When a coronavirus vaccine comes on the market, people will likely need two doses, not just one -- and that could cause real problems, CNN says in a fresh article.

Some of the potential problems are logistical. Difficulties procuring test kits and protective gear throughout the pandemic point to supply chain issues that could also plague distributing double doses of vaccines for an entire country.

Other potential concerns are more human. Convincing people to show up to get a vaccine not once, but twice, could be a formidable undertaking.

"There's no question that this is going to be the most complicated, largest vaccination program in human history, and that's going to take a level of effort, a level of sophistication, that we've never tried before," said Dr. Kelly Moore, a health policy professor at Vanderbilt University.

So far, Operation Warp Speed, the federal government's effort to get a vaccine on the market, has given money to six pharmaceutical companies.

Two of those companies, Moderna and Pfizer, are now in Phase 3, large-scale clinical trials. The 30,000 volunteers in each of the trials are getting two doses, with Moderna spacing their shots out 28 days apart and Pfizer spacing theirs out by 21 days.