Efficacy of a pandemic (H1N1) 2009 virus vaccine in pigs against the pandemic influenza virus is superior to commercially available swine influenza vaccines.

In April 2009 a new influenza A/H1N1 strain, currently named "pandemic (H1N1) influenza 2009" (H1N1v), started the first official pandemic in humans since 1968. Several incursions of this virus in pig herds have also been reported from all over the world. Vaccination of pigs may be an option to reduce exposure of human contacts with infected pigs, thereby preventing cross-species transfer, but also to protect pigs themselves, should this virus cause damage in the pig population. Three swine influenza vaccines, two of them commercially available and one experimental, were therefore tested and compared for their efficacy against an H1N1v challenge. One of the commercial vaccines is based on an American classical H1N1 influenza strain, the other is based on a European avian H1N1 influenza strain. The experimental vaccine is based on reassortant virus NYMC X179A (containing the hemagglutinin (HA) and neuraminidase (NA) genes of A/California/7/2009 (H1N1v) and the internal genes of A/Puerto Rico/8/34 (H1N1)). Excretion of infectious virus was reduced by 0.5-3 log(10) by the commercial vaccines, depending on vaccine and sample type. Both vaccines were able to reduce virus replication especially in the lower respiratory tract, with less pathological lesions in vaccinated and subsequently challenged pigs than in unvaccinated controls. In pigs vaccinated with the experimental vaccine, excretion levels of infectious virus in nasal and oropharyngeal swabs, were at or below 1 log(10)TCID(50) per swab and lasted for only 1 or 2 days. An inactivated vaccine containing the HA and NA of an H1N1v is able to protect pigs from an infection with H1N1v, whereas swine influenza vaccines that are currently available are of limited efficaciousness. Whether vaccination of pigs against H1N1v will become opportune remains to be seen and will depend on future evolution of this strain in the pig population. Close monitoring of the pig population, focussing on presence and evolution of influenza strains on a cross-border level would therefore be advisable.