Non-specific effects of maternal and offspring rabies vaccination on mortality and antibiotic use in a Danish pig herd : a randomized trial

INTRODUCTION : Human non-live vaccines have been associated with detrimental non-specific effects (NSE), particularly in females. A large trial found 2-fold increased overall mortality in girls receiving a new malaria vaccine compared to the rabies vaccine used as a control; a beneficial NSE of the rabies vaccine was proposed. Conversely, in dogs increased mortality was seen in females but not males following rabies vaccination of puppies born to immunized mothers. We investigated NSE of non-live rabies vaccine in piglets and the potential modifying effect of maternal priming with rabies vaccine. METHODS : In a Danish herd of commercial rabies virus-free pigs, 575 pregnant sows (2–3 weeks before scheduled farrowing) and 5747 of their offspring (median 6-day-old) were allocated (1:1) to non-live rabies vaccine (Versiguard rabies vet) or no rabies vaccine. Outcomes were overall mortality and antibiotic treatment until departure from the nursery (approximately age 12 weeks/30 kgs). RESULTS : Until weaning, overall offspring mortality was 2.2% (127 piglets died, rabies vaccine: n = 69; control: n = 58), the proportion ratio (PR) being 1.19 (95% confidence interval: 0.84–1.68). Until end of follow-up, mortality was 4.1% (233, rabies vaccine: n = 115; control = 118, PR: 0.97 (0.76–1.25)). Prior sow rabies vaccination did not affect piglet mortality. For mortality as well as risk of antibiotic treatment before weaning, there was indication of a beneficial effect of rabies vaccine in female piglets, but a negative effect in (castrated) male piglets from rabies-naïve sows. Prior sow vaccination significantly modified the vaccine effect estimate in female piglets toward a detrimental effect of rabies vaccine on treatment risk. These effects had waned by 12 weeks of age. CONCLUSION : The study did not support the hypothesized beneficial NSE of rabies vaccine. Although under-powered for subgroup analyses, the study indicated effect modification by sex and maternal vaccination. Results could be different in a herd with higher mortality and infectious burden.