By Ahmed Aboulenein and Katharine Jackson

WASHINGTON (Reuters) - Americans vaccinated and boosted against COVID-19 can be with family over the holidays but attending large gatherings is not safe, even for those who received a booster dose, top U.S. infectious disease expert Anthony Fauci said on Wednesday.

The United States faces a second Christmas of upended holiday plans, with a surge in infections fueled by the now-dominant Omicron variant of the coronavirus forcing many to cancel travel, reconsider visiting loved ones, and question attending holiday parties.

"There are many of these parties that have 30, 40, 50 people in which you do not know the vaccination status of individuals. Those are the kind of functions in the context of Omicron that you do not want to go to," Fauci said at a White House briefing.

Early evidence indicates Omicron is less severe than the Delta variant, said Fauci, citing studies from South Africa and Scotland, but warned Americans must remain cautious.

"This is good news. However, we must wait to see what happens in our own population which has its own demographic considerations," he said.

The seven-day average of COVID-19 cases in the United States rose 25 percent from the previous week to about 149,300 cases per day, said U.S. Centers for Disease Control and Prevention Director Rochelle Walensky, with average daily deaths up 3.5 percent at 1,200.

Omicron represents approximately 73 percent of cases across the country, said Walensky, and as high as 90 percen of cases in some areas, such as the eastern Atlantic states, parts of the Midwest, South, and northern Pacific states.

"This increase in Omicron proportion is what we anticipated and what we have been preparing for," she said.

The U.S. government will have 265,000 treatment courses of Pfizer Inc's COVID-19 anti-viral treatment available by January and 10 million by late summer, said White House COVID-19 response coordinator Jeff Zients.

The Food and Drug Administration on Wednesday authorized Paxlovid, Pfizer's pill for at-risk people aged 12 and above.

The government will provide any resources Pfizer needs for production and will distribute treatments to states and localities at no charge as soon as they are delivered, he said.

(Reporting by Ahmed Aboulenein and Katharine Jackson; Additional reporting by Caitlin Webber in Washington and Kanishka Singh in Bengaluru; Editing by Leslie Adler and Alistair Bekk)