When Should Foals Be Vaccinated? First-Year Schedule and Booster Timing

By Dr Duncan Houston

Deciding when to vaccinate a foal is more complicated than choosing a single age on the calendar.

The correct timing depends on the mare’s vaccination history, whether the foal received and absorbed enough colostrum, which diseases occur locally, the foal’s likely exposure and the instructions for the specific vaccine being used.

For most healthy foals born to appropriately vaccinated mares, many first-year vaccines begin at around four to six months of age. However, selected vaccines may be started earlier in foals born to unvaccinated mares, foals with inadequate passive transfer or foals facing an unusually high risk of exposure.

Quick Answer

Most healthy foals born to appropriately vaccinated mares and with adequate colostral antibody transfer begin many vaccines between four and six months of age.

Selected vaccines may be started at three to four months in foals from unvaccinated or inadequately vaccinated mares, or when exposure risk is unusually high. Starting early does not mean that one injection is enough. Foals usually need a complete primary series, often including a second dose four to six weeks later and a further dose at approximately 10 to 12 months of age. (AAEP)

There is no single worldwide foal vaccination schedule. Vaccine availability, disease risks and product instructions differ considerably between countries. (AAEP)

Why Does the Mare’s Vaccination Status Matter?

Foals are immunocompetent, meaning their immune system can respond to infection and vaccination, but they are born immunologically naive and essentially without useful circulating immunoglobulin. Their immediate protection comes primarily from antibodies absorbed from the mare’s first milk, known as colostrum. (Merck Veterinary Manual)

Vaccinating a broodmare against appropriate diseases approximately four to six weeks before foaling can increase the concentration of specific antibodies in her colostrum. This helps protect the foal during the first few months of life, before the foal has completed an active vaccination series. (AAEP)

However, a vaccinated mare does not automatically mean a protected foal.

Protection still depends on:

• The mare producing good-quality colostrum

• The foal standing and nursing promptly

• The foal consuming enough colostrum

• The foal successfully absorbing the antibodies

• The mare having immunity against the specific disease in question

Premature lactation, poor-quality colostrum, a weak foal, maternal rejection, prematurity, difficult birth and delayed nursing can all compromise passive transfer. (Merck Veterinary Manual)

When Should a Newborn Foal Have an IgG Test?

A blood IgG test is commonly performed as part of the newborn foal examination, usually at approximately 12 to 24 hours of age.

Testing at six to 12 hours may be appropriate when the foal is high risk because it leaves more time for oral colostrum supplementation while the intestine can still absorb large antibody molecules. By approximately 18 to 24 hours, oral colostrum becomes substantially less effective, and foals with inadequate passive transfer may require intravenous equine plasma. (Merck Veterinary Manual)

A practical interpretation is:

Foal IgG result	Interpretation	Typical response
Above 800 mg/dL	Adequate passive transfer	Usually no antibody supplementation is required in a healthy, low-risk foal
400 to 800 mg/dL	Partial failure of passive transfer	Assess the foal’s health, environment and infection risk; supplementation may be advised
Below 400 mg/dL	Failure of passive transfer	Prompt veterinary treatment is generally required
Below 200 mg/dL	Profound or complete failure	High infection risk and urgent veterinary management

These figures must be interpreted alongside the foal’s clinical condition. A weak, premature or unwell foal with an IgG of 600 mg/dL is more concerning than a bright, vigorous foal with the same result on a very clean, well-managed property. (Merck Veterinary Manual)

Vaccination Does Not Treat Failure of Passive Transfer

This is one of the most important distinctions for owners to understand.

A vaccine stimulates the foal to develop its own active immune response over time. It does not immediately replace missing colostral antibodies.

A newborn foal with low IgG cannot simply be vaccinated and considered protected. Failure of passive transfer requires immediate assessment and, where indicated, colostrum or plasma treatment. The foal’s active vaccination program is planned later. (Merck Veterinary Manual)

What Is Maternal Antibody Interference?

The antibodies a foal receives through colostrum are extremely valuable, but they create a vaccination dilemma.

Maternal antibodies can recognise and bind to vaccine antigens before the foal’s immune system develops a strong response of its own. This is known as maternal antibody interference.

The result is that a vaccine administered too early may produce:

• A weak antibody response

• An incomplete immune response

• Shorter-lasting immunity

• The appearance of vaccination without reliable protection

This is why many conventional foal vaccination programs begin at four to six months rather than during the neonatal period. Multiple doses are then used to improve the likelihood that the foal develops active immunity as maternal antibodies decline. (AAEP)

Unfortunately, this can also create an immunity gap. Maternal antibodies may have fallen below reliably protective levels while still interfering with vaccination. During this period, biosecurity and exposure control remain critical. (AAEP)

Does Research Support Vaccinating Foals at Three Months?

A frequently referenced Kansas State University study compared 12 healthy foals that began a multivalent three-dose vaccination program at either 90 or 180 days of age. Both groups received an additional booster at approximately 11 months.

The researchers found evidence of cellular immune activation in both groups and demonstrated memory responses following the later booster. This showed that three-month-old foals can mount measurable immune responses despite maternal antibodies. (PubMed)

However, the study does not prove that every foal should routinely begin every vaccine at three months.

Important limitations include:

• Only 12 foals were studied

• Immune markers were measured rather than clinical disease prevention

• The foals were not challenged with each disease to confirm protection

• The researchers specifically stated that immune activation does not automatically equal protection

• The foals received a complete three-dose series and an 11-month booster, not a single early injection

The practical interpretation is that vaccination beginning at three months may be reasonable in selected high-risk situations. It should not be treated as a universal replacement for current disease-specific guidelines, many of which still recommend beginning at four to six months. (PubMed)

How Does the Mare’s Status Change the Foal’s Schedule?

Foal From an Appropriately Vaccinated Mare

When the mare received appropriate pre-foaling boosters and the foal has adequate passive transfer, many vaccines begin at approximately four to six months.

Waiting helps reduce maternal antibody interference, but the foal still needs the complete primary series.

Foal From an Unvaccinated Mare

Selected vaccines may be started at three to four months because the foal is less likely to have high concentrations of disease-specific maternal antibodies.

Earlier vaccination does not always apply to every vaccine. The schedule still depends on the product, disease and local risk.

Foal With Unknown Mare History

Treat the vaccination history as uncertain until records can be confirmed. An individual schedule should be created based on the foal’s IgG result, disease exposure and the specific vaccine product.

Foal With Failure of Passive Transfer

Treat the passive transfer problem first. Vaccination is not an emergency source of immunity.

Once the foal is stable, its vaccination program may begin earlier for selected diseases, but the timing should be directed by the attending veterinarian.

High-Risk Foal

Earlier vaccination may be considered when there is:

• Early mosquito exposure

• A significant local disease threat

• Planned movement to a breeding, training or sales facility

• Frequent introduction of new horses

• Poor passive transfer

• An unvaccinated mare

• A recognised farm-specific disease history

The increased exposure risk must be weighed against maternal interference and the limitations of early vaccination.

Which Vaccines Are Core for Foals in the United States?

Under American Association of Equine Practitioners guidance, the core equine vaccines in the United States are:

• Tetanus

• Eastern and Western equine encephalomyelitis

• West Nile virus

• Rabies

Equine herpesvirus, equine influenza, strangles, botulism, Potomac horse fever and several other vaccines are classified as risk-based rather than core. That does not mean they are unimportant. It means their use depends on geography, age, property type and exposure. (AAEP)

Typical United States Foal Vaccination Schedule

This table provides a practical overview, not a replacement for the attending veterinarian or the individual product label.

Vaccine	Foal from vaccinated mare	Foal from unvaccinated or unknown mare	Important notes
Tetanus	Begin at 4 to 6 months; second dose 4 to 6 weeks later; third at 10 to 12 months	Begin at 3 to 4 months; second dose 4 to 6 weeks later; third at 10 to 12 months	Tetanus exposure can occur through the umbilicus, wounds and surgery
EEE/WEE	Begin at 4 to 6 months; second dose 4 to 6 weeks later; third at 10 to 12 months	Usually the same, although earlier vaccination may be considered in high-risk circumstances	Time the series before mosquito season where possible
West Nile virus	Three-dose series beginning at 4 to 6 months; third dose at 10 to 12 months	May begin at 3 to 4 months with a complete three-dose series	Exact spacing varies with product and mosquito season
Rabies	First dose at 4 to 6 months, followed by a second dose 4 to 6 weeks later	One dose at 4 to 6 months, subject to product instructions	The additional dose in foals from vaccinated mares addresses maternal antibody interference
EHV-1/EHV-4	Three-dose series beginning at 4 to 6 months; second dose approximately four weeks later; third at 10 to 12 months	Individual risk-based schedule	Risk-based but commonly considered in young horses and breeding populations
Equine influenza	Product and risk dependent; often delayed because of maternal antibody interference	Some injectable products may begin at 4 to 6 months	Particularly relevant for travelling, showing, sales and training horses
Strangles	Product and risk dependent	Product and risk dependent	Vaccination must be combined with quarantine and biosecurity

The tetanus, West Nile, rabies and encephalomyelitis recommendations differ slightly because maternal interference, vaccine technology and available evidence are not identical for every disease. (AAEP)

Equine Herpesvirus Vaccination

Current AAEP guidance recommends a three-dose primary EHV series beginning at four to six months, with approximately four weeks between the first two doses and a third dose at 10 to 12 months.

EHV vaccination can reduce respiratory disease, nasal shedding and viraemia, but it does not guarantee protection. There is currently no licensed vaccine labelled to prevent the neurological form of EHV-1, known as equine herpesvirus myeloencephalopathy.

Equine Influenza Vaccination

Influenza vaccination is particularly relevant for foals that will enter training, attend sales, travel, compete or live on properties with regular horse movement.

The starting age and number of primary doses vary with the product. Injectable vaccines commonly require two or three doses, while intranasal products have separate age restrictions and protocols. The product label and veterinarian’s recommendation should therefore be followed rather than assuming every influenza vaccine uses the same schedule. (AAEP)

Strangles Vaccination

Strangles vaccination is risk-based. It may be useful on breeding farms, boarding properties and facilities with regular horse movement, but it cannot replace quarantine, testing and carrier control.

Vaccinating a foal simply because an outbreak has already begun is not automatically appropriate. The choice of vaccine, exposure history and stage of the outbreak must be considered by the attending veterinarian.

What Vaccinations Do Foals Need in Australia?

Australian owners should not copy an American foal vaccination chart.

Australia remains free from rabies, equine influenza and Eastern and Western equine encephalomyelitis. Those vaccines are therefore not routinely included in domestic Australian foal programs. (Micor)

Australian vaccination plans commonly consider the following.

Tetanus

Australian tetanus vaccine labelling allows primary vaccination from three months of age.

For a commonly used tetanus-only product, the primary course is:

1. First dose from three months

2. Second dose four to six weeks later

3. Booster 12 months later

4. Annual boosters thereafter

Tetanus protection is important even for foals that never travel because exposure comes from environmental contamination of wounds rather than horse-to-horse transmission. (Zoetis)

Strangles

A commonly used Australian strangles vaccine can begin from three months of age.

The primary course consists of three doses administered at intervals of at least two weeks. The first booster is given six to 12 months after the primary course, followed by boosters every six or 12 months depending on exposure risk. (Zoetis Australia)

Hendra Virus

The Hendra vaccine is registered for horses four months of age and older. However, foals born to vaccinated mares are directed to begin vaccination at six months.

The primary course consists of:

1. First vaccination

2. Second vaccination three to six weeks later

3. Third vaccination six months after the second

4. Annual boosters thereafter

Hendra vaccination must be administered by a veterinarian and recorded in the central registry. The Australian Veterinary Association considers horse vaccination the most effective way to manage Hendra virus risk and reduce potential transmission to people. (Zoetis)

EHV-1 and EHV-4

The Australian Duvaxyn EHV-1,4 product is generally recommended from five months of age, followed by a second dose four to six weeks later.

When there is insufficient colostrum or a substantial risk of early EHV exposure, an additional early dose may be given from three months. The foal must still receive the full primary course beginning at five months. Six-monthly boosters are recommended following completion of the course. (Zoetis)

Salmonella

On properties where Salmonella risk justifies vaccination, the Australian Equivac EST vaccine may be started from four months.

Foals receive two doses four weeks apart, followed by a booster six months after the second dose and then annual boosters. This is a risk-based program rather than a routine requirement for every foal. (Zoetis)

Rotavirus

Rotavirus vaccination is generally directed at the pregnant mare rather than the newborn foal.

The Australian product is administered to previously unvaccinated mares during the eighth, ninth and tenth months of pregnancy. This increases antibodies in colostrum and milk, helping reduce rotavirus diarrhoea risk in the foal. (Zoetis)

A Practical First-Year Foal Timeline

Birth to 24 Hours

Focus on passive immunity, not routine vaccination.

• Confirm that the foal stands and nurses

• Assess colostrum intake and quality

• Arrange the newborn examination

• Test IgG at an appropriate time

• Treat inadequate passive transfer promptly

Three to Four Months

Earlier vaccination may be appropriate for selected foals, including those from unvaccinated mares, those with poor passive transfer or those facing significant early exposure.

The exact vaccines depend on the country and product.

Four to Six Months

This is the most common starting period for many foal vaccines.

Depending on location and risk, the first doses may include tetanus, mosquito-borne vaccines, rabies, EHV or other property-specific vaccines.

Four to Six Weeks After the First Dose

Administer the required second doses.

Missing this appointment is one of the most common reasons a foal remains inadequately protected despite having “been vaccinated.”

Six to Nine Months

Introduce or continue risk-based vaccines such as influenza, strangles or region-specific products where appropriate.

This period often overlaps with weaning, transport, sales preparation and movement into larger groups, all of which may increase exposure.

Ten to Twelve Months

Administer third doses where required and confirm that the primary series has been completed.

Many foals require a third dose at this age even if they received two earlier injections. The third vaccination is not an optional extra. It is part of the primary immunisation program for several diseases.

How Urgent Is Your Foal’s Situation?

Lower Risk

The foal is healthy, the mare was appropriately vaccinated, passive transfer was adequate and the foal lives on a closed property.

Action: Arrange the routine four-to-six-month vaccination plan.

Moderate Risk

The mare’s vaccination timing was incomplete, the property has regular horse movement or the foal will soon be weaned, sold or transported.

Action: Review the schedule with your veterinarian before the exposure occurs. Do not wait until the day before transport.

High Risk

The mare was unvaccinated, records are unavailable, IgG was low, the foal is entering mosquito season or a relevant infectious disease is circulating locally.

Action: Arrange a prompt veterinary risk assessment. Selected vaccines may need to begin earlier or use an adjusted primary series.

Critical

The newborn foal has not nursed, is weak, has low IgG, is premature or is showing signs of infection.

Action: This is not primarily a vaccination problem. The foal requires urgent veterinary examination and possible colostrum, plasma, diagnostics and treatment.

When Is It an Emergency?

Contact a veterinarian urgently when a newborn foal:

• Has not stood or nursed successfully within the expected first few hours

• Is weak, dull or repeatedly recumbent

• Has a poor suckle reflex

• Has a rectal temperature that is abnormally high or low

• Develops diarrhoea

• Has rapid or difficult breathing

• Develops a swollen or painful joint

• Has an enlarged, wet or painful umbilicus

• Has an IgG result below 400 mg/dL

• Has an IgG between 400 and 800 mg/dL and is unwell or high risk

Weak or sick foals that fail to stand or nurse normally within two to four hours require early intervention. Delaying until the next morning can allow the window for effective oral colostrum absorption to close. (Merck Veterinary Manual)

Should a Sick Foal Be Vaccinated?

Routine vaccination is intended for healthy animals.

Vaccination may need to be postponed if the foal has:

• Fever

• Diarrhoea

• Respiratory disease

• Suspected sepsis

• Marked lethargy

• Reduced nursing

• An active inflammatory or immune-mediated condition

• Treatment with immunosuppressive medication

The urgency of vaccination must be balanced against the foal’s current health and exposure risk. Vaccinating an unwell foal will not treat the existing disease and may produce an inadequate immune response. (AAEP)

What Reactions Are Normal After Vaccination?

Mild, temporary reactions can include:

• Soreness at the injection site

• A small local swelling

• Mild stiffness

• Reduced appetite

• Temporary lethargy

• A low-grade fever

These signs should remain mild and begin improving rather than becoming progressively worse.

When Is a Vaccine Reaction an Emergency?

Seek immediate veterinary attention if the foal develops:

• Difficulty breathing

• Collapse

• Facial or throat swelling

• Widespread hives

• Severe colic

• Rapidly increasing injection-site swelling

• Marked weakness

• Neurological abnormalities

• A high or persistent fever

Severe systemic reactions such as anaphylaxis are uncommon but can be life-threatening. This is one reason vaccines are best administered by, or under the direct supervision of, a veterinarian. (AAEP)

What Should You Do Next?

1. Find the Mare’s Records

Confirm the actual vaccine names and dates. “She was vaccinated sometime during pregnancy” is not enough to determine the foal’s likely maternal antibody status.

2. Arrange the Newborn Examination

Passive transfer, congenital abnormalities, the umbilicus, eyes, heart, lungs, joints and general neonatal adaptation should be assessed.

3. Record the Foal’s IgG Result

Keep the numerical result rather than simply writing “passed.” The actual concentration may influence decisions when the foal becomes unwell.

4. Assess the Foal’s Exposure

Tell your veterinarian about:

• Planned weaning

• Sales or transport

• Shows or competitions

• Boarding or training

• New horses arriving

• Mosquito season

• Breeding-farm exposure

• Previous disease on the property

5. Create a Written Schedule

Record every required dose, including the second and third injections. A vaccine course that is started but not completed may leave unreliable protection.

6. Complete Vaccination Before Exposure

Where possible, the primary course and relevant boosters should be completed at least 14 days before likely exposure. Active immunity is not immediate. (AAEP)

Common Foal Vaccination Mistakes

Assuming the Mare’s Vaccination Protects the Foal Automatically

The mare may have been vaccinated too early, missed a relevant vaccine or produced inadequate colostrum. The foal may also have failed to absorb it.

Using Vaccination Instead of an IgG Test

Vaccination cannot identify or correct failure of passive transfer.

Starting Every Vaccine at Three Months Because of One Study

The three-month study demonstrated immune activation, not guaranteed protection against every included disease. Current schedules remain vaccine and risk specific.

Forgetting the Second or Third Dose

One injection is frequently only the beginning of the primary course.

Copying an American Schedule in Australia

Disease status and vaccine availability differ. Vaccinating against diseases that are not present domestically while missing tetanus, Hendra or property-specific risks is the vaccination equivalent of bringing an umbrella to a bushfire.

Vaccinating Immediately Before Weaning or Transport

The foal may not have developed protection and could also become temporarily sore or lethargic during an already stressful event.

Treating Vaccination as a Replacement for Biosecurity

Vaccines reduce risk, but they do not guarantee that a foal cannot become infected or spread disease.

Protecting the Foal While Immunity Develops

The period between declining maternal antibodies and completion of active vaccination requires careful management.

Reduce exposure by:

• Quarantining new arrivals

• Avoiding shared buckets and grooming equipment

• Separating horses with fever or respiratory signs

• Monitoring temperatures following travel or exposure

• Cleaning trailers between horses

• Avoiding unnecessary movement during outbreaks

• Maintaining separate equipment for mare-and-foal groups

• Completing vaccines before weaning, sales or transport where possible

Vaccination works best as one part of a prevention program. It cannot replace isolation, hygiene, movement control and early recognition of illness. (AAEP)

Frequently Asked Questions

Can a three-month-old foal be vaccinated?

Yes, selected vaccines may be given from three months when product labelling allows it or the foal has increased risk. This may apply to foals from unvaccinated mares, foals with poor passive transfer or foals facing early exposure. It should not automatically be applied to every vaccine.

Does a foal need vaccines if the mare was vaccinated?

Yes. Maternal antibodies are temporary. The foal still needs to develop active immunity through its own complete vaccination series.

What happens if the mare was not vaccinated before foaling?

The foal should have passive transfer assessed and may need selected vaccines started earlier. Vaccinating the newborn immediately does not replace the missing maternal antibodies.

How many vaccinations does a foal need in the first year?

There is no single number. Several vaccines require two or three primary doses, while others require only one initial dose. The number depends on the country, vaccine product, mare’s status and the foal’s exposure.

Is the 11-month booster always required?

Many first-year schedules include a third dose at approximately 10 to 12 months, particularly for tetanus, West Nile virus, encephalomyelitis and EHV. Other vaccines use different schedules, so the product-specific plan must be followed.

Can a foal travel after its first vaccination?

A first dose rarely provides immediate or complete protection. Ideally, required primary doses should be completed and the final relevant vaccination administered at least 14 days before likely exposure.

Final Thoughts

Most healthy foals born to appropriately vaccinated mares and with adequate passive transfer begin many vaccines between four and six months of age.

Earlier vaccination at three to four months can be useful for selected foals, but it is not a universal rule. Maternal antibodies, local disease risk, product labelling and the mare’s disease-specific vaccination history all change the correct timing.

The most important first step is not the first vaccine. It is making sure the newborn foal receives and absorbs adequate colostrum. From there, the goal is to build a written vaccination schedule and complete every required dose before the foal faces significant exposure.

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ASK A VET™ can help you organise mare and foal vaccination records, track IgG results and booster dates, and seek veterinary guidance when a missed dose, unknown history or early exposure makes the correct schedule unclear.