Execute macro only if specific cell value changes

Question

I have a simple countif formula in Cell E2 that will check for a specific text. Once it's true, it'll execute a macro that will prompt for a msgbox. This code works fine, but when making any other changes to the worksheet will execute the macro again, even though Cell E2's value hasn't changed. How do I stop the macro from executing any further if E2 does not change at all?

Sub Worksheet_Change(ByVal Target As Range)

If Not Intersect(Target, Range("E2")) Is Nothing Then
    If Target.Value = "True" Then
        Application.EnableEvents = False
        a = MsgBox("Test", vbYesNo, "Test")
        If a = vbYes Then
            Range("E3") = "003"
        Else
            Range("E3") = "001"
        End If

        Call ApplyMG
        Application.EnableEvents = True
    End If
End If
End Sub 

EDIT: Thanks to the comment below, removed 'old code' setting target to the same range in the intersect line. However, the macro is not being triggered anymore.

Answer 1

Well turns out I was overcomplicating things. After some research, what I'm trying to do won't work on Worksheet_Change in the first place as this is a formula. So, I only need to move my code onto Worksheet_Calculate instead. No need for unnecessary intersect or anything, as my code only needs to determine if Cell E2 is True, which is determined by the cell formula. Nothing else would matter anyway.

Private Sub Worksheet_Calculate()
Dim trigger As Range
Set trigger = Range("E2")

If trigger.Value = "True" Then
    Application.EnableEvents = False
    a = MsgBox("Test", vbYesNo, "Test")
    If a = vbYes Then
        Range("E3") = "003"
    Else
        Range("E3") = "001"
    End If

    Call ApplyMG
    Application.EnableEvents = True
End If
End Sub

Answer 2

A Real Solution

The result of a CountIf formula is a whole number or an error. So you probably have an If statement that is evaluating a CountIf formula in cell E2.
Use True without the quotes (NOT "True") if the result is a boolean (True or False).
You should always use Option Explicit to be forced to declare all variables (referring to 'a').
Use constants at the beginning of the code to be able to quickly change values in one place only, if necessary.
In your answer, the code is still running all the time (when the worksheet is being calculated) if the value in E2 is True, regardless if the value has changed or not.
The following code might have been written using a static variable inside the procedure instead of the chosen module level variable (blnCheck) outside the procedure. (Should be investigated.)

Option Explicit

Private blnCheck As Boolean

Private Sub Worksheet_Calculate()
  
  Const cStrRangeCheck As String = "E2"
  Const cStrRangeWrite As String = "E3"
  Const cStrResultYes As String = "003"
  Const cSTrResultNo As String = "001"
    
  Dim Msg As Variant
  Dim blnTarget As Boolean
  
  If IsError(Range(cStrRangeCheck).Value) Then GoTo TargetHandler
  
  blnTarget = Range(cStrRangeCheck).Value
  
  If blnTarget = True Then
    If blnCheck = False Then
      blnCheck = True
      Application.EnableEvents = False
      Msg = MsgBox("Test", vbYesNo, "Test")
      If Msg = vbYes Then
          Range(cStrRangeWrite) = cStrResultYes
        Else
          Range(cStrRangeWrite) = cSTrResultNo
      End If
      ApplyMG
      Application.EnableEvents = True
    End If
   Else 'blnTarget = False
    If blnCheck = True Then
      blnCheck = False
    End If
  End If

ProcedureExit:

Exit Sub

TargetHandler:
  MsgBox "blnTarget has to be a boolean."
  GoTo ProcedureExit

End Sub