but fine."
Cases in people who were fully vaccinated with either Pfizer or Moderna's vaccine, but not boosted, remained mild, but slightly more intense.
"More fatigued. More fever. More coughing. A little more miserable overall. But no shortness of breath. No difficulty breathing," he wrote.
For those with Johnson & Johnson who were not boosted, he wrote the patients "felt horrible," with fevers, fatigue, coughs and shortness of breath, but did not require hospitalization or oxygen.
In the unvaccinated, however, the symptoms were more severe.
"Almost every single patient that I’ve taken care of that needed to be admitted for Covid has been unvaccinated," Spencer wrote. "Every one with profound shortness of breath. Every one whose oxygen dropped when they walked. Every one needing oxygen to breath regularly."
But despite reports of milder infections among the vaccinated, in Illinois, hospitalizations have been rising.
Pediatric hospitalizations are also rising across several states in the U.S.
"No vaccine is perfect, for one thing," said Dr. Egon Ozer, who works in infectious diseases at Northwestern Medicine. "No vaccine is going to be a magic bullet. So, especially with so much virus that’s still circulating, there’s always potential that there’s going to be some breakthrough, that people are still going to be able to get some degree of the virus. That’s certainly been the case with delta, as well.”
But two doses of Pfizer and BioNTech's vaccine are highly effective at protecting children 12 to 18 from a severe inflammatory condition associated with COVID infection, a new study found.
The Centers for Disease Control and Prevention, in a report published Friday, found that Pfizer's vaccination was 91% effective at protecting adolescents against multisystem inflammatory syndrome, or MIS-C.
The CDC study looked at 283 hospitalized patients ages 12 to 18 across 24 pediatric hospitals in 20 states from July through December 2021 when delta was the predominant variant. The analysis focused on the 12- to 18-year age group because Pfizer shots weren't available to younger kids until November.
The CDC noted that vaccine efficacy against MIS-C caused by the omicron variant, which is now dominant in the U.S., could not be determined due to the timing of the study.
What about booster shots?
Moderna is working on a booster shot that will target the omicron variant of COVID for this fall as nations around the world prepare to distribute annual vaccinations against the virus.
"We are discussing with public health leaders around the world to decide what we think is the best strategy for the potential booster for the fall of 2022. We believe it will contain omicron," CEO Stephane Bancel told CNBC's "Squawk Box" on Monday.
Also Monday, Pfizer CEO Albert Bourla said a vaccine that targets the omicron variant will be ready in March, and the company has already begun manufacturing the doses.
"This vaccine will be ready in March," Bourla told "Squawk Box." "We [are] already starting manufacturing some of these quantities at risk."
Bourla said the vaccine will also target the other variants that are circulating. He said it is still not clear whether or not an omicron vaccine is needed or how it would be used, but Pfizer will have some doses ready since some countries want it ready as soon as possible.
Bourla said it's not clear whether a fourth dose is needed. He said Pfizer will conduct experiments to determine if another dose is necessary.
But Bancel on Thursday said the efficacy of boosters against COVID-19 will likely decline over time, and people may need a fourth shot in the fall to increase their protection.
Bancel said people who received their boosters last fall will likely have enough protection to get them through the winter, when new infections surge as people gather indoors to escape the cold.
However, Bancel said the efficacy of boosters will probably decline over the course of several months, similar to what happened with the first two doses.
"I will be surprised when we get that data in the coming weeks that it's holding nicely over time — I would expect that it's not going to hold great," Bancel said, referring to the strength of the booster shots.
Israel has made a fourth dose of Pfizer and BioNTech's vaccine available to people over the age of 60, people with compromised immune systems and health-care workers.
Israel found that a fourth dose of the vaccine increases antibodies that protect against the virus fivefold a week after receiving the shot.
Who is eligible for a booster shot and when?
The timing for when people can get their booster shot has shifted in the midst of the omicron surge.
Meanwhile, the U.S. is urging that everyone 12 and older get a COVID-19 booster as soon as they're eligible, to help fight back the hugely contagious omicron mutant that's ripping through the country.
The Illinois Department of Public Health on Thursday announced it would adopt the CDC's latest recommendation for those aged 12-15 years to get a booster dose five months after receiving the second dose.
“We know that COVID-19 vaccine booster doses can help provide ongoing protection against the Omicron variant,” IDPH Director Dr. Ngozi Ezike said in a statement. “Following the CDC’s recommendation, booster doses are encouraged for those aged 12-15 years who received their second dose of Pfizer-BioNTech vaccine five months ago.”
Chicago's health department followed suit Thursday.
"The Chicago Department of Public Health supports these decisions and encourages all eligible Chicagoans, age 12 and up, to get their booster dose now – no matter the type of COVID-19 vaccine they originally received," the Chicago Department of Public Health said in a release.
Boosters already were encouraged for all Americans 16 and older, but Wednesday the Centers for Disease Control and Prevention endorsed an extra Pfizer shot for younger teens — those 12 to 15 — and strengthened its recommendation that 16- and 17-year-olds get it, too.
The CDC is also recommending that children between the ages of 5 and 11 that are moderately or severely immunocompromised should get a third dose of the COVID vaccine 28 days after their second dose.
New U.S. guidelines say anyone who received two Pfizer vaccinations and is eligible for a booster can get it five months after their last shot, rather than the six months previously recommended.
The vaccine made by Pfizer and its partner BioNTech is the only U.S. option for children of any age.
On Friday, the FDA and CDC extended that timing to Moderna, shortening the timing for people seeking a booster shot of both mRNA vaccines to five months. The Johnson & Johnson booster interval is two months.