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The U.S. is working to vaccinate a high percentage of its population against COVID-19 as soon as possible to stop the spread of the disease and end the outbreak in the country.
The mission becomes even more urgent as coronavirus variants emerge around the world, raising concerns that the virus could evade our efforts to control it, if the spread is not curbed quickly.
Since vaccine distribution began in the U.S. on Dec. 14, more than 110 million doses have been administered, reaching 21.7% of the total U.S. population, according to federal data collected by the Centers for Disease Control and Prevention. The U.S. is currently administering over 2.4 million shots a day.
In addition to the states, the federal government distributes vaccines to four federal agencies, five U.S. territories and three freely associated states.
Currently, two of the three COVID-19 vaccines that have been authorized for emergency use require a two-shot regimen spaced out by three or four weeks. A third vaccine that requires just one shot was authorized on February 27.
Strategies for distribution — along with the efficiency and equity of the process — vary from state to state.
Getting millions of people vaccinated, in order of priority, is a big logistical challenge for states. As a result, there's often a delay between when states receive their federal shipments of vaccines and when they get all the shots into people's arms.
The speed of vaccination has improved since December, but there are still millions more doses distributed to states than have been administered to people.
States receive weekly vaccine allocations from the federal government based on their total adult populations. Each state has its own plan for how to get those vaccines out to its residents — through county health offices, hospital systems, pharmacies, mass vaccination sites and mobile clinics — and some states are making more efficient use of their supplies than others.
The federal government also sends vaccine allotments directly to some large retail pharmacies and community health centers.
Some state officials have argued that the CDC figures don't accurately represent how efficiently they are administering the vaccine doses they receive. The CDC says its data may reflect a reporting lag of up to five days.
President Biden has declared a goal of getting 100 million shots into arms in the first 100 days of his administration — an effort that stretches from late January through April.
To speed up the effort to get the U.S. population vaccinated, Biden says the country needs more vaccine supply from manufacturers and more efficiency from states.
On February 11, President Biden announced the federal government has inked deals to purchase 600 million vaccine doses from Pfizer and Moderna, to be delivered by the summer. The federal government also announced that it is using the Defense Production Act to help smooth other bottlenecks, such as limited supplies of syringes or protective gear.
In late January, the Biden administration promised to provide states with more reliable projections of the vaccine supply at least three weeks ahead of time, to increase the number of vaccinators and to blanket the nation with thousands of new government-supported sites where people can go to get vaccines.
Zach Levitt, Selena Simmons-Duffin, Ruth Talbot, Thomas Wilburn and Carmel Wroth contributed to this report.