This is true. From Equine Respiratory Diseases (link below), the section on EVA written by Timoney:
“Two vaccines have been developed against EVA, a modified live virus product (Arvac®, Ft. Dodge Animal Health) and an inactivated adjuvanted virus product (Artervac®, Ft. Dodge Animal Health) [8,73]. Extensive use of the modified live virus vaccine since 1985 has confirmed its safety and immunogenicity for stallions, non-pregnant mares, fillies and colts. The vaccine manufacturers advise against its use in pregnant mares and foals less than 6 weeks of age, unless under conditions of significant risk of natural exposure to EAV. The inactivated vaccine, though safe for use in pregnant mares, is not as strongly immunogenic as the modified live virus vaccine. Two or more vaccinations are frequently required to stimulate a detectable neutralizing antibody response. The durability of immunity produced by this vaccine has yet to be established.”
ftp://ftp.aave.inv.org.ar/IVIS/IVIS%202003/Equine%20Viral%20Arteritis.pdf
My mare was vaccinated with Arvac in February 2010, then inseminated with Florencio in June and again in July 2010, so had three opportunities to create an immune response.
I asked Dr. Timoney about the risks to the foal in my situation (mare being bred to Florencio in 2011 with this vaccination history). In a Nov 2010 email, he wrote: “a mare vaccinated against EVA for the first time and then bred with equine arteritis virus infective semen need not be revaccinated for at least 1-2 years after that. The rationale is that the mare’s immune response to vaccination will be boosted significantly following exposure to the virus in Florencio’s semen, leaving her protected against reinfection for at least the following 1-2 years. If you have further concern over this, it would be very simple to check the current virus neutralizing antibody titer of the mare which should confirm what I have said. If you decide to proceed with this suggestion, we would be pleased to test a sample of blood for you. Assuming the mare to be strongly seropositive to the virus, then in my opinion, she should not represent a risk to her foal.” [emphasis mine]