Scroll based camera movement on curve with tween.js

Hi everyone!
I have a scene where I have a catmullromcurve spiral, and if the user scrolls down the camera should move on the curve using tween.js. I tried to implement the camera movement from this example:

I setup a listener for the scroll with 2 direction (up and down). It looks like this:

window.addEventListener(‘wheel’, function(event) {
if (event.deltaY < 0){

         else if (event.deltaY > 0)
            var camPos = curve.getPoint(camPosIndex / 50);
            var camRot = curve.getTangent(camPosIndex / 50);
            var tween = new TWEEN.Tween(camPosIndex).to(camPosIndex++, 2000); 
                camera.position.x = camPos.x;
                camera.position.y = camPos.y;
                camera.position.z = camPos.z;
                camera.rotation.x = camRot.x;
                camera.rotation.y = camRot.y;
                camera.rotation.z = camRot.z;
                camera.lookAt(curve.getPoint((camPosIndex+1) / 50));

For some reason when the scroll event is happening it just jumps to the next step, without easing. I know I’m doing something wrong, but cannot figure out, how to define the next “step” in the curve.
You guys can see the live example here:

Any help is well appreciated.