Current Vaccination Strategies and Challenges

Swine Influenza A viruses (swIAV) cause significant respiratory diseases in pigs, impacting modern pig production economically. In Europe, there are no mandatory reporting requirements for swIAV infections in pigs, and coordinated monitoring and control measures are lacking. Common strategies to control swIAV include biosecurity, herd management, and vaccination of sows with commercial licensed or autologous adjuvanted whole inactivated vaccines (WIV). Suckling piglets may receive some immune protection via maternally derived antibodies (MDA) following uptake of colostrum, but in herds enzootically affected by swIAV, these MDA-positive piglets can become asymptomatic carriers, spreading the virus without developing clinical signs and failing to mount active immunity. Vaccination in young piglets is hindered by MDA interference, limiting the effectiveness of WIVs, which can be used only in pigs above 56 days of age. While WIVs can reduce clinical signs, their efficacy infection and shedding can be compromised by viral antigenic drift.

Novel Vaccination Strategies: Bat-IAV and VSV Replicon Vaccines

To address the limitations of WIVs, new vaccine candidates aim to (i) induce broader immunity, (ii) stimulate mucosal defense, and (iii) be used in piglets in the presence of MDA. Live-attenuated influenza vaccines (LAIV) and RNA replicon particle vaccines are promising strategies. LAIVs mimic natural infections without causing disease, stimulating comprehensive and robust immunity. Chimeric bat-IAVs, expressing the hemagglutinin and neuraminidase proteins of the target IAV on a bat-IAV backbone, have shown potential as they can replicate in various hosts without reassortment risks, preventing virulence reversion. RNA replicon particle vaccines use propagation-defective vesicular stomatitis virus (VSV) vectors, eliciting strong humoral and cellular immune responses.

Study Overview - Efficacy of Novel Vaccines in Piglets

This study evaluates the efficacy of novel LAIV and replicon vaccines in piglets from an enzootically swIAV-infected farm. These vaccines, in addition to commercial heterologous and autologous WIVs, were applied to piglets in a prime-boost regime in the presence of MDA. Piglets were challenged by exposure to swIAV-infected unvaccinated pigs.

Figure 1. Schematic overview of experimental study design.

Key Findings - Vaccine Efficacy

Both LAIV and replicon vaccines significantly reduced virus replication in the respiratory tract with a better performance compared to WIVs. Yet, none of the vaccines induced sterile immunity. The VSV replicon vaccine notably reduced virus replication in the lower respiratory tract.

Conclusions and Future Directions

Vaccination remains the primary tool to combat swIAV infections in pigs. The study highlights the potential of LAIV and VSV replicon vaccines for improved control of swIAV infections in piglets compared to WIV vaccines. Although these vaccines may require updates to antigenically match circulating virus strains, they provide flexible platform technologies. Further studies are needed to assess long-term efficacy at herd level, safety, and optimized vaccination strategies, including cross-boosting for broader protection. Optimized vaccines and vaccination regimes are expected to improve swIAV control leading to enhanced animal health and to reduced risks of zoonotic swIAV transmission.

Practical Implications

From a practical standpoint, intranasal applications of LAIV are feasible in week-old piglets but challenging in older pigs. The study suggests that efficient vaccination of all piglets, not just sows, may be necessary to build resilient herd immunity and interrupt continuous swIAV transmission.