Can a vaccine or antidote be administrated via gases or sprays?
There are a number of currently used aerosolized vaccines throughout the world. Generally, these are studied and administered in single individual doses. There are advocates for the use of larger, sealed exposure chambers for rapid administration of vaccines to large numbers of people, possibly in the field, for example in a tent like this figure from the earlier linked reference.



The primary barriers for a general release of a vaccine or other medication as, e.g., an aerosolized spray, would be ethical (difficult to get consent from all potential exposed individuals) and dose related (difficult to ensure a specific safe and effective dose).  

The closest natural experiment I'm aware of would be the use of live attenuated Sabin oral poliovirus vaccine. Unlike other currently used vaccines, the Sabin vaccine is transmissible. In developing countries, vaccine is transmitted from vaccinated individuals to un-vaccinated individuals through the fecal contamination of food and water, using the same route that wild type poliovirus is transmitted. This can result in extending immunity beyond those who directly receive the vaccine. You could conceptualize this as blanketing a local population with vaccine. It's not without problems, though, because in cases where the overall initial vaccine coverage is low, circulation of the vaccine can eventually result in reversion of the reservoir of previously non-virulent (attenuated) vaccine to a virulent type. This is how you get vaccine derived polio outbreaks, and one of the reason not to use transmissible viruses as vaccines.