Swine influenza A viruses (swIAVs) with a truncated NS1del126 protein were strongly attenuated in previous laboratory-based safety studies and therefore approved for use as LAIVs for swine in the United States. In the field, however, the LAIV strains were detected in diagnostic samples and could regain a wild-type NS1 via reassortment with endemic swIAVs.

Surveillance of swine influenza A viruses (swIAV) allows timely detection and identification of new variants with potential zoonotic risks. In this study, we aimed to identify swIAV subtypes that circulated in pigs in Belgium and the Netherlands between 2014 and 2019, and characterize their genetic and antigenic evolution. We subtyped all isolates and analyzed hemagglutinin sequences and hemagglutination inhibition assay data for H1 swIAV, which were the dominant HA subtype. We also analyzed whole genome sequences (WGS) of selected isolates. Out of 200 samples, 89 tested positive for swIAV. swIAV of H1N1, H1N2 and H3N2 subtypes were detected. Analysis of WGS of 18 H1 swIAV isolates revealed three newly emerged genotypes. The European avian-like H1 swIAV (lineage 1C) were predominant and accounted for 47.2% of the total isolates. They were shown to evolve faster than the European human-like H1 (1B lineage) swIAV, which represented 27% of the isolates. The 2009 pandemic H1 swIAV (lineage 1A) accounted for only 5.6% of the isolates and showed divergence from their precursor virus. These results point to the increasing divergence of swIAV and stress the need for continuous surveillance of swIAV.

We report a zoonotic infection of a pig farmer in the Netherlands with a Eurasian avian-like swine influenza A(H1N1) virus that was also detected in the farmed pigs. Both viruses were antigenically and genetically characterized. Continued surveillance of swine influenza A viruses is needed for risk assessment in humans and swine.

In a previous study in influenza-naïve pigs, heterologous prime-boost vaccination with monovalent, adjuvanted whole inactivated vaccines (WIV) based on the European swine influenza A virus (SwIAV) strain, A/swine/Gent/172/2008 (G08), followed by the US SwIAV strain, A/swine/Pennsylvania/A01076777/2010 (PA10), was shown to induce broadly cross-reactive hemagglutination inhibition (HI) antibodies against 12 out of 15 antigenically distinct H3N2 influenza strains. Here, we used the pig model to examine the efficacy of that particular heterologous prime-boost vaccination regimen, in individuals with pre-existing infection-immunity. Pigs were first inoculated intranasally with the human H3N2 strain, A/Nanchang/933/1995. Seven weeks later, they were vaccinated intramuscularly with G08 followed by PA10 or vice versa. We examined serum antibody responses against the hemagglutinin and neuraminidase, and antibody-secreting cell (ASC) responses in peripheral blood, draining lymph nodes, and nasal mucosa (NMC), in ELISPOT assays. Vaccination induced up to 10-fold higher HI antibody titers than in naïve pigs, with broader cross-reactivity, and protection against challenge with an antigenically distant H3N2 strain. It also boosted ASC responses in lymph nodes and NMC. Our results show that intramuscular administration of WIV can lead to enhanced antibody responses and cross-reactivity in pre-immune subjects, and recall of ASC responses in lymph nodes and NMC.

Most H1 influenza A viruses (IAVs) of swine are derived from past human viruses. As human population immunity against these IAVs gradually decreases, the risk of reintroduction to humans increases. We examined 549 serum samples from persons 0–97 years of age collected in Belgium during 2017–2018 for hemagglutination inhibiting and virus neutralizing antibodies against 7 major H1 swine IAV (swIAV) clades and 3 human progenitor IAVs. Seroprevalence (titers > 40) rates were > 50% for classical swine and European human-like swIAVs, > 24% for North American human-like δ1a and Asian avian-like swIAVs, and < 10% for North American human-like δ1b and European avian-like swIAVs, but rates were age-dependent. Antibody titers against human-like swIAVs and supposed human precursor IAVs correlated with correlation coefficients of 0.30–0.86. Our serologic findings suggest that European avian-like, clade 1C.2.1, and North American human-like δ1b, clade 1B.2.2.2, H1 swIAVs pose the highest pandemic risk.

No abstract

Heterologous prime-boost vaccination with experimental or commercial influenza vaccines has been successful in various animal species. In this study, we have examined the efficacy of alternating 3 different European commercial swine influenza A virus (swIAV) vaccines: the trivalent Respiporc® FLU3 (TIV), the bivalent GRIPORK® (BIV) and the monovalent Respiporc® FLUpan H1N1 (MOV). Five groups of 6 pigs each received 3 vaccinations at 4–6 week intervals in a homologous or heterologous prime-boost regimen. A sixth group served as a mock-vaccinated challenge control. Four weeks after the last vaccination, pigs were challenged intranasally with a European avian-like H1N1 (1C.2.1) swIAV, which was antigenically distinct from the vaccine strains. One heterologous prime-boost group (TIV–BIV–MOV) had higher hemagglutination inhibition (HI) and neuraminidase inhibition antibody responses against a panel of antigenically distinct H1N1, H1N2 and H3N2 IAVs than the other heterologous prime-boost group (BIV–TIV–MOV) and the homologous prime-boost groups (3xTIV; 3xBIV; 3xMOV). Group TIV–BIV–MOV had seroprotective HI titers (≥ 40) against 56% of the tested viruses compared to 33% in group BIV–TIV–MOV and 22–39% in the homologous prime-boost groups. Post-challenge, group TIV–BIV–MOV was the single group with significantly reduced virus titers in all respiratory samples compared to the challenge control group. Our results suggest that the use of different commercial swIAV vaccines for successive vaccinations may result in broader antibody responses and protection than the traditional, homologous prime-boost vaccination regimens. In addition, the order in which the different vaccines are administered seems to affect the breadth of the antibody response and protection.

I nfluenza A viruses (IAVs) of the H3 subtype are endemic to humans, swine, and wild birds; they also cause outbreaks in horses and are often detected in domestic birds. An H3 IAV that crosses the species barrier from animals to humans can result in a pandemic if the virus carries a hemagglutinin (HA) against which humans lack protective antibodies and the virus readily replicates in and spreads among humans. For example, in 1968, transmission of an IAV with an avian-origin H3 HA to humans caused the influenza A(H3N2) pandemic (1).The natural IAV reservoir is considered to be wild waterfowl, but transmission to domestic poultry is frequent. Avian H3 IAVs are classified as Eurasian and North American lineages, although the HA of these viruses is antigenically closely related (2,3). In contrast, after being introduced to humans in 1968, the HA of human H3 IAVs quickly drifted away from that of the avian precursor IAV. Consequently, contemporary human H3 IAVs are antigenically divergent from those in birds (2). Similarly, avian H3 IAVs were introduced into horses in the 1960s, after which their HA antigenically drifted. That evolution was, however, different and slower than for human H3 IAVs (4). Equine H3 IAVs of Florida clade 1 (FC1) are currently predominant (5). All swine H3 IAVs derived their HA from human IAVs.H3 IAVs from swine in Europe originated from a human IAV that circulated in the late 1970s. Of the 2 major lineages cocirculating in North America, cluster IV-A was derived from human IAVs from the late 1990s and novel human-like swine H3 IAVs from human IAVs from the early 2010s (6). H3 IAVs undergo slower antigenic drift in swine than in humans. Consequently, persons born after the swine viruses' human ancestor IAV had circulated are unlikely to have cross-reactive antibodies against the swine H3 IAVs. Therefore, with time, human population immunity against swine H3 IAVs decreases, increasing the pandemic risk (7-10).The infectious potential of swine H3 IAVs for humans is evident from >400 recorded zoonotic infections in the United States caused by North American cluster IV-A or novel human-like H3 swine IAVs. Four zoonotic infections with H3 IAVs from swine in Europe have also been reported (6,(11)(12)(13). H3 IAVs from equids can infect humans under experimental conditions, but there are no confirmed cases of natural transmission (14). Animal H3 IAVs might, however, become more adapted to humans by accumulating mutations in their viral proteins, reassortment of gene segments with IAVs of different species, or both (6,15). Avian H3 IAVs can infect humans directly or via an intermediate host, such as poultry or swine (2,15). In 2019, an H3N1 IAV that