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Coronavirus vaccine likely will require two doses

(CNN)— When a coronavirus vaccine comes on the market, people will likely need two doses, not just one -- and that could cause real problems.

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.

AstraZeneca is expected to start Phase 3 trials this month. Their Phase 1 and Phase 2 trials used two doses given 28 days apart.

Novavax also has yet to begin Phase 3 trials but used two doses in their earlier trials.

In Johnson & Johnson's upcoming Phase 3 trials, some participants will take one dose and others will take two doses.

Sanofi hasn't made announcements about whether their vaccine will be in one or two doses.

It's not surprising that the coronavirus vaccine will likely need two doses. Many vaccines -- including childhood vaccines for chickenpox and Hepatitis A and an adult vaccine for shingles -- require two doses.

Some require even more -- children get five doses of the DTaP vaccine, which protects them against diphtheria, tetanus, and pertussis.

There's also a precedent for developing mass vaccination programs on short notice. In the spring of 2009, when a new strain of flu emerged, a vaccine program vaccinated 161 million Americans within months, according to the US Centers for Disease Control and Prevention.

That means the upcoming coronavirus vaccine program will be difficult -- but not impossible -- to pull off.

"I have faith we can do it, but it is a big ask and we have to work with people to make it work," Moore said.

FILE - In this March 16, 2020, file photo, a subject receives a shot in the first-stage safety study clinical trial of a potential vaccine by Moderna for COVID-19, the disease caused by the new coronavirus, at the Kaiser Permanente Washington Health Research Institute in Seattle. According to results released on Tuesday, July 14, 2020, early-stage testing showed the first COVID-19 vaccine tested in the U.S. revved up people’s immune systems the way scientists had hoped. The vaccine is made by the National Institutes of Health and Moderna Inc. (AP Photo/Ted S. Warren, File)