Most foals begin their core vaccinations between 4 and 6 months of age, but the exact starting point depends on whether the mare was properly vaccinated before foaling. Foals from unvaccinated mares need to start earlier, sometimes as young as 2 to 3 months, because they lack the protective antibodies that colostrum normally provides. Getting this timing right is one of the most important decisions in a young horse’s first year.
Why the Mare’s Vaccination Status Matters
A foal’s immune protection starts before it’s even born. When a pregnant mare receives booster vaccines about one month before her expected foaling date, she concentrates antibodies in her colostrum. The foal absorbs those antibodies during the first 12 to 24 hours of life, gaining temporary protection against the diseases the mare was vaccinated for. This is called passive transfer, and it’s the foal’s only immune defense in the early weeks.
Those maternal antibodies are a double-edged sword, though. While they protect the foal, they also interfere with vaccines. If you vaccinate too early, the maternal antibodies neutralize the vaccine before the foal’s own immune system can respond to it. Research on equine influenza found that even small amounts of maternal antibodies blocked the vaccine from working, and that foals born to well-vaccinated mares shouldn’t receive their first influenza vaccine before 24 weeks (about 6 months) of age. After the first and second doses, foals in that study showed no measurable antibody increase at all. Only after a third dose did they mount a strong, lasting immune response.
This is why foals from vaccinated mares start their series later, typically at 5 to 6 months, while foals from unvaccinated mares (or foals that didn’t get enough colostrum) can start at 3 to 4 months. Without maternal antibodies in the way, earlier vaccination is both safe and necessary.
Core Vaccines and When to Start
Core vaccines are the ones every foal needs regardless of location or lifestyle. These protect against diseases that are widespread, potentially fatal, or both.
Tetanus
Foals from vaccinated mares get their first tetanus dose at 5 to 6 months. Foals from unvaccinated mares should receive it as soon as possible, with a booster 4 weeks later. The Merck Veterinary Manual recommends doses at 3, 4, and 6 months for foals lacking maternal protection. Tetanus is nearly always fatal in horses once symptoms appear, so this vaccine is non-negotiable.
Eastern and Western Encephalomyelitis
These mosquito-borne brain infections carry mortality rates above 80% for Eastern encephalomyelitis. Foals from vaccinated mares begin at 5 to 6 months with a three-dose series at roughly 6, 7, and 9 months. Foals from unvaccinated mares start at 3 to 4 months, with a second dose 4 to 6 weeks later and a third at 10 to 12 months.
West Nile Virus
West Nile follows a similar schedule to encephalomyelitis since it’s also mosquito-transmitted. Foals from vaccinated mares receive a three-dose series starting around 6 months. Timing the primary series so that protection is solid before mosquito season is worth discussing with your veterinarian, especially in the southern United States where the season starts early.
Rabies
The AAEP recommends the first rabies dose at 4 to 6 months of age. Foals from vaccinated mares get a two-dose series, with the second dose 4 to 6 weeks after the first. Foals from unvaccinated mares receive a single dose in that same 4 to 6 month window, then revaccinate annually. Some university guidelines place the first dose a bit later, at 7 to 8 months, so your vet may adjust based on local rabies risk.
Risk-Based Vaccines by Age
Beyond the core vaccines, several others are recommended based on geography, travel, and exposure risk. These have their own age minimums that are important to respect.
Equine Influenza
Influenza timing varies more than any other foal vaccine. Foals from vaccinated mares should wait until 9 months for their first intranasal dose, with a second at 11 months. Starting earlier is counterproductive because maternal antibodies suppress the response. Foals from unvaccinated mares can begin at 3 to 4 months with a booster 4 weeks later, since there are no maternal antibodies to contend with.
Equine Herpesvirus (Rhinopneumonitis)
EHV-1 and EHV-4 cause respiratory disease in young horses and can spread quickly in group housing. The first dose goes in at 5 to 6 months for foals from vaccinated mares, or 3 to 4 months for foals from unvaccinated mares, with a booster 4 to 6 weeks later and a third dose at 10 to 12 months.
Strangles
The intranasal modified-live strangles vaccine is labeled for use starting at 9 months. The primary series is two doses given 3 weeks apart. Using this vaccine in foals younger than 9 months is considered off-label, and the AAEP cautions practitioners about doing so. Annual revaccination begins at 12 months.
Potomac Horse Fever
This vaccine is labeled for foals 3 months and older, but clinical disease is rare in very young foals, and maternal antibodies can interfere. For most foals, starting after 5 months makes more sense. The primary series is two doses given 3 to 4 weeks apart, followed by a third dose at 12 months. If you do start before 5 months, additional doses up to 6 months are recommended to ensure the foal actually responds.
Botulism
In regions where botulism is endemic (the mid-Atlantic states, in particular), foals are at risk for a toxic-infectious form of the disease. This is the one vaccine that can start remarkably early. Foals at high risk can begin the series at just 2 weeks of age, with additional doses at 4 weeks and 8 weeks. Research suggests foals develop adequate immune responses at these ages even when maternal antibodies are present, which is unusual among equine vaccines.
The Three-Dose Rule
A single vaccine dose does very little for a foal. Unlike adult horses that need only a booster, foals require a primary series of two or three doses to build real immunity. The first dose primes the immune system, the second amplifies the response, and in many cases a third dose at 10 to 12 months solidifies long-term protection. Skipping or delaying boosters in the primary series can leave a foal functionally unprotected even though it technically received a vaccine.
Intervals between doses matter too. Most primary series call for 4 to 6 weeks between the first and second doses. Giving them too close together (less than 3 weeks) doesn’t allow the immune system enough time to respond to the first dose. Spacing them too far apart isn’t dangerous, but it delays full protection.
Preparing the Mare Before Foaling
The easiest way to protect a foal in its first months of life is to vaccinate the mare on schedule. Boosters for core and relevant risk-based vaccines should be given about 4 to 6 weeks before the expected foaling date. This timing maximizes antibody concentration in the colostrum. A mare vaccinated too early in pregnancy will have declining antibody levels by the time she foals, and one vaccinated too late may not produce peak colostral antibodies in time.
Equally important is making sure the foal nurses within the first few hours of life. The foal’s gut can only absorb large antibody molecules during roughly the first 12 to 24 hours after birth. After that window closes, even antibody-rich colostrum passes through without being absorbed. If there’s any concern about whether a foal nursed adequately, a blood test measuring immunoglobulin levels (IgG) can confirm whether passive transfer was successful. Foals with IgG levels below 200 mg/dL are considered to have failed passive transfer and should be treated as if born to an unvaccinated mare, with vaccines starting at 3 to 4 months rather than 5 to 6.
Quick Reference by Age
- 2 to 4 weeks: Botulism series (endemic areas only)
- 3 to 4 months: Tetanus, encephalomyelitis, EHV, and influenza for foals from unvaccinated mares or those with failed passive transfer
- 4 to 6 months: Rabies (all foals), Potomac horse fever where applicable
- 5 to 6 months: Tetanus, encephalomyelitis, EHV, and strangles for foals from vaccinated mares
- 9 months: Influenza (vaccinated-mare foals), strangles intranasal series
- 10 to 12 months: Third doses and annual boosters begin