While it's impossible to prove that PREbiotics can help protect against Covid infection, this study certainly suggests that it is possible, with the authors stating:

"Prebiotics may also have an excellent potential effect against COVID-19 by enhancing probiotics growth and survivability. "

This certainly fits with many other studies suggesting PREbiotics can improve our immune response to harmful pathogens.

The study can be found here:

The potential application of probiotics and prebiotics for the prevention and treatment of COVID-19 | npj Science of Food (nature.com)