Valacyclovir is a brand name of acyclovir, an anti-viral med used to treat a few forms of herpes virus such as shingles and lip sores. I did use it to treat two of my pullets, deciding it might be worth a shot since Marek's is a herpes virus. I can't say it worked or didn't because I was also using three different antibiotics one right after the other. Who knows what effect each had? Some folks tell their doctor they have lip sores and need a prescription for acyclovir. I keep a supply on hand since I get shingles as well as lip sores from time to time.
There are different strains of Marek's and I imagine the symptoms could be different as well. It might account for why some chickens recover from the symptoms and others die. Marek's virus isn't just in the soil but can be transmitted on air currents, so it could reach your chickens if the weather conditions are right.