# Assessment of Immunological Interference Between Live Infectious Bursal Disease Virus and Avian Reovirus Vaccines in SPF Chickens

## TL;DR

This study shows that giving two poultry vaccines too close together can reduce their effectiveness, but spacing them by at least five days avoids this issue.

## Contribution

The study provides the first definitive evidence of asymmetric immune interference between live IBDV and ARV vaccines in SPF chickens.

## Key findings

- The IBDV vaccine provided full protection regardless of timing, while the ARV vaccine was less effective when given too soon after the IBDV vaccine.
- A five-day interval between vaccines eliminated interference and restored ARV vaccine efficacy.
- Shorter intervals (same day or three days apart) led to significantly reduced ARV protection rates.

## Abstract

Vaccination is a key tool for preventing infectious diseases in poultry, but the timing of different vaccines can influence how well they work. In this study, we investigated whether two widely used live vaccines, one targeting infectious bursal disease and the other targeting viral arthritis, affect each other when they are given at the same time or within a short period. Using chickens raised without prior exposure to pathogens, we evaluated immune responses and disease protection under different vaccination schedules. The results showed that the infectious bursal disease vaccine always provided full protection and was not influenced by the second vaccine. In contrast, the viral arthritis vaccine was less effective when both vaccines were administered on the same day or only three days apart, resulting in weaker immune responses and lower protection against disease. When the interval between the two vaccinations was extended to five days or longer, this negative effect disappeared and strong protection was restored. These findings demonstrate that vaccine timing is critical for achieving optimal protection. Adjusting vaccination schedules to include a sufficient interval can improve disease prevention, reduce production losses, and support better health and welfare in poultry flocks, contributing to more efficient and sustainable poultry production.

Infectious bursal disease virus (IBDV) and avian reovirus (ARV) are major immunosuppressive pathogens controlled through the widespread use of live attenuated vaccines. Concerns persist regarding potential immune interference when these vaccines are co-administered, though comprehensive in vivo data are lacking. Here, we reported the immunogenicity and protective efficacy of a live IBDV vaccine (W2512G-61) and a live ARV vaccine (ZJS) administered simultaneously or sequentially at 3-, 5-, and 7-day intervals in specific-pathogen-free (SPF) chickens. The IBDV live vaccine elicits strong, interval-independent humoral immunity and conferred 100% protection, demonstrating no compromise from ARV co-administration. Conversely, ARV-specific immunity was severely impaired by close temporal vaccination. ARV protection rates fell from 86.7% (ARV-only) to 46.7% with simultaneous administration and from 93.3% to 66.7% with a 3-day interval. Extending the interval to five or seven days eliminated this interference, restoring ARV antibody titers and protection to levels equivalent to ARV-only control vaccinated groups. This study provides the first definitive evidence of asymmetric immune interference between live IBDV and ARV vaccines. The results establish a minimum safe interval of five days to prevent interference and ensure robust ARV vaccine efficacy. These findings offer critical, evidence-based guidance for optimizing vaccination schedules to improve disease control in commercial poultry production.