Levels of neutralizing antibodies against resident farm strain or vaccine strain are not indicators of protection against PRRSV-1 vertical transmission under farm conditions

Background Vertical transmission is key for the maintenance of porcine reproductive and respiratory syndrome virus (PRRSV) infection in endemic farms. In most cases, neutralizing antibodies (NAb) more effectively neutralize the homologous strain, but some reports indicate the existence of broadly NAb (bNAb). In vaccinated farms, where all breeders have some levels of immunity, vertical transmission can still occur. The present study correlates the age of sows and the amplitude of NAb with PRRSV-1 vertical transmission (VT) through sampling two vaccinated farms. Results The proportion of VT events in the two examined farms ranged from 18.9–23.0%. Young sows (parity 1–2) were 1.7 times more likely to have VT than older sows ( p < 0.05). Despite higher ELISA S/P antibody ratios in younger sows ( p < 0.05), NAb against the resident farm strain were at a similar level between sows delivering infected and healthy piglets although mostly with low titres (2–3 log 2 ). The titres of NAb against the vaccine virus were also low, and no correlations with VT were observed. When a panel of another 4 strains (1 isolated in the 1990s, and 3 contemporary strains) were used for the neutralization test, most sows were not capable of neutralizing the contemporary strains. Conclusions Titres of NAb could not be correlated with the occurrence of PRRSV VT. The amplitude of NAb present in most vaccinated sows is limited with a considerable proportion unresponsive regarding NAb production.