Haunted--I'm sure it was a very difficult time; I recently lost about 20 birds to coccidia.
I looked at the vial of vaccine that I have and the Series 3 is the only listing. I know flu shots for humans can vary as they try to determine the most likely strains of the flu to include.So I can beleive there are other strains of merecks for vaccinating chickens.
Modified live vacicines come in different forms. ANd storage time varies. It depends on the vaccine. Some can be multi dose bottles like I had EVA for my horses that was a vial of 10 doses that could be kept refrigerated, using one dose at a time. Or individual doses like the typical DHLPP vaccines for dogs are individual vials, mixing sterile water into the vial of dried and pulling up ONE dose per dog. Some vaccines are more fragile than others. For the best effect of the vaccine, follow the manufacturers instructions, they did the studies. As for the mereks vaccine specifically, the mixed vaccine must be kept at 70-80 degrees for no more than an hour. Peter Brown says you can extend that time to 2-2.5 hours IF the temperature is reduced by putting jar in a cold water bath.
As for children and vaccinations, don't get me started. I made the apptments and dealt with scared and crying kids to make sure they got there vaccinations. I don't want them exposed to the risk of those children that are not vaccinated--they put my kids at risk. SHesh. Simple. Get vaccinated. Massachusetts has a free vaccine program so there is NO excuse financially to not get vaccinated. School rules mandate it as far as I know here in Mass.; Just got a notice requiring verification of physical and vaccinations. When everyone is vacinated we can just about wipe out a number of diseases. Polio is one. Unfortuantely the primates other than humans are the primary source of polio; when they are in close contact to unvaccinated folks, there is an out break. Vaccination keeps polio a thing of the past.