# Duration of immunity following infection with moderately virulent ASFV

## TL;DR

Pigs recovering from a moderately virulent African swine fever virus infection gain long-lasting immunity, surviving a lethal challenge six months later.

## Contribution

Demonstrates that immunity from a moderately virulent ASFV strain lasts at least six months and involves the complement system.

## Key findings

- Recovered pigs showed robust protection against a lethal ASFV challenge six months post-infection.
- Complement system activation, particularly via the lectin pathway, correlates with immune responses to ASFV.
- No chronic disease was observed in convalescent pigs, and antibody levels remained high.

## Abstract

African swine fever virus (ASFV) poses a significant threat to pork production and wild pig populations worldwide. The study assessed the long-term fate and immunity of animals recovering from a moderately virulent ASFV infection, following the principles of a duration of immunity study for live vaccines. Pigs inoculated with the moderately virulent ASFV strain Estonia14 largely developed mild clinical signs and only transient viremia. Six months after the initial inoculation and once fully recovered, all the animals were challenged with highly virulent ASFV Armenia08. Only one of the previously exposed pigs exhibited mild clinical signs, while all control animals showed typical signs of acute, lethal ASF. Moreover, only a subset of pigs inoculated with the ASF strain Estonia14 displayed temporary detectability of ASFV genomes following challenge infection. Virus isolation corroborated these findings, with low levels of infectious virus in organs of previously inoculated pigs (28 days post challenge). Furthermore, monitoring of ASFV-specific IgM and IgG kinetics enabled the analysis of humoral responses. IgG levels were sustained over the study period and increased slightly upon challenge infection. Lastly, plasma analysis revealed elevated complement factor C3a levels post inoculation and challenge in the recovered pigs, directly correlating with challenge virus presence. In contrast, both C3a and C5a levels were increased in the control group. It could be shown that complement system activation was mediated by the lectin pathway, possibly by interaction of mannose-binding lectins and ASFV particles. This study suggests that protective immunity following recovery can last at least six months. No cases of persistent or chronic disease were observed in convalescent pigs. These findings have implications for both vaccine development and assessment, as well as for disease control strategies including surveillance actions.

African swine fever virus (ASFV) causes the typically lethal African swine fever (ASF) in pigs, and its spread threatens pig populations worldwide. Vaccines can efficiently protect animals but require detailed understanding of the disease and responses in affected animals. For example, little is known about how long recovered animals are protected from lethal infection. Therefore, we infected pigs with the well-characterized moderately virulent ASFV strain Estonia14, which most animals survived with only minor clinical signs. Six months later, we infected the same pigs together with a group of non-immune animals with the highly virulent ASFV strain Armenia08. Remarkably, all pigs that had previously recovered from moderately virulent ASFV survived with minor or no clinical signs at all. In contrast, all non-immune pigs died of ASF. Further analyses, like low viral loads and continually high antibody levels in survivors, underscore effective protection. We also show that a part of the innate immune system, the complement system, plays a role during these responses. These results demonstrate that recovery from infection with moderately virulent ASFV can induce robust protection for at least six months.