As for how many? Doctors said the herd immunity threshold has yet to be established.
"We've been able to figure that out for other diseases such as measles, which approximately 95% of the population needs to be vaccinated to protect the other 5%," said Dr. Zachary Rubin.
Dr. Zachary Rubin is a clinical immunologist. He said scientists are getting closer to finding that magic number.
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"My feeling is that we're going to need somewhere around the line of somewhere under that 80%," Dr. Rubin said.
As for how long it will take to get whatever that herd immunity threshold is? Some experts have estimated the population could reach herd immunity as early as April, while others are still projecting that won't come until 2022.
CLICK HERE: Illinois COVID vaccine tracker
CLICK HERE: Chicago COVID vaccine tracker
"I believe it's somewhere in the middle," Dr. Rubin said. "Probably by fall we'll get to somewhere close to that point, but that's also going to come down to how fast we can vaccinate people."
According to the CDC's latest numbers, about 18.8 million people in the United States have been fully vaccinated against COVID-19.
In Illinois, over 2 million vaccine doses have been administered, according to the Illinois Department of Health.
Despite the numbers, most timelines put the uncontrolled stop of the spread at months, if not a year away.
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However, there is good news as health officials see case counts dropping.
"The cases across the country, [and] in the state of Illinois, have been going down -- which is very promising. And if we continue to follow the mitigations of being physically distant, wearing masks, washing your hands and getting vaccinated, those case numbers will likely continue to go down into the spring and summer," Dr. Rubin said.