The Science Behind Veterinarian-Administered Vaccines

Modern, commercially produced, government-approved vaccines are generally safe and effective, but occasionally I have a client ask why their vaccinated horse still developed the disease they were hoping to prevent.

To understand why a vaccine may fail to protect, it’s important to first understand how vaccines work.

How Vaccines Work

When germs—bacterial or viral—invade the horse’s body, they attack and multiply. This invasion is what causes the horse to become sick.  Vaccines work with the horse’s natural defenses to safely develop immunity against specific diseases by imitating an infection. This infection doesn’t cause illness; rather, it causes the horse’s immune system to react.

The Natural Response to Bacterial and Viral Invasions

The horse’s immune system uses several tools to fight infection. Like humans’, a horse’s blood contains red blood cells, which carry oxygen to tissues and organs; and white blood cells, which fight infection.

White blood cells are primarily comprised of three infection-fighting tools:

  • Macrophages – swallow and digest germs and dead or dying cells, leaving behind antigens
  • B-lymphocytes – respond to the antigens by producing antibodies that attack and kill the disease-causing germs
  • T-lymphocytes – stimulated by the antigens to attack and destroy cells that have been attacked by the invading bacteria or virus

The first time the horse’s body encounters a germ, it can take several days to make and use all the infection-fighting tools needed to overcome the infection.  However, after the infection, the immune system creates and keeps a few unique T-lymphocytes, called memory cells, that stimulate a much quicker response if that same germ attempts to infect the horse again.

The Role of Vaccines in Immunity

Vaccines imitate an infection that doesn’t cause the disease but does stimulate the horse’s body to produce T-lymphocytes and antibodies against bacteria or viruses that produce that infection. This leads to the body’s production of memory cells (T-lymphocytes) that will remember the infectious agent introduced by the vaccine and be prepared to recruit B- and T-lymphocytes to fight that agent if it infects the horse in the future.

The Exception

It’s important to remember that the immune system takes several days or weeks after vaccination against an infectious agent to produce enough white blood cells to fight off the infection. Therefore, it’s possible that a horse infected with the disease just prior to, or immediately after, vaccination can still develop symptoms of the disease because the vaccine has not had adequate time to provide protection. This is often seen when horses are vaccinated only a few days before leaving for an event where they encounter horses that are shedding germs.

Why Veterinarian-Administered Vaccines are Important

Commercially available vaccines are regulated by the federal government and must meet rigid standards for stability, effectiveness and safety. If handled and administered properly, they are seldom the reason for vaccine failure.

Storage & Damage

Proper storage of vaccines is one of the most critical aspects in assuring they will provide the desired disease protection. The label recommendations for storage of vaccines read as follows:  Store in dark at 35 to 45 F.  Avoid freezing.  Shake well to assure uniform suspension of the vaccine prior to administration. Lack of adherence to the label directions can result in lack of vaccine effectiveness, vaccine failure and an increased rate of local reactions after vaccine administration.

Damage to the vaccine is most often due to exposure to heat, light or freezing. Common causes include a faulty refrigerator that is either not cool enough or is too cold and freezes the vaccine, or the vaccine being left at room temperature for an extended period of time.

Your veterinarian will be familiar with these risks and recommendations and have a system in place to ensure the vaccine is stored properly and delivered undamaged.

Proper Dosing

If you are vaccinating a foal or horse for the first time, it is critical that they receive a second dose three to four weeks after the initial dose to build more complete immunity.  Failure to do so dramatically diminishes the horse’s immune response and is a common cause of vaccine failure.

For most vaccines, the horse’s immunity to disease will gradually decrease over time so boosters are usually recommended at least annually.  In the case of influenza and rhinovirus (Equine Herpes 1 & 4), vaccination every six months is usually recommended in order to maintain a solid immune response.

Work with your veterinarian to develop a proper vaccination schedule for your horse.


With the exception of intranasal influenza and strangles vaccines, most vaccines should be administered deep in the muscle. Subcutaneous administration (just under the skin) can decrease their effectiveness and the horse’s response.

In the case of live, attenuated vaccines, their use concurrently with antibiotics, chemical sterilization or reuse of syringes, as well as excessive use of alcohol on the skin, can decrease their effectiveness.

Veterinarians are well-versed in proper administration techniques and possess the knowledge of any potential contraindications.

Maternal Antibodies

Foals are born with a limited ability to mount an immune response to disease during the first few months of life.  To protect them from disease, they depend on antibodies in the mare’s colostrum (first milk) that they ingest during the first few hours of life. The maternal antibodies will gradually diminish over time as the foal’s immune system matures. Vaccines administered while maternal antibodies are still high are blocked by the antibodies.

To prevent maternal antibody block and ensure a good immune response, consult your veterinarian about the proper time to start your foal’s vaccination programs.

Preexisting Conditions

Heavily parasitized or malnourished horses may not be able to mount a good immune response when vaccinated. Stress, including extreme cold or heat, can also decrease a horse’s response to vaccination. In addition, geriatric horses, especially those suffering Cushing’s Disease, may have a decreased ability to respond to vaccination.

It is important to note that vaccination alone, in the absence of good nutrition and management practices directed at infection control, are not sufficient in preventing infectious disease.

Your veterinarian will be able to assess the horse’s condition and determine a proper care plan for overall wellness, including a timely vaccination schedule.

In cases where the horse’s immunity, gained through vaccination, doesn’t completely protect the animal, the vaccine reduces the severity of the clinical signs and duration of illness.

Vaccines are safest and most effective when administered by your veterinarian. Your horse’s vaccination schedule should be tailored specifically to its needs and location, so ask your veterinarian to help you develop a vaccination program to best fit your horse. For additional information on vaccinating your horse, visit the AAEP Vaccination Guidelines.

By Tom Lenz, DVM, MS, DACT