December has been the worst month on record for cases, hospitalizations, and deaths. Even with delays in reporting because of the holidays, hundreds of thousands of Americans are still testing positive for the virus every day.

The U.S. has logged more than 5.6 million cases in the first 28 days of December -- more than any month on record, according to ABC News’ analysis of data compiled by the COVID Tracking Project.

According to the CDC, California currently has the highest average of daily cases per 100,000 people in the last seven days, followed by Tennessee, Oklahoma and Arizona.

Across the U.S., 121,235 people are currently hospitalized with COVID-19 -- setting a new record high since the pandemic began. That total is double the previous hospitalization peaks from April and July.

ABC News’ Arielle Mitropoulos contributed to this report.

Vice President-elect Kamala Harris received her first dose of the Moderna vaccine on Tuesday morning at United Medical Center in Washington, D.C.

"That was easy!" she said after. "I barely felt it."

"I want to encourage everyone to get the vaccine," Harris said. "It is literally about saving lives. I trust the scientists, and it is the scientists who created and approved this vaccine."

"Right in your community is where you can take the vaccine," Harris said. "Where you will receive the vaccine by folks you may know, folks who otherwise are working in the same hospital where your children were born, folks who are working in the same hospital where an elderly relative received the kind of care that they needed. So I want to remind people that they have trusted sources of health, and that's where they will be able to go to get the vaccine, so I encourage them to do that."

Harris’ husband, Doug Emhoff, also received the vaccine.

Staff and residents of Holyoke Soldiers' Home in Holyoke, Massachusetts, started receiving COVID-19 vaccinations Tuesday morning.

Staff looked on as the first person -- 78-year-old man U.S. Air Force Veteran Robert Aucoin -- was inoculated.

The long-term care facility has suffered one of the worst coronavirus outbreaks in the country. In the spring, 76 veterans who lived at the home died of the virus. A resident who had been living in an off-site nursing facility since April died earlier this month.

Two former top administrators pleaded not guilty to criminal negligence charges connected to the residents' deaths, WCVB reported.

U.S. states have begun vaccinating patients at long-term care facilities in an effort to keep the most vulnerable Americans safe from the virus. The government's goal is to have 20 million Americans vaccinated by the end of the year.

The TSA screened 1,111,751 travelers at checkpoints across the U.S. on Monday. This marked the seventh day within the last 11 days that more than 1 million people were traveling.

ABC News’ Sam Sweeney contributed to this report.

Public Health England does not recommend mixing COVID-19 vaccines from different suppliers, according to the agency's head of immunizations, Dr. Mary Ramsay.

"We do not recommend mixing the COVID-19 vaccines -- if your first dose is the Pfizer vaccine you should not be given the AstraZeneca vaccine for your second dose and vice versa," Ramsay said in a statement Saturday. "There may be extremely rare occasions where the same vaccine is not available, or where it is not known what vaccine the patient received. Every effort should be made to give them the same vaccine, but where this is not possible it is better to give a second dose of another vaccine than not at all."

The clarification comes amid questions over the British government's updated guidance on COVID-19 vaccines, which now says that if individuals who received the first shot go to an immunization site where that same vaccine is not available for a second shot, or if the first vaccine received is unknown, "it is reasonable to offer one dose of the locally available product to complete the schedule."

"This option is preferred if the individual is likely to be at immediate high risk or is considered unlikely to attend again," the guidance adds.

The guidance, which was updated Thursday, also notes that "there is no evidence on the interchangeability of the COVID-19 vaccines although studies are underway," and thus "every effort should be made to determine which vaccine the individual received and to complete with the same vaccine."

Next week, the United Kingdom is set to begin distribution of a COVID-19 vaccine developed by England's University of Oxford and manufactured by British-Swedish pharmaceutical giant AstraZeneca, after the drug was approved Wednesday for emergency supply. Another COVID-19 vaccine developed by U.S. pharmaceutical giant Pfizer and its German partner BioNTech was approved in the U.K. on Dec. 2 and rollout began a week later.

ABC News' Zoe Magee contributed to this report.