Tomorrow, President Biden is expected to announce a stricter COVID-19 testing requirement for travelers entering the United States as part of his enhanced winter COVID-19 strategy. The new requirement would mandate that all travelers, regardless of vaccination status, take a COVID-19 test no more than 24 hours before a flight entering the US (and show a negative result).
Previously, vaccinated travelers could take the test up to 72 hours before the flight, with only unvaccinated people subject to the 24-hour test requirement.  A mandatory post-travel quarantine requirement was considered but is not expected to be adopted for U.S. citizens.