Immune responses to modified live virus vaccines developed from classical or highly pathogenic PRRSV following challenge with a highly pathogenic PRRSV strain

• PRRSV is a costly swine virus that can be controlled through vaccination.
• Modified live virus (MLV) vaccines are leading candidates for novel PRRSV vaccines.
• The CH-1R vaccine (classical PRRSV) provided some protection against HuN4 infection.
• The HuN4-F112 vaccine (highly pathogenic PRRSV) protected against HuN4 infection.
• Both vaccines elicited humoral and cellular immune responses.

Modified live virus vaccines (MLVs) are used on swine farms to control porcine reproductive and respiratory syndrome virus (PRRSV). MLVs from classical PRRSV (C-PRRSV) provide some protection against emergent highly pathogenic PRRSV (HP-PRRSV). This study characterized the protective efficacy and immune response to MLVs from C-PRRSV (CH-1R) or HP-PRRSV (HuN4-F112) in a challenge using HP-PRRSV (HuN4). The outcomes were clinical signs of disease, pathological changes in the thymus and lungs, viremia, and humoral and cellular immune responses. CH-1R provided some protection against challenge with HuN4, while HuN4-F112 was protective in the HuN4 challenge. Compared to unvaccinated piglets, the vaccinated piglets had milder symptoms and fewer pathological changes in the lung and thymus. Piglets vaccinated with HuN4-F112 had higher antibody titers and lower viral loads than piglets vaccinated with CH-1R post challenge. The differences in outcome between the MLVs suggested that underlying differences in the immune responses might warrant further study.