Los Angeles' new vaccine mandate goes into effect Monday, requiring proof of vaccination to enter most public indoor spaces, including: any establishments where food or beverages are served; gyms and fitness venues; entertainment and recreation venues; personal care establishments; and buildings owned or operated by the city.

The mandate also requires proof of vaccination for large outdoor events with 5,000 or more attendees.

Enforcement will begin on Nov. 29.

-ABC News' Catherine Thorbecke

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 U.S. daily case average has jumped by 15% since the end of October, according to federal data.

Twenty states have seen daily cases jump by at least 10% in the last two weeks: Arizona, California, Colorado, Illinois, Indiana, Iowa, Maine, Massachusetts, Michigan, Minnesota, Mississippi, Missouri, Nebraska, Nevada, New Hampshire, New Mexico, New York, Utah, Vermont and Wisconsin.

Cases are still falling in most of the South, which was the first region to get hit hard by the delta surge over the summer. In Florida, where high transmission was reported in every county over the summer, now only 1 out of the 67 counties is reporting high transmission, according to federal data.

-ABC News' Arielle Mitropoulos