the actual label does not say that - well at least on the one I foundexcept for one thing... http://www.drugs.com/vet/mycoplasma-gallisepticum-vaccine.html
under "Precautions" states clearly Do not use in breeders.
it's a modified live virus which will give the birds a modified version of the disease. the immunity they build against this version also works against the more virulent types, but they are still carriers of the modified version now, which can transmit to their eggs/chicks. whether it causes other problems with fertility or development I don't know.
or... *tinfoil hat time* maybe that immunity is passed to the offspring, just as MG does, so the pharmaceutical companies say don't use on breeders, because then they won't ever sell more. LOL
ZOETIS INC.
333 PORTAGE STREET, KALAMAZOO, MI, 49007
Telephone: 269-833-4000
Customer Service: 800-733-5500 and 800-793-0596
Veterinary Medical Investigations & Product Support: 800-366-5288
Technical Services (USA): 800-366-5288
Website: www.zoetis.com

MG-BAC

Zoetis
ZOETIS
For Veterinary Use Only
For the subcutaneous or intramuscular vaccination of chickens and turkeys as an aid in the prevention of clinical signs associated with Mycoplasma gallisepticum infection.
DOSE: Inject 0.5 mL (0.5 cc) subcutaneously (in the lower neck region) for birds 1 to 10 weeks of age using aseptic technique. For birds 10 weeks of age or older, may vaccinate intramuscularly or subcutaneously. Vaccinate only healthy birds prior to field exposure. The vaccination program should be completed prior to the point of lay. For optimum results, vaccinate twice, allowing at least 4 weeks between vaccinations. Revaccinate birds during molt before the second laying period.
CAUTION: Store in the dark at 36° to 45°F (2° to 7°C). Do not freeze. Warm to 72°F (22°C) and shake well before using. Do not vaccinate within 42 days before slaughter. Do not vaccinate broilers or meat turkeys by the intramuscular route of administration. If birds are vaccinated during lay, a drop in egg production may occur. Use entire contents when first opened.
Penicillin added as a preservative.
WARNING: In case of accidental human injection seek immediate medical attention, stating the vaccine is an oil emulsion type.
Zoetis Inc., Kalamazoo, MI 49007
U.S. Vet. License No. 190
List No.
2173
500 mL
1,000 Doses
11641800
NAC No.: 36902952
As far as preventing transmission through eggs - some studies show that a "double vaccinated" leghorn hens transmission of MG to her eggs reduces by 67% and other articles talk of total breaking of the cycle.
Here is the link to one http://www.oie.int/doc/ged/D9108.PDF