The Omicron variant of COVID-19 took the world by surprise and inspired serious concern after researchers learned it could still infect fully vaccinated persons. New research from the Centers for Disease Control and Prevention (CDC) now shows just how many more Omicron infections happened last year, including among those that received a booster shot.

On Wednesday, the CDC published data showing a decline in protection provided by COVID-19 vaccines at a time when the Omicron variant was beginning to appear in the United States. As Omicron grew into the predominant strain nationwide, an increasing number of vaccinated Americans became hospitalized albeit at a much lower rate than those who remained unvaccinated.

As to why this was the case, the agency suggests that the existing COVID vaccines’ protection against infection is waning after they began to be widely administered last year.

To illustrate the increased susceptibility of vaccinated Americans to infection by Omicron, the CDC compared infection rates during the peak of the Delta variant’s wave last summer and Omicron’s during the winter. Compared to unvaccinated Americans, vaccinated Americans were 15 times less likely to be infected during the Delta variant but this figure was sliced in half by the time Omicron emerged.

Booster shots for COVID-19 began to be administered in the United States in September 2021 as a way to address the waning protection of previous shots. However, there were still reports of boosted adults becoming infected, but it was five times less likely for them to get sick than their unvaccinated peers.

But despite this all, vaccines unquestionably offer significantly more protection against COVID-19 and they dramatically reduce the odds of getting seriously ill or dying. Vaccine manufacturers have also acknowledged the challenges posed by Omicron and have been at work fine-tuning their shots to offer improved protection.

Recent research also shows that booster shots will wane in effectiveness after about four months, prompting discussions about whether or not a fourth dose may be needed. In the United States, a fourth dose is already available to those with compromised immune systems, but they are not yet widely available for more Americans.

About 81% of Americans are vaccinated with at least one shot against COVID-19 and total case numbers have been trending downward in recent days.

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