Charter flights bringing Pfizer's COVID-19 vaccine to the United States from Belgium began on Friday, the start of what the Federal Aviation Administration calls the first "mass air shipment" of a coronavirus vaccine.
There are no authorized coronavirus vaccines in the U.S. yet, but preparations for distribution are ramping up. Under FDA rules, vaccine cannot be shipped to actual administration sites until it was been either licensed or authorized by FDA.
Pfizer's vaccine is anticipated to be authorized by the Food and Drug Administration by mid-December. 
While it can't be delivered to doctors, vaccine can be pre-positioned at distribution sites to allow for quicker delivery once it is authorized, which is what began on Friday.
Operation Warp Speed, the White House-led initiative to develop and distribute vaccines, plans to begin the first vaccine deliveries within 24 hours of FDA authorization.
Pfizer has two plants making its COVID-19 vaccine, one in Kalamazoo, Michigan, and one in Puurs, Belgium.
In a statement Sunday, Health and Human Services confirmed that shipments were coming from Europe. 
"Operation Warp Speed leaders are aware of and facilitating vaccine shipments coming to the U.S. from Belgium. In an effort to minimize the potential risk to delivery and distribution, we are unable to provide specific details regarding where vaccines are produced and stored," the statement read.
The Wall Street Journal reported Friday the charter flights on United Airlines were to position doses of the vaccine for quick distribution should it be authorized.
Pfizer has a distribution site in Pleasant Prairie, Wisconsin, where its vaccine is being stored. There may also be other sites around the nation where the vaccine will be stored prior to authorization.