Mine are just taken off the dark, no gradual lengthening of daylight hours. I confine them to the loft for 2-3 days to adjust and take this opportunity to change their feeding times.
My Guess is that the vaccination you gave them knocked them right back and something else has taken advantage of their weakened state. Your only course of action is to get either a dead youngster or one thats showing symptoms to a vet asap
3 or 4 from 20 miles then onto 45 miles, have done this for the last 20+ years. I always start them of on nice days with broken cloud, I avoid days when its wall to wall sunshine.