COVID updates: New Zealand imposes restrictions amid omicron outbreak

Despite waning immunity over time, vaccines still dramatically reduced the risk of severe illness caused by COVID-19 through at least the end of the delta wave, according to data updated by the Centers for Disease Control and Prevention Thursday evening.

In November, unvaccinated adults had a four times greater risk of testing positive for COVID-19, and a 15 times greater risk of dying from COVID-19, compared to vaccinated individuals, according to federal data pulled from 28 states and jurisdictions.

Additionally, unvaccinated adults had a 13 times greater risk of testing positive for COVID-19 and a 68 times greater risk of dying from it as compared to fully vaccinated individuals with a booster.

The emergence of omicron does appear to have increased the occurrence of breakthrough cases. Between late November and late-December, the rate of infections among the fully vaccinated increased more than fourfold. Even so, unvaccinated Americans remained twice as likely to test positive for the virus.

Similarly, the rate of infections among the fully vaccinated and boosted Americans testing positive increased by nearly tenfold. However, unvaccinated Americans remained 3.8 times as likely to test positive for the virus.

-ABC News' Arielle Mitropoulos

A new CDC study finds that the omicron variant was likely in the U.S. more than a week before the first case was detected. 

The first official case was detected on Dec. 1 in California. But a review of the national wastewater surveillance system indicates that the variant could have been present as early as Nov. 21, according to samples collected in New York City. 

-ABC News' Arielle Mitropoulos, Sony Salzman

More than 160,000 COVID-19-positive Americans are currently in hospitals -- a pandemic high and double the number from about three weeks ago, according to federal data.

It's still not clear how many were admitted with COVID-19 and how many tested positive for the virus after they were admitted for other reasons.

The U.S. is reporting an average of 760,000 new cases per day, according to federal data.

Although case levels remain high, there's growing evidence to suggest the omicron surge is receding in the parts of the country first struck by the variant.

In New York, daily cases have dropped by 33% in the last week, while in New Jersey new cases are down by 43.7%.

Wisconsin now leads the nation in new cases per capita, followed by Rhode Island, Utah and South Carolina.

-ABC News' Arielle Mitropoulos

In San Francisco, COVID-19 cases are "dropping rapidly" following record highs that appeared to peak on Jan. 9, officials announced.

While "cases are still extremely high," they "have plateaued and are starting to go down," said Dr. Grant Colfax, director of the Department of Public Health.

"We're looking at data from other places … the consistent data seem to show that cases go up very fast, they started to come down very fast. So we're on that downward trend now," Colfax said.