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.

An accommodation facility for refugees in the German city of Cologne has been hit by new, more contagious variants of the novel coronavirus, city officials said.

At least 41 residents of the Herkulesstrasse facility have tested positive for COVID-19. Variants first identified in South Africa or Brazil have been detected in 31 of them so far, according to a statement from the city government on Sunday.

At least 16 staff members at the facility have also tested positive for COVID-19, with the South Africa variant detected in 11 of them so far. Variant analysis is still pending on the other five, according to the statement.

All residents of the facility have been under quarantine since Friday, while the site has been under surveillance since Sunday. Entry into the refugee accommodation center, which can house up to 600 people, is currently banned, according to the statement.

The first known COVID-19 cases were confirmed among two employees at the facility about 10 days ago, while the first case of a variant was confirmed last week, according to the statement.

Dr. Harald Rau, head of Cologne's public health department, said the detection of variants at the refugee accommodation center "is a clear alarm signal for all of us."

"I ask all people in Cologne to avoid contact even more consistently than before and to follow the distance and hygiene rules of infection protection," Rau said in a statement Sunday.

Zanzibar's First Vice President Seif Sharif Hamad, his wife and a number of his close aides have tested positive for COVID-19, according to a statement from his party.

Hamad, 77, was admitted to a hospital in Tanzania's semi-autonomous archipelago on Friday evening for precautionary measures, after doctors advised him to remain under close medical supervision while he receives treatment, according to the statement.

Hamad's party noted in the statement that the medical conditions of him and his wife are improving.

The number of cases, hospitalizations and deaths are continuing to decline nationwide, according to a new report by the U.S. Department of Health and Human Services.

There has been a four-week downward trend of reported COVID-19 cases – resulting in a 50% decline since the peak on Jan. 8.

However, the rates of adult occupancy in intensive care units remain high in several states, the report found.

So far, 39,037,964 vaccine doses have been administered, with 9% of the population (30.3 million people) having received one or more doses and 3% of the population (8.3 million people) having received two doses.

Since President Joe Biden was sworn in on Jan. 20, 22,512,683 doses have been administered toward his 100 million goal.

ABC News’ Josh Margolin and Arielle Mitropoulos contributed to this report.