CDC study confirms not getting updated COVID booster leaves Americans vulnerable this winter

The new bivalent COVID booster this fall was relatively 80% effective at keeping older adults out of the hospital due to complications from the virus compared to seniors who were fully vaccinated with at least two shots but had let several months pass, according to a new study by the Centers for Disease Control and Prevention.

The new study is the latest in real-world assessments of the newly formulated shot and confirms what researchers have been saying for months: Immunity wanes with time. It also highlights the dire warning from doctors that people who were vaccinated early in the pandemic but opted to forgo boosters this fall are vulnerable this winter.

While the benefits are most dramatic with people 65 and older, there was a benefit for all adults in keeping them away from emergency rooms, according to the CDC.

"These studies show pretty unequivocally that the vaccines provide about 40 to 80% extra protection. So put another way that can cut an individual's risk of hospitalization due to COVID-19 and about half," said Dr. Ruth Link-Gelles, program lead on a CDC task force examining vaccine effectiveness.

Early in the pandemic, the CDC considered a person "fully vaccinated" if they got the first two shots in a series, although it later recommended getting a third shot, and then a bivalent booster to shore up a person's immunity.

But despite public health officials pleading with Americans to get "up-to-date" on their shots, many people remain skeptical on the effectiveness while others haven't given much thought to going beyond those first two shots.

According to the latest Kaiser Family Foundation COVD-19 Vaccine Monitor, only about four in 10 adults say they've gotten the new bivalent booster or will get it as soon as they can. Among people 65 and older who haven't gotten the booster, about a third of respondents said that they don't think they need it and another third said they didn't think the benefit of the updated booster was worth it.

About a quarter said they have been too busy or have not had time to get the new booster yet.

Link-Gelles said it's not uncommon for someone to get COVID or the flu after getting a shot for it. But the focus is on preventing hospitalization, not infection.

"What we're really looking for is does that vaccine keep people out of the hospital," she said. "And what we know from the (flu) vaccine as well as from these COVID vaccines is that you may get infected, but your symptoms are likely to be a lot less severe. You're much less likely to need to go to the hospital if you got your updated booster."

The CDC study released Friday confirms unvaccinated patients are most at risk of hospitalization. It also looked at people who were fully vaccinated with at least two shots but had let months go by without a booster.

The biggest impact was in seniors: The relative effectiveness of the bivalent was 78% at keeping people 65-plus out of the hospital compared to patients who had gotten their primary series six to 11 months earlier. If the patient was fully vaccinated a year ago, the relative effectiveness of the bivalent was 83%.

The impact on adults was less dramatic but still there: Adults who got the bivalent with two to four months of their last dose -- a timeline on par with CDC recommendations -- were a third less likely to wind up visiting emergency room or at urgent care.

ABC News' Cheyenne Haslett contributed to this report.