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U.S. researchers have opened another safety test of an experimental COVID-19 vaccine, this one using a skin-deep shot instead of the usual deeper jab.

The pinch should feel like a simple skin test, a researcher told the volunteer lying on an exam table in Kansas City, Missouri, on Wednesday.

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“It’s the most important trial that we’ve ever done,” Dr. John Ervin of the Center for Pharmaceutical Research told The Associated Press afterward. “People are beating down the door to get into this trial.”

The experiment, using a vaccine candidate developed by Inovio Pharmaceuticals, is part of a global hunt for much-needed protection against a virus that has triggered an economic shutdown and forced people indoors as countries try to stem the spread.

A different vaccine candidate began safety testing in people last month in Seattle, one developed by the U.S. National Institutes of Health. About two-thirds of that study’s participants have gotten the first of two needed doses.

Inovio’s study is set to test two doses of its vaccine, code-named INO-4800, in 40 healthy volunteers at the Kansas City research lab and the University of Pennsylvania. Inovio is working with Chinese researchers to also begin a similar study in that country soon.

These early-stage studies are a first step to see if a vaccine appears safe enough for larger tests needed to prove whether it will protect. Even if the research goes well, it is expected to take more than a year before any vaccine could be widely available.

Dozens of potential vaccines are being designed in labs around the world, expected to begin this testing process over the next several months.

“The good thing is we’ve got a bunch of candidates,” Dr. Anthony Fauci, the NIH’s infectious diseases chief, said during a podcast for the Journal of the American Medical Association Wednesday.

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Most of the vaccines under development have the same target: A spike protein that studs the surface of the virus and helps it invade human cells. Yet many work in quite different ways, making it crucial to test different options.

Inovio researchers packaged a section of the virus’ genetic code inside a piece of synthetic DNA. Injected as a vaccine, the cells act as a mini-factory to produce harmless protein copies. The immune system makes protective antibodies against them — primed if the real virus ever comes along.

Inovio research and development chief Kate Broderick likens it to giving the body an FBI wanted poster so it can recognize the enemy.

But after the skin-deep injection, researchers must hold a device over the spot that gives a little electrical zap. The synthetic DNA is large when it comes to penetrating human cells, and the pulse helps the vaccine more easily penetrate and get to work, Broderick said.

DNA vaccines are a new technology. But Inovio has experimental vaccines against other diseases that are made the same way that have passed initial safety testing.

And at least one showed hints that going skin-deep somehow sped the immune system’s development of protective antibodies, University of Pennsylvania's Dr. Pablo Tebas told The AP. Tebas leads the this latest new COVID-19 study.

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The NIH’s vaccine candidate, manufactured by Moderna Inc., works similarly, except it uses a type of genetic code called messenger RNA and is injected deeper — into the muscle.

Neither NIH’s nor Inovio’s potential vaccines are made using the actual virus, meaning there’s no chance of getting infected from the vaccines -- and it’s possible to make far more quickly than traditional shots.

AP Video Producer Kathy Young contributed to this report.