The present invention provides a one phase, aqueous vaccine composition for immunizing an animal against infection by Mycoplasma hyopneumoniae, comprising: an immunizing amount of a Mycoplasma hyopneumoniae bacterin, an acrylic acid polymer in the concentration range between 0.8 and 1.2 mg/ml, and a pharmaceutically acceptable carrier, and substantially no oil. It is especially useful for immunizing a pig against infection by Mycoplasma hyopneumoniae for at least 20 weeks after a single administration, which effective immunity is reached within 4 weeks after vaccination.