The U.S. has approved two vaccines for emergency use developed by Pfizer and BioNtech and another by Moderna.
But doses are being rationed out over the next few months, starting with high-risk groups and health care workers.
And the head of the World Health Organization warns of more challenges in the year ahead.
"For example, new variants of COVID-19 and helping people who are tired of the pandemic continue to combat it. ... Only if countries are looking and testing effectively will you be able to pick up variants and adjust strategies to cope," WHO Director-General Dr. Tedros Adhanom Ghebreyesus said.
Monday marked a year since the WHO first learned about COVID-19 from scientists in China.
According to Johns Hopkins University, in that time, more than 81 million people have been infected worldwide.
More than 19 million of them in the U.S. alone, and hospitals and intensive care units are overwhelmed.
The COVID Tracking Project says more people have been hospitalized with COVID in the past week than ever during this pandemic.