Transcript for Winter weather hampers COVID-19 vaccine deliveries
Now to the pandemic and the weather hampering delivery of vaccines to all 50 states. 6 million doses delayed. Shipping companying struggling to keep up. FedEx and others rerouting deliveries. More than 42 million nationwide receiving at least one dose of the vaccine. That's 13% of the population. This as the U.S. Approaches a new painful nile stone -- nearly 500,000 dead from the virus, amid growing concerns over spreading mutations. ABC's Stephanie Ramos with the latest. Reporter: Tonight, the rush to get back on track. Covid vaccination centers from Colorado to Pennsylvania scrambling to catch up after those deadly ice storms slowed shipments. I can't do anything about the cold weather. Just asking people to dress warm. Reporter: Overnight in Philadelphia, a group of black doctors holding a 24-hour vaxathon to reach those eligible in the hardest hit areas. The weather hampering the rollout in all 50 states and creating a backlog. At least 6 million doses unable to be delivered. Days' worth of shipping delays. Today, the white house revealing the majority of those doses have been shipped. FedEx and U.P.S. Making deliveries all weekend. Vaccination sites now working overtime. We're asking them to extend their hours. We're asking them to make more appointments. Reporter: The supply shortages fueling ongoing debate over whether second doses should be delayed to get more Americans their first shot. A new study finding the pfizer vaccine was 85% effective in preventing symptomatic disease 15 to 28 days after the first dose. The white house remaining firm on following the two-dose regimen. Even though you can get a fair degree of "Protection" after a single dose, it clearly is not durable. Reporter: But seemingly positive signs emerging. The number of patients in hospitals dropping to numbers not seen since the spring and summer. The U.S. Though still inching toward that unfathomable milestone -- 500,000 confirmed covid deaths. Kristen Chaves is part of a mariachi band. He says his California organization has lost 60 of its 400 members to covid, including his godfather, who died after his band returned to work in We thought it was okay to start working again, and we started working. Then I got infected and then my father got sick as well, and two other people, or coworkers from my mariachi got sick and they ended up dying. Reporter: The white house says the fda plans to release guidance to pharmaceutical companies on how to test updated vaccines targeting variants. Scientists say they may not need to be tested in large clinical trials made up of tens of thousands of volunteers like the initial vaccine, making the process faster. Whit? Stephanie Ramos, our thanks to you tonight.
This transcript has been automatically generated and may not be 100% accurate.