@Adhina1 I highly recommend continuing with the duvet covers.
My experience over 15 years is that people will sleep on top of the bed, under the duvet but not within the sheets, within the sheets, or directly on the mattress cover. And they are not just sleeping...
We have 3 sets of linens, pillow protectors, duvet covers and duvet inserts for each bed. The linens, pillow protectors, duvet covers and duvets are washed after EACH guest. If the bed looks like there has been an adventure in it, the mattress cover goes as well. If I did not allow enough time to do the laundry onsite, I (or my housekeeper) take the linens home to wash.
I recently stayed at a hotel. The duvet on the bed did not have a cover on it. I called housekeeping and asked that the entire bed be made up with fresh linens and a washed duvet with a cover. As I was staying for a week, I did not ask housekeeping to change the bed again until I left, but I WAS NOT going to sleep on suspect bedding.