How do you handle a ComboBox SelectionChanged in MVVM?

For those doing pure MVVM, how do you handle a ComboBox SelectionChanged event without reverting to code behind?

I tried e.g. AttachedBehaviors but Event="SelectedChanged" is not supported:

<ComboBox>
    <ComboBoxItem Content="Test1">
        <c:CommandBehaviorCollection.Behaviors>
            <c:BehaviorBinding Event="SelectionChanged" 
                               Command="{Binding SelectedChanged}"
                               CommandParameter="MainBorder123"/>
        </c:CommandBehaviorCollection.Behaviors>
    </ComboBoxItem>
    <ComboBoxItem Content="Test2"/>
    <ComboBoxItem Content="Test3"/>
</ComboBox>