I really am against throwing antibiotics at chickens when you hear a sneeze. You have no idea what you are treating, if anything at all. Sneezing can be environmental and the death may not be related to that. Antibiotics do NOTHING at all for anything viral and only create antibiotic resistance. You will weaken your entire flock by treating respiratory illness. Plus, you have to know that many viruses they get are herpes type...yes, they are permanent, even if the bird seems to recover. We have to stop this insane antibiotic party or we will create lots of super-germs. I will never treat respiratory illness in my flock if it ever happens. I want birds with healthy immune systems, not ones that come down with some crud every three or four months. This is the time to think about these things and what you really want out of your flock. Isolate, observe, provide supportive care, but don't treat with antibiotics, is my recommendation.
And VetRx is not a medication, per se. It's Vick VapoRub for birds, is all.
Be sure they are in a draft free place. My six week olds are in a coop with high ceilings and lots of ventilation, but no drafts at their level. Yours should be okay at this age.
Chickens do not get the common head cold like humans do.