I think you're borrowing trouble from tomorrow. The vaccination is not difficult. First prepare everything you need within hand's reach and practice with your loaded siring delivering 1 dose into the sink to know how hard the plunger of your siring moves. Take your time and have an assistant...
I recommend vaccinating the chicks and putting them back in under momma. Also do Fowl Pox, New Castle, and Infectious Bronchitis. Do the entire flock for the last 3 at the appropriate time for the chicks. That will put you on track for a fully vaccinated flock. Per Dr Waters, head of the...
Since my original posting I have learn from the Dept of Ag and the Univ of Ill this:
Vaccinate ASAP all birds.
Mereks is best given to day olds but will be effective to lessening degrees on bird up to 6 weeks.
Edited because I can't spell sometimes