PARTNER FEATURE ▶▶▶
Large-scale vaccination against swine flu (part II)
The first episode in this two-part series showed that influenza viruses are circulating in a large proportion of the pig herd in Europe. This article describes how pig vaccination could help avert the next global influenza pandemic.
BY KATHRIN LILLIE-JASCHNISKI DVM, PHD AND FRIEDERIKE SCHMELZ, DVM, PHD, CEVA SANTÉ ANIMALE
W
hile sows make up only about 10% of the total pig population on-farm, their non-vaccinated offspring represent a vulnerable group of animals that can keep virus circulating. Mater-
nally derived antibodies that piglets receive from their vacci- nated mothers are able to protect piglets from clinical signs, but infection still occurs. Vaccination strategies implemented on-farm should therefore also include piglets and fatteners: not only can they be a source of reassortment, but infection also causes huge economic impact due to reduced daily weight gain and increased use of medication. Trials have shown that vaccination is effective in preventing clinical disease from all the circulating swine flu strains. However, to protect against most lineages of influenza A cir- culating in pigs in Europe, a combination of two vaccines – a trivalent covering H1N1, H1N2 and H3N2 and a vaccine with H1N1 pandemic strain – is required. While it is true that vaccines cannot totally eliminate virus, they prevent clinical disease and significantly reduce the amount of virus in circulation, thereby reducing the opportu- nity for viruses to mix, exchange genetic material and emerge as new, potentially pandemic strains. Vaccines against swine flu for pigs are much more effective in preventing clinical disease than flu vaccines used in people. It is therefore ironic that the widespread use of the vaccines in people is widely accepted, while the use of much more effective vaccines in pigs is sometimes questioned.
Increasing efficacy of flu vaccines Several strategies can be used to further increase the efficacy of swine flu vaccines used in pigs. All pig vaccines against swine flu licensed in Europe include the use of adjuvants in the vaccine composition. Adjuvants are certain types of
18 ▶ PIG PROGRESS | Volume 38, No. 3, 2022
chemicals that specifically stimulate the immune system and may potentiate vaccine efficacy. The disadvantage of adjuvants is that they may cause more local reactions, such as swelling at the site of injection, and also short-lived systemic reactions, such as fever or pain. For that reason, adjuvants are not often used in most human flu vaccines. Only in exceptional cases, an adjuvant could be used because immune responses of older people are generally weaker than those of younger people. Most influenza vaccines for use in people contain small frag- ments of virus particles from a number of different strains. Trivalent or, increasingly, tetravalent vaccines, containing viral particles from three or four different strains, are most commonly used throughout the world. In contrast, pig vaccines are made from whole, inactivated viruses: Ceva’s RespiporcFLU3 vaccine contains whole inactivated viruses of three strains of swine flu virus which between them cover three of the four major lineages of influenza A virus; Resiporc- FLUpanH1N1 contains whole inactivated virus from a strain originally isolated from a human case of influenza. So, while the human vaccines mostly contain just the viral proteins that are the major determinants that induce protective immuni- ty, the pig vaccines contain the full range of viral structural proteins. Some of those additional proteins may also induce immunity and thereby boost the efficacy of the vaccines, and may also provide broader protection against a wider range of strains.
New seasonal flu vaccines Each year, new seasonal flu vaccines are produced for people which are designed to closely match the influenza strains circulating that year. The regulatory authorities allow human vaccine manufacturers to replace the stains included in annu- al flu vaccines based on the recommended strains, providing certain standard procedures are followed. Current regulations controlling the use of veterinary vaccines in Europe pre- clude that option: changing the strains used in the currently licensed pig vaccines would necessitate a mandatory and lengthy approval process which would be impractical and u neconomic for vaccine manufacturers. As noted previously, pigs can become infected with swine flu strains from other pigs, birds and people. Of those, people are likely to be the major source of infection: although the disease is called “swine flu”, from a pig’s perspective it would actually be human flu. For that reason, pig farmers and other
PHOTO: RONALD HISSINK
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