# High serological barriers may contribute to restricted Influenza-A-virus transmission between pigs and humans

## TL;DR

The study finds that pre-existing antibodies in both humans and pigs may act as a barrier to Influenza A virus transmission between the two species.

## Contribution

The study identifies serological barriers, not just genetic factors, as a key mechanism limiting cross-species influenza transmission.

## Key findings

- Urban children without pig contact showed neutralizing antibodies against swine IAV.
- Swine sera contained neutralizing antibodies against human IAV strains.
- Serological barriers may be more significant than genetic factors in restricting IAV transmission.

## Abstract

Influenza A viruses (IAV) circulate in both humans and pigs, with bidirectional transmission potentially driving viral evolution. Despite frequent contact and genetic compatibility, observed cross-species transmission remains rare, suggesting the presence of unexplored or little-known barriers. The study investigated transmission dynamics and mechanisms restricting IAV spread at the human-swine interface in Germany. We analyzed 3070 porcine and 333 human nasal swabs from 135 swine farms via RT-qPCR and full-genome sequencing. Concurrently, we conducted serological surveys: 1) Children's sera (urban, no pig contact) for antibodies against circulating swine IAV, and 2) Swine sera for antibodies against human-adapted IAV. Molecular surveillance identified only one zooanthroponosis event and sporadic anthropozoonosis (primarily in children) despite swine IAV strains carrying zoonotic-propensity genetic markers (MxA resistance). Serologically, urban children without pig exposure exhibited marked neutralizing activity against swine IAV, whereas swine sera contained neutralizing antibodies against human IAV strains. Pre-existing cross-reactive immunity—evidenced by unexpected antibody prevalence in both species—creates a more complex interspecies barrier than genetic factors alone. This serological “shield” may critically limit IAV transmission between humans and pigs, reshaping our understanding of zoonotic risk.

•IAV with zoonotic propensity circulate in swine populations in Germany.•Multifaceted investigations revealed a complex human-swine interspecies barrier.•Serological barriers may restrict IAV transmission between pigs and humans.•Continuing surveillance of IAV in swine remains mandatory.

IAV with zoonotic propensity circulate in swine populations in Germany.

Multifaceted investigations revealed a complex human-swine interspecies barrier.

Serological barriers may restrict IAV transmission between pigs and humans.

Continuing surveillance of IAV in swine remains mandatory.