COVID updates: New Zealand imposes restrictions amid omicron outbreak

The CDC says three new studies show the importance of vaccines and boosters in protecting against severe COVID-19 infection from delta or omicron. 

The first study found that a third shot slashed the risk of an emergency room or urgent care trip by 94% during delta and 82% during omicron. 

The second study found that, during the delta and omicron waves, the unvaccinated were nearly 14-times more likely to be infected and 53-times more likely to die compared to people who are vaccinated and boosted. 

The third analysis found that three doses of a COVID-19 mRNA vaccine -- as opposed to two doses or none -- provided protection against both delta and omicron against symptomatic infection. However, the protection was greater against delta compared to omicron. 

-ABC News' Arielle Mitropoulos

The U.S. is now reporting an average of about 745,000 new cases per day, down by about 5% in the last week, according to federal data. It is still unclear whether the nation is experiencing a true plateau or whether data is skewed from the holiday weekend.

In the last month alone, the U.S. recorded 18.2 million new cases, according to Johns Hopkins University data -- more than one-quarter of the nation's total number of confirmed cases reported since the onset of the pandemic.

There's growing evidence to suggest the latest omicron surge continues to recede in the parts of the country first struck by the variant.

In New York, daily cases have dropped by nearly 38% in the last week, and in New Jersey, new cases are down by 45.8%, according to federal data.

-ABC News' Arielle Mitropoulos

Beginning Saturday, the Department of Homeland Security is requiring non-U.S. citizens entering the U.S. via land or ferry to be fully vaccinated.

This rule, outlined in October, applies to people arriving from Mexico's and Canada's borders.

This requirement does not apply to U.S. citizens traveling over the border. 

-ABC News' Luke Barr

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