Are the rods used for scoliosis surgery more flexible than the harrington rod?

No: Modern fixation and correction is structuraly stiffer than harrington rod fixation fors everal reasons material used and number of fixation sites why do you ask?

Variable: Modern scoliosis rods have different properties based on their thickness (cross sectional area) and their metallurgy (ie titanium is more flexible). Another important bit of knowledge about the implants are whether or not they are MRI safe.

Rods are malleable: Harrington rods were straight, stainless steel rods that acted like a car jack to straighten the spine in one direction. Which was and advancement at that time in history, modern spinal instrumentation is malleable (bendable) with special tools so surgeons can customize the rod to correct and stabilize the individual patient's condition. Rods are not flexible in the body.