Can I safely cover the wall vents in my living room?

I have two vents in my small ground floor flat, located near the bottom of the walls.they are basically 6 inch holes in the wall and it gets quite drafty. I have a gas fire in one room which is turned off and not used.could these vents be covered safely if I don't use the fire and have a carbon-monoxide detector and smoke alarm. also have a gas combi boiler in the adjoining kitchen which vents directly outside