By Betty Yu

SAN FRANCISCO (CBS SF) — The COVID-19 vaccine rollout is falling millions of doses short.

The Trump administration promised 20 million doses would be given by the end of 2020, but the CDC says just over 4 million people have gotten the shot.

“There are vaccines sitting around. It’s not that we don’t have the shots in the US. We just don’t have the well-oiled machinery to administer those shots,” said UCSF Infectious Disease Specialist Dr. Peter Chin-Hong.

Dr. Chin-Hong says there are a variety of factors at play, including a lack of cold storage space and manpower. States also need more money from the federal government to support distribution.

Former FDA Commissioner Scott Gottlieb spoke on Face the Nation Sunday.

“We were hoping we would have more injection at this point and the vaccine could be starting to have an impact on the trajectory of the epidemic,” he said.

The UK has delayed the second doses of the Pfizer vaccine, in order to reach more people with initial doses. That strategy has prompted debate in the US.

“So the choices are vaccinate a few really well, or vaccinate the whole population kind of well,” said Dr. Chin-Hong. “My preference is really to go for the science first, because I’m not really sure if delaying the vaccine will result in the same efficacy in some people in the population.”

Meanwhile, airports were packed Sunday night. The TSA says it’s screened more than 14 million travelers in the last two weeks.

Dr. Chin-Hong says there is another option up for debate – giving half doses of the Moderna vaccine to people between 18 and 55 to accelerate the rollout.