Six months after the start of an extraordinary national vaccination campaign, the United States has dethroned "COVID haven" countries like New Zealand and Singapore as the best place to live as the world begins to reopen from the coronavirus pandemic.

The nation's high ranking--based on Bloomberg's COVID Resilience Ranking system which tracks metrics like how much air travel has recovered, mortality and infection rates, freedom of movement, and economic growth--comes as a dramatic pivot from being once the world's worst outbreak.

Still, there are some looming threats to U.S. economic recovery and pandemic reopening--chief among them being the Delta variant which first emerged in India and is now the dominant global strain.

Last Friday, the World Health Organization (WHO) urged fully vaccinated people to continue to wear masks, social distance and practice other public health safety protocols as the highly contagious Delta variant threatens to infect even the inoculated.

People cannot feel safe just because they had the two doses. They still need to protect themselves," Dr. Mariangela Simao, WHO assistant director-general for access to medicines and health products, said during a press briefing from the global health agency's headquarters in Geneva.

"Vaccine alone won't stop community transmission," Simao added. "People need to continue to use masks consistently, be in ventilated places...[and] avoid crowding. This still continues to be extremely important, even if you're vaccinated, when you have a community transmission ongoing."

The WHO's warning comes as some countries with high vaccination rates, including the United States, have eased their mask requirements and other pandemic-related restrictions as infection rates decline. However, the Delta variant can easily threaten that progress.

Dr. Anthony Fauci, the nation's top infectious disease expert, recently stated that the Delta variant makes up about 20% of all new COVID infections in the U.S., doubling in about two weeks. The variant strain seems to be following the same outbreak pattern as the Alpha strain, which was first discovered in the United Kingdom, Fauci stated during a White House press briefing, with infections doubling about every two weeks. That means that the Delta variant could be the dominant U.S. strain in an alarmingly short amount of time.

"Similar to the situation in the U.K., the Delta variant is currently the greatest threat in the U.S. to our attempt to eliminate COVID-19," Fauci stated.