Is it necessary to protect as well, the duvet and/or the pillows with a 

 dust mite proof cover? 

The use of anti-dust mite covers for the pillows and duvets is based on the following rule:

1° you can wash your duvet and your pillows, regularly every three weeks, approximatively at more than 55°C:

In this case, it is not necessary to use anti-dust mite covers for these duvet and pillows that you wash regularly.

2° you cannot wash your duvet and your pillows, regularly every three weeks, approximatively at more than 55°C:

In this case, it is necessary to enclose the duvets and pillows that you cannot wash at 55°C, in anti-dust mite covers such as the ProtecSom's ones.