I am entering all of my hens into the state fair. I want to vaccinate them for marek's. This vaccine https://www.valleyvet.com/ct_detail...PrX0CShoGDefftVFNuslOWgERxoC0hTw_wcB#tab-info says only to vaccinate 1 day old chicks only and I got my birsa this spring. Can I still vaccinate them? My flock is 7 hens only. What syringes should I use? How long does teh vaccine last after being opened because I might offer to vaccinate my neighbors chickens.