pigs of H1pdm lineages that are antigenically significantly distinct from current human strains. It is difficult to extrapolate such results to an actual risk of zo- onotic transmission. That risk was assessed by testing adult human sera against the variant SIV lineages, which still found a reasonable degree of cross-reactivity. That is likely due to the broad immunological memory generated in adult hu- mans following various episodes of influenza virus infections and/or vaccinations. Similar cross-reactivity could also be noted against other SIV lineages such as the H1av subtype. However, the study has not been able to include sera of children who have limited in- fluenza exposure. In fact, a substantial proportion of the few sporadic human SIV infections described in Europe have been reported in children and in immunocompromised adults.
Avoiding new reassortants Pigs are known as a mixing vessel of IAV of porcine, human and even avian origin. Historically, all human pandemic virus- es (with the sole exception of the 1957 H2N2 IAV) have been transmitted by humans to pigs in a “reverse zoonotic” fashion. These strains were and are maintained in pig populations over decades. That enlarges the reservoir of genetic building blocks for future zoonotic IAV. The true rate of mutual IAV transmission at the human–porcine interface is not known and is difficult to assess. On the basis of the few and sporadic cases of SIV causing dis- ease in individual human beings in Europe, SIVs are not easily transmitted to humans, though there may be asymptomatic
infections that have been neglected in the past. Reducing the load of IAV present in swine populations and in humans dur- ing the influenza season certainly would decrease the risk of mutual exposure and hence transmission of viruses. Inhibiting infections and/or reducing the load of virus excretion after in- fection are the main goals of vaccination. Therefore, vaccina- tion of swine against SIV and of staff interacting with swine against human seasonal influenza is highly recommended, including from a one-health perspective.
Vaccination against influenza The difficulty with influenza vaccines is that the variation in the antigenic make-up is a notorious feature of all IAV strains. Attempts to find a universal vaccine, a “one size fits all” solu- tion, has failed so far even for human seasonal influenza. Therefore it is a common and useful strategy in influenza vaccine design to combine the major circulating strains of a season or of a region into multivalent vaccines.
The role of haemagglutinin Haemagglutinin (H) is the major viral determinant inducing protective immunity through the induction of antibodies neutralising viral infectivity. Those antibodies are believed to play an important role in inhibiting infection of a vaccinated individual, especially if such antibodies are present on the surface of the respiratory tract. Current concepts of human seasonal influenza vaccines therefore focus on as high as pos- sible homology between the H proteins of the vaccine and the circulating strains. On that basis a vaccine based on
▶PIG PROGRESS | Volume 36, No. 7, 2020 11
A nose swab is being taken from a piglet. In order to keep stress with the animal as low as possible, this can be done by one person as well.
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