Twenty years after the last human Lyme disease vaccine was pulled from the market, outdoor enthusiasts could get a new vaccine to protect against the widespread tick-borne illness.

According to NPR, the new Lyme vaccine candidate is called VLA15 and it entered its third phase of a clinical trial in humans this week. The vaccine aims to protect people as young as 5.

Though it's expected to take years to reach the market, VLA15 would become the only vaccine for Lyme disease available to humans in the United States, if approved.

If the phase III trial is a success, Pfizer and French pharmaceutical company Valneva — the vaccine's creators — would likely seek official approval in 2025, NPR reports.

"We are extremely pleased to reach this important milestone in the development of VLA15," Valneva Chief Medical Officer Juan Carlos Jaramillo said in a statement. "Lyme disease continues to spread, representing a high unmet medical need that impacts the lives of many in the Northern Hemisphere."

Though it had a high efficacy, the human Lyme disease vaccine Lymerix was withdrawn from the market 20 years ago amid consumer concerns it caused adverse reactions such as arthritis.

The National Institute of Allergy and Infectious Diseases (NIAID) said, "Analysis by the FDA and others did not support that conclusion," but use of the vaccine plunged regardless, which led to its being discontinued by the manufacturer.

Vaccines for Lyme disease in dogs are currently available on the U.S. market, however.

VLA15 works in a similar way to Lymerix, targeting an outer surface protein of the Borrelia burgdorferi bacteria which causes Lyme disease, according to the NIAID. The new human vaccine candidate "lacks the region of the proteins that some had attributed to adverse events," according to the institute.

Pfizer and Valneva told NPR that 6,000 people in the U.S. and Europe who are at least 5 years old are participating in the current study of VLA15. These participants live in places where Lyme disease is "highly endemic."

In earlier trials, the vaccine candidate triggered a strong immune response "with acceptable safety and tolerability profiles," the drug companies said.

Spread by black-legged ticks, people face a greater risk of Lyme disease if a tick remains attached to their skin for a prolonged period of time. Experts urge people to check themselves for ticks after spending time outside.

According to the federal Centers for Disease Control and Prevention, early symptoms of Lyme disease include fever, headache and a circular rash that can resemble a bull's-eye. Later symptoms include joint pain, facial palsy, and inflammation of the brain and spinal cord. The disease is treatable with antibiotics, especially in its early stages.

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