Infectious bovine rhinotracheitis (IBR) is a worldwide endemic viral disease (BoHV-1) of cattle. Various vaccination plans against IBR are implemented in field; however, little is known about their ability to induce neutralizing antibody immune responses. The objective of this study was to evaluate this type of immune responses in calves after they had been vaccinated with several licensed (gE−) modified live virus (MLV) marker vaccines in the field. BoHV-1 gE/gB seronegative calves of 6 to 7 mo of age were selected from a fattening farm. Animals were randomly allocated into groups and vaccinated with either the gE−/tk− MLV HIPRABOVIS IBR MARKER LIVE (G1, n = 15) or 2 other licensed gE− MLV (G2, n = 15; G3, n = 14). One group, G4 (n = 12) did not receive any vaccine. G1 was boost-vaccinated with a further dose of the same vaccine as specified by the manufacturer’s instructions. Sera were collected before and 42 d post-vaccination (dpv) in all groups and tested by a virus neutralization assay against “Los Angeles” BoHV-1 strain. Titers higher than 1/2.5 were considered positive. Statistical comparisons were performed using the Mann-Whitney U-test. Neutralizing antibody responses against BoHV-1 were detected at 42 dpv in all vaccinated calves and titers ranged from 1/5 to 1/240. The highest titers were observed in G1 and differences were statistically significant. In particular, a subpopulation of animals (25%) with titers higher than 1/120 was only detected in G1. G2 did not show any statistically significant difference when compared with G3. The licensed MLV marker vaccines studied were able to induce a neutralizing antibody immune response against BoHV-1 providing a complete protection to the selected herds. Strength of the responses varied based on the type of vaccine used, although results showed that HIPRABOVIS IBR MARKER LIVE conferred the highest protection level. Table 1. Neutralizing antibody responses against BoHV-1 at 42 dpv

Group	Arithmetic Mean	Standard deviation	Geometric Mean
1	78.33a	60.11	60.89
2	33.57b	17.26	29.61
3	25.00b	22.66	17.35
4	0,00c	0,00	0,00

^a–cStatistically significant difference between groups (P < 0.05).