Regeneron's monoclonal antibody treatment can lower the risk of contracting COVID-19 by nearly 82% for up to eight months, the company said.

Regeneron's monoclonal antibody treatment, given as an intravenous infusion or a subcutaneous injection, is already FDA-authorized as a way to prevent COVID-19 among people who have recently been exposed and are considered "high risk" for more serious illness. This new data shows that when given as preventative measure, it dramatically reduces the risk of infection for up to eight months.

Regeneron and the FDA say that vaccination is the best way to prevent infection and serious illness. However, Regeneron's antibody treatment can be a good option for people who are immune compromised or haven't mounted an adequate immune response through the vaccine.

-ABC News' Sony Salzman, Sasha Pezenik

The U.S. reopened borders to vaccinated travelers on Monday after 20 months of being closed to many countries, including the United Kingdom, Brazil, China, India, South Africa and most of Europe.

In January, as the vaccine was distributed on both sides of the Atlantic, the ban was kept in place, with the Biden administration stating concerns about the delta variant.

On Oct. 20, the Biden administration announced it was lifting the ban on vaccinated travelers.

The ban, which only applies to vaccinated travelers, still excludes many countries where the vaccine is not yet easily available or recognized by the U.S.

-ABC News' Zoe Chevalier

The worldwide number of people who have been diagnosed with COVID-19 surpassed 250 million on Monday, according to a count kept by Johns Hopkins University.

The United States, India and Brazil account for about a third of the recorded cases, Johns Hopkins data shows.

The grim milestone came as some countries in Eastern Europe, including Russia, Ukraine and Greece, grapple with record levels of newly reported cases.

The pandemic began less than two years ago after the virus was first detected in Wuhan, China, in December 2019.

The Biden administration sent letters to superintendents and principals across the United States on Monday, urging them to set up COVID-19 vaccination clinics inside their elementary schools.

"Parents rely on their children’s teachers, principals, school nurses, and other school personnel to help keep their students safe and healthy every school year," U.S. Secretary of Health and Human Services Xavier Becerra and U.S. Secretary of Education Miguel Cardona wrote in the letters. "We need your help now more than ever to continue to protect our communities and our children."

They also asked the school leaders to distribute information "from trusted sources" about COVID-19 vaccines to all families with children ages 5 to 11, and to host community engagements with parents in partnership with local pediatricians and "other trusted medical voices" in the community.

"The communications you issue -- in languages accessible to your parents -- will be critical in helping families learn more about the vaccine," Becerra and Cardona wrote.

The letters went out on the same day that first lady Jill Biden and U.S. Surgeon General Vivek Murthy plan to visit an elementary school in McLean, Virginia, that was used as one of the first sites in the country to begin administering the polio vaccine in 1954.

School officials would not be responsible for handling COVID-19 vaccines or giving shots to students. Instead, they would partner with a local vaccine provider already administering shots, such as a pharmacy or community health clinic.

The schools would be allowed to use federal dollars through the American Rescue Plan to offset any costs with providing the space and organizing the vaccine drive.

-ABC News' Anne Flaherty

As COVID-19 cases increase across the country, at least four states are moving to recommend booster shots for all adults ahead of federal authorization.

Colorado Gov. Jared Polis signed an executive order on Thursday declaring the entire state at high risk from COVID-19, thereby making all fully vaccinated adults eligible to receive a booster shot.

"We want to ensure that Coloradans have every tool they need to protect themselves from this deadly virus and to help reduce the stress on our hospitals and health care workers," Polis said in a statement.

Every Coloradan ages 18 and up who is at least six months past the second dose of the Pfizer-BioNTech or Moderna vaccine, or two months past the single-dose Johnson & Johnson vaccine, can now receive a booster shot.

"I was relieved to get the booster two weeks ago," Polis said, "and strongly encourage you to get it too."

On Friday, the governor of New Mexico issued a similar order, and officials in California and West Virginia have previously encouraged residents to receive a booster shot.

Meanwhile, Pfizer asked the Federal Drug Administration for booster authorization for all adults on Tuesday.