Serological responses in 11 Italian herds after vaccination with a combined vaccine against H1N1, H3N2 and H1N2 swine influenza virus subtypes

Protection against viral infections is achieved through a large array of immune mechanisms. Priming pigs toward antigens is able to produce specific memory cells later responsible for a faster onset and a higher intensity of the immune response when animals are infected with the field viruses. The aim of the present study was to evaluate on a large scale and under Italian current field conditions the serological responses of pigs vaccinated or not vaccinated with a commercial inactivated Swine Influenza (SI) vaccine containing H1N1, H1N2 and H3N2 strains. A secondary objective was to assess if a common field off label use of the two vaccines, i.e. dilution of the dry pellet of a Pseudorabies (PR) MLV vaccine in the SI vaccine was not impairing the safety nor the immune response of the SI vaccine. Eleven farrow-to-finish farms in the North of Italy were studied during the year 2013: in each farm two groups of 12-14 pigs were vaccinated with GRIPOVAC®3 against SI concomitantly to PR vaccination (AKIPOR®6.3) scheme i.e. twice before turn-to fattening (70-90 days of age (DoA) and 3 weeks later) and at 180 DoA during fattening: one group received separated injections of the two vaccines and one group was vaccinated following dilution of the AKIPOR 6.3 dry pellet in GRIPOVAC 3. A control group of 12-14 pigs was also included in the study and were vaccinated only with AKIPOR 6.3 at the same dates. Blood samples were roughly collected at around 70, 125, 180 and 250 DoA (just before slaughter) for antibody titration against recent Italian SIV subtypes H1N1, H3N2, and H1N2 by inhibition of hemagglutination test (HI). A sample was considered positive for one specific subtype if the titre was ≥20; one farm was classified as positive against one given subtype if at least 2 sera showed an HI titre ≥20 against this subtype. Mitigation of the results was performed to account for cross-reactivity between strains. Statistical analyses were performed using nonparametric tests. No adverse event related to safety was seen in any of the farm for any of the vaccination schedules. No clinical signs evoking a flu passage was observed except in one farm in which a H3N2 flu passage was confirmed both virologically following nasal swabbing and serologically. However, a raise of the SIV antibodies titres in control pigs, induced a suspicion of a field infection in all herds: 2/11 herds for H1N1, 4/11 for H3N2, 4/11 for H1N2, and 1/11 for both H1N1 and H3N2, thus stressing a high incidence of swine influenza cases in fatteners. In this context, almost all pigs were negative towards SIV at 70 days of age before primo-immunization. One month after the second vaccination, a clear seroconversion was seen in both groups vaccinated against SI: the pigs vaccinated with GRIPOVAC 3 displayed indeed significantly higher HI titres as compared to the controls (p<0.001). Seroconversion was then definitely observed in the control groups from mid-fattening but in the groups vaccinated with GRIPOVAC 3 HI titres were significantly higher than in the controls (p<0.04) except for H3N2 titres at the third sampling but with a trend of significance (p=0.06). No difference between the vaccinated groups was evidenced in HI titres over the whole fattening period except in H1N2 titres at 125 days of age which tended to be higher when the vaccine were injected separately (p=0.06). Despite no severe SI clinical signs were detected during the study, seroconversion of control pigs indicated frequent SIV infections. Whatever the vaccination protocol, GRIPOVAC 3 appeared capable of inducing a significant antibody response against the SIV strains circulating in Italy.