The same day the nation celebrated the arrival of the long-awaited coronavirus vaccine, with the first doses of the Pfizer-BioNTech COVID-19 jab distributed on Monday morning, the nation faced a grim reality: the novel virus continues to kill, with the U.S. surpassing 300,000 deaths.

Estimates from Johns Hopkins University show that the U.S. has recorded 300,267 coronavirus deaths as of Monday afternoon. The country continues to lead the world in the highest amount of such fatalities, ahead of Brazil, India and Mexico.

The saddening milestone comes less than a month after the nation surpassed 250,000 deaths in November.

While the coronavirus vaccine has been touted as an extraordinary achievement by health officials, Health and Human Services Secretary Alex Azar warned Americans to not let their guard down against the deadly disease.

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"We need you to be vigilant, we need you to do that because we want everybody who is here now to be here next year for the holiday season," Azar said. "Now is not the time to let our guard down. This is not the end of our battle against COVID but today marks a critical milestone in the ultimate defeat of COVID-19."

The first shipments of the coronavirus vaccine developed by Pfizer and BioNTech's arrived at their destinations on Monday, and the rollout officially began with the first shot being administered to a critical care nurse in New York. The vaccine is the first in the country to receive emergency use approval from the U.S. Food and Drug Administration (FDA).

The Centers for Disease Control and Prevention (CDC) has recommended that the initial doses be administered to health care workers and residents and staff of long-term living facilities. Though states are not required to follow the federal agency’s recommendations, many will follow them.

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The FDA is set to meet Thursday to review the Moderna vaccine. A third candidate, from Johnson & Johnson, which would require just one dose, is working its way through the pipeline. Behind that is a candidate from AstraZeneca and Oxford University. U.S. health experts are hoping a combination of vaccines will ultimately enable the U.S. to conquer the outbreak.

“This is the beginning of the end,” said U.S. Surgeon General Dr. Jerome Adams of the vaccine rollout during an appearance on “Fox and Friends” on Monday.

The Associated Press contributed to this report.