With the Omicron variant surging across the US, the Food and Drug Administration (FDA) on Monday expanded the emergency authorization of a booster dose of the Pfizer-BioNTech coronavirus vaccine to include youths ages 12 to 15.

The booster dose is the same strength as the dose previously authorized by the FDA for emergency use for those 16 years of age and older.

The vaccine advisory committee of the Centers for Disease and Control and Prevention (CDC) is expected to meet this week to review the FDA moves and decide whether to recommend the changes. If it does, CDC Director Dr Rochelle Walensky is expected to quickly sign off on them.

A booster dose "may help provide better protection against both the Delta and Omicron variants", said Dr Peter Marks, who oversees the agency's vaccines office. "In particular, the Omicron variant appears to be slightly more resistant to the antibody levels produced in response to the primary series doses from the current vaccines."

Regulators also shortened by a month the amount of time that adults and adolescents should wait between second and third doses, allowing them to seek booster shots five months after second injections. They also allowed some immunocompromised children ages 5 to 11 to seek a third shot, including those who have undergone solid organ transplants or have conditions that amount to "an equivalent level of immunocompromise", the FDA said.

The number of daily new coronavirus cases now average 400,000 in the US, according to The Associated Press.

A major snowstorm moved across the South and mid-Atlantic regions on Monday. It caused nearly 850,000 power outages, shut the federal government in Washington and compounded flight crew and airline staffing shortages connected to the Omicron variant.

Across the country, more than 2,800 domestic flights were canceled Monday, and more than 3,400 have been delayed, according to FlightAware, which tracks flight status in real time.

In Washington, Defense Secretary Lloyd Austin said Sunday night in a statement that he has tested positive for the virus and was experiencing mild symptoms while quarantining at home.

Austin said he last met with President Joe Biden on Dec 21, more than a week before he began to experience symptoms and had tested negative the morning of that day.

"My staff has begun contact tracing and testing of all those with whom I have come into contact over the last week," he said.

Austin, 68, said he was fully vaccinated and received a booster in October. He said he requested a test Sunday morning after experiencing symptoms while at home on leave and, given the result, planned to remain in quarantine for five days, per CDC guidelines.

Congress' top doctor, Brian Monahan, urged lawmakers on Monday to move to a "maximal telework posture", citing surging numbers of coronavirus cases at the Capitol that he said are mostly breakthrough infections.

The seven-day average rate of infection at the Capitol's testing center has grown from less than 1 percent to more than 13 percent, he wrote in a letter to congressional leaders obtained by The Associated Press.

In what he said was a limited sampling as of Dec 15, about 61 percent of the cases were the new, highly contagious Omicron variant, while 38 percent were the Delta variant.

Nearly 2,200 public schools have announced that they will be closed starting Monday because of coronavirus outbreaks and staffing shortages, according to Burbio, a data company that is monitoring K-12 school closures in 5,000 districts across the country. Many have switched to remote instruction for periods lasting from a few days to several weeks.

In New York City, the largest public school system in the country with more than 1 million students reopened from the holiday break on Monday. The city Education Department's website listed eight schools that were closed Monday because of the virus, out of about 1,700.

Mayor Eric Adams said Monday, his third day in office: "The safest place for our children is in a school building, and we are going to keep our schools open."