 [ Symptoms ] my 12 yr old has chicken pox vaccinated at 2 at 4 then refused booster shots will he infect his 20 month old sister [ Disease ] the vaccine is not 100 protective so clearly your 12 year old was not adequately protected yes it is possible that he may infect his 20 month old sister since she has probably just had one vaccine but i suspect she will be fine the incubation period is about two three weeks so you will find out at that time