Two new reports from the U.S. Centers for Disease Control and Prevention detail who received COVID-19 vaccines during the first month of the country's mass immunization program. Of the 13 million Americans who got at least one dose between Dec. 14 and Jan. 14, 63% were women and 55% were aged 55 or older, according to the reports, which were released Monday.

The new data revealed a troubling trend in nursing home settings. While nearly 78% of residents got at least one vaccine dose, only 38% of staff members got vaccinated, which suggests barriers to vaccination for staff that "need to be overcome," according to the CDC.

The racial breakdown of who got vaccinated is less clear. Since multiple jurisdictions aren't reporting a breakdown by race, there's missing data on about half of the people who were vaccinated. Based on the data that's currently available, 60% of those people who received vaccines were white, 12% were Hispanic and 5% were Black, the CDC reported.

As the vaccination rollout continues, "it is critical to ensure efficient and equitable administration to persons in each successive vaccine priority category, especially those at highest risk for infection and severe health outcomes," the CDC said.

ABC News' Sony Salzman contributed to this report.

January marked the nation's deadliest month of the coronavirus pandemic, with a death total approximately four times the reported number of COVID-19 deaths recorded by the U.S. in any month between June and October 2020, according to an ABC News' analysis of data compiled by The COVID Tracking Project, a volunteer-run effort to track the U.S. outbreak.

In total, the virus has claimed the lives of over 440,000 people in the U.S., which translates to about 1 in every 747 Americans, according to an ABC News analysis of data collected by Johns Hopkins University.

The U.S. death toll is approximately 147 times the total lives lost during the terrorist attacks on Sept. 11, 2001, and is around 65% of the total number of deaths that were recorded nationwide during the 1918 influenza pandemic.

Although the country's seven-day average of daily COVID-19 deaths appears to be plateauing, the U.S. is still reporting an average of just over 3,100 new deaths from the disease per day, according to The COVID Tracking Project data.

More than 6 million COVID-19 infections were diagnosed nationwide during the month of January, making it the country's second-worst month of the pandemic in terms of confirmed cases, data shows.

Over the weekend, the country's cumulative tally of confirmed cases surpassed 26 million, which signifies that one in every 12 Americans has now tested positive for COVID-19. However, the national seven-day average of daily COVID-19 cases has dropped by 32.3%, the lowest average the country has seen since mid-November, according to The COVID Tracking Project data.

COVID-19 hospitalizations also continue to decline rapidly across the country. The number of COVID-19 patients currently hospitalized nationwide is the lowest since late November.

In total, more than 800,000 people have been hospitalized with COVID-19 in the U.S. since the start of the pandemic, according to The COVID Tracking Project data.

ABC News' Brian Hartman and Arielle Mitropoulos contributed to this report.

The European Union's law enforcement agency, Europol, is warning member nations to be on the lookout for fake COVID-19 test certificates.

An increasing number of countries in the EU and beyond are requiring travelers to present proof of a negative COVID-19 test to be allowed entry when coming from a high-risk area. In an early warning notification issued Monday, Europol said several cases have emerged of fraudulent test certificates being sold to travelers, including the arrest of a forgery ring selling negative test results to passengers at France's Paris Charles de Gaulle Airport. The amount charged for the fake documents ranged between 150 to 300 euros ($181-362).

Fraudsters were also caught in the United Kingdom selling bogus test documents for 100 pounds ($137), faking the name of a genuine laboratory on the false certificates, according to Europol.

"The detection of fake COVID-19 negative test certificates confirms that criminals -- be it organised crime groups or individual opportunistic scammers -- seize profitable opportunities once they arise," the agency said in the notification. "As long as travel restrictions remain in place due to the COVID-19 situation, it is highly likely that production and sales of fake test certificates will prevail. Given the widespread technological means available, in the form of high-quality printers and different software, fraudsters are able to produce high-quality counterfeit, forged or fake documents."

Dr. Stephen Hoge, president of American biotechnolgy company Moderna, said he's hopeful that vaccines can help the U.S. population achieve herd immunity against the novel coronavirus by mid-year.

"It really depends what you think herd immunity needs to be. But if you assume 50 to 70% of the population, then we're working hard ourselves and the other manufacturers to make sure that's a possibility really in the late spring, early summer," Hoge told ABC News chief anchor George Stephanopoulos in an interview Monday on "Good Morning America."

"It's ultimately going to depend upon the delivery of those vaccines, and so that's something that the states and the health care providers in this country are ultimately leading the way on as well as Americans deciding they want to receive that vaccine," he added. But we're optimistic that by the middle of the year, we'll be able to achieve those sorts of numbers."

Moderna is ramping up production of its COVID-19 vaccine and is working to clear any "bottlenecks" in the supply chain, according to Hoge.

"At this point, a lot of the logistical bottlenecks that we're running into are problems we can solve on our own," he noted. "We're in good shape."

Hoge, who was a resident physician in New York City, said data currently shows that existing vaccines are still effective against all emerging strains of the virus. But the variant first identified in South Africa "is of some concern because it looks like it could hide from the vaccine a little better than others," he said.

"So our approach in Moderna is going to be to develop a booster vaccine so that if the South African variant or any other variant becomes a concern, we'll be able to offer a way to identify that, prevent it from hiding from the vaccine," he said.