This paper introduces WAVE (Worm Gear-based Adaptive Variable Elasticity), a novel variable stiffness actuator (VSA) that employs a non-backdrivable worm gear to decouple the driving motor from external forces. By integrating compliance directly into the mechanical design, WAVE allows robots to switch continuously between high stiffness (for precise actuation and payload handling) and low stiffness (for safe interaction and impact absorption). The worm gear transmits torque for joint actuation while passively absorbing positional errors through spring deformation, effectively protecting the motor. The authors developed a modular WAVE unit and validated its stiffness model experimentally, showing that motor load approaches zero at rest even under external forces. Tests confirmed robust behavior under impact loads, contact tasks, and position tracking, demonstrating that manipulators equipped with WAVE can operate safely in contact-rich and high-load environments. While limitations such as frictional losses and worm gear locking under heavy loads remain, WAVE highlights a promising approach for safe, resilient, and versatile robotic manipulation in unstructured settings.