Influenza is among the most common upper respiratory diseases of horses. Foals are not usually clinically infected with influenza because the dam's colostrum provides the newborn with protective maternal antibodies (assuming the colostrum has antibodies to influenza).
According to research and studies cited by Dr. Robert E. Holland, Jr. and Dr. Thomas M. Chambers, at the Maxwell H. Gluck Equine Research Center, the maternal antibodies decay as foals get older and they become susceptible to influenza, especially as yearlings and two-year-olds.
Veterinary practice has been to begin an immunization schedule for equine influenza between two and six months of age. However, researchers in Europe have questioned the effectiveness of this strategy due to possible interference from maternal antibodies.
Recent research gauged the effectiveness of influenza vaccination starting at different ages of the foal to determine any maternal antibody interference. This information is needed by practitioners to determine an effective influenza program in foals.
By giving a number of foals influenza vaccine at various ages over a period of time, researchers found that the vaccine was ineffective in younger foals that continued to have maternal antibodies derived from their dams in their system. This is the first direct evidence that maternal antibodies are responsible for the failure of foal vaccinations.
Since virtually every dam in the US has prior exposure to influenza, thereby having some level of maternal antibody to it, it is expected that in most foals, the influenza vaccination is relarively ineffective.
At present the only known solution is to wait until foals are old enough, i.e. weanlings of at least 8 months age, before beginning a vaccination program for influenza.
This information should not be extrapolated to other vaccinations until specific studies are completed to determine the influence of maternal antibody on specific vaccine antigens.