For the small flock owner, vaccination is generally necessary only if the birds have had disease problems in the past, may possibly be exposed to other birds (eg, at poultry shows, meat swaps, or wild bird access), or if new birds are introduced to the flock (open flock). Birds should not be vaccinated for a disease not present in their local area, because this will only introduce new organisms into the flock. Also, a sick bird’s immune system is compromised and unable to withstand the stress of vaccination.
If certain diseases are a problem in a backyard flock, vaccination may be recommended after veterinary consultation. Marek's disease Marek's Disease in Poultry Marek disease is a highly contagious viral disease of poultry characterized by T-cell lymphomas and peripheral nerve enlargement. Standard criteria used for diagnosis include history, clinical... read more 
is present in almost every flock, and vaccination of chickens is strongly recommended in all cases; vaccination is key for control and is inexpensive. Backyard poultry owners may purchase chicks from hatcheries and request their chicks be vaccinated at hatch with serotype 3, or they can vaccinate their own chicks if hatched onsite. Because the virus is ubiquitous and spreads through feather dander, vaccinating birds at hatch before they are most susceptible (2–7 months) is critical to establish early immunity. There are three serotypes of Marek disease: 1, 2, and 3. Because most backyard chickens are vaccinated only for serotype 3, they may not be fully protected. In addition, vaccination does not prevent infection or shedding of the field virus.
Newcastle disease Newcastle Disease in Poultry Newcastle disease is a severe, systemic, and fatal viral disease of poultry due to virulent strains of avian paramyxovirus type 1. Clinical signs in unvaccinated birds include sudden death,... read more 
virus vaccination may be recommended in certain geographic areas and in birds that frequent shows and exhibitions. However, commercial vaccines are only sold at large quantities in lots of 10,000 dose vials, which presents a challenge for small flock owners that only require small amounts (eg, less than 100). In addition, some live vaccines are only approved for administration by approval of the state veterinarian, and regulations differ by state. If certain vaccines (eg, infectious bronchitis virus Infectious Bronchitis in Poultry Infectious bronchitis is an acute, highly contagious upper respiratory tract disease in chickens. In addition to respiratory signs, decreased egg production and egg quality are common, and nephritis... read more 
, infectious bursal disease Infectious Bursal Disease in Poultry Infectious bursal disease (IBD) is seen in young domestic chickens worldwide and is caused by infectious bursal disease virus (IBDV). Signs can include depression, watery diarrhea, ruffled feathers... read more 
, etc) that might be a challenge for accessibility are desired for specific needs, it may be best to tailor those needs and receive vaccinated chicks directly from the hatchery.
