Andy Slavitt said at a news conference that all 50 states in the country are currently affected.
"The 6 million doses represents about three days of delayed shipping, and many states have been able to cover some of this delay with existing inventory," he said.
The advisor cited three reasons for the delay in shipment. Logistic firms including FedEx, UPS, and McKesson faced challenges because workers have been unable to get to work to package and ship vaccines kits.
Road closures between manufacturing sites to distribution, and to shipping hubs were the second reason. Third, more than 2,000 vaccine sites, said Slavvit, are located in areas with power outages.
"Now, as weather conditions improve, we're already working to clear this backlog,” he said. Of the 6 million, “1.4 million doses are already in transit today, and we anticipate that all the backlog doses will be delivered within the next week.”