set 2 axis on roation

I have a C# code on the camera that makes it follow the player by rotation and position from first and third person view:

Code (CSharp):

1.     void LateUpdate () {
2.         if (thirdPersonView)
3.         {
4.             transform.position = Vector3.Lerp(transform.position, camPos1.position, Time.deltaTime * smoothFollow);
5.             transform.forward = Vector3.Lerp(transform.forward, camPos1.forward, Time.deltaTime * smoothFollow);
6.         }
7.         else
8.         {
9.             transform.position = camPos2.position;
10.             transform.forward = camPos2.forward;
11.         }
12.     }
13.  
14.     void Update() {
15.         if (Input.GetButtonDown("View"))
16.             thirdPersonView = !thirdPersonView;
17.  
18.         rotationY = Input.GetAxis("Mouse Y");
19.  
20.         transform.Rotate(Vector3.right, -rotationY);
21.     }

What I am trying to do is make the camera follow the player and when I use the mouse and moves it up and down the camera rotation should move too up and down.
As you can see I have tried that but this is not working right beacouse of the line:

Code (CSharp):

1. transform.forward = camPos2.forward;

I have tried to split the forward to 3 part: the X Y Z and sets the X to transform.rotation but it is not working too.
How can I make it work?