Burger King is bringing its vegan Impossible Whopper nationwide, starting Aug. 8.

The nation's second-largest burger chain began testing the plant-based burger from Impossible Foods at locations in St. Louis in April. Those Burger King locations saw traffic outperform national averages by 18.5% that month, according to a report from inMarket.

Chris Finazzo, president of Burger King North America, declined to share any specific numbers about foot traffic or sales in an interview.

Since April, Burger King has brought the Impossible Whopper to six more markets. Jose Cil, CEO of Burger King's parent company, Restaurant Brands International, said in late May that the chain will roll it out nationwide to its 7,200 U.S. locations.

McDonald's, the largest burger chain in the U.S., has yet to offer a plant-based burger in its home market. CEO Steve Easterbrook has said that the chain is waiting to learn more about how the vegan item can drive foot traffic.

The Impossible Whopper has been bringing in new customers to the chain, Finazzo said. Burger King restaurants testing the burger have been drawing people who usually shop at Trader Joe's, Whole Foods and Panera Bread, he said.

The popularity of alternatives that taste and look like meat has largely been driven by flexitarians, people who are trying to reduce their meat intake — a fact that surprised and impressed Finazzo when Burger King started thinking about adding a plant-based burger a year ago.

According to data from the NPD Group, 95% of plant-based burger buyers have bought a beef burger within the last year. Thanks to chains such as Burger King, White Castle and Bareburger, servings of plant-based burgers at fast-food chains are up 10% in the year ending in May, the NPD Group found.

As part of its plan to promote the burger, Burger King will offer a limited-time taste test box through its mobile app and delivery partner DoorDash. For $7, customers can receive a Whopper made with beef and an Impossible Whopper.

"I think one of the insights we had during the entire process is that it really does taste like beef," Finazzo said.

He added that some people in Burger King's office have been unwittingly served the Impossible Whopper instead of its beefy counterpart.

A similar version of that story played out in Brooklyn in June. Eater reported that a Burger King location had been advertising that it sells the Impossible Whopper on GrubHub's Seamless delivery platform and then sending a Whopper that contained beef. Burger King said in a statement at the time that the product error was "due to a technology error."

"It was an unfortunate glitch that's been rectified but is not expected to be a problem for the national launch," Finazzo said.

The Impossible Whopper will only be available for a limited time, which Finazzo said is typical for new products.

On Wednesday, Impossible announced a manufacturing deal with OSI Group, a large meat supplier that also makes patties for fast-food chains, as demand soars. The company is also planning to bring its Impossible Burger to grocery stores in September.