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The objective of this study was to determine Ro of flu in non-immune populations and in populations of pigs vaccinated with homologous or heterologous vaccines and to assess how vaccination may affect transmission.
Ten groups of eleven 3 week-old pigs, free of most significant pathogens including influenza virus, PRRS virus, and Mycoplasma hyopneumoniae, and born from unvaccinated influenza sows, were housed in separated isolation rooms and distributed in 3 treatment groups as follows: a) an unvaccinated naïve control group (3 replicates); b) a group vaccinated with the same viral strain as the challenge strain (termed the homologous group) (3 replicates); and c) a group vaccinated with a commercially available licensed vaccine containing current viral strains from alternative lineages from the challenge strain (termed heterologous) (4 replicates). Pigs were vaccinated at arrival and two weeks later, and one pig in each room was left unvaccinated as a seeder. Two weeks after the second vaccine and in a separate room, the 10 seeder pigs were challenged with a triple-reassortant H1N1 strain (A/Sw/IA/00239/04 H1N1) named IA04, which was genetically different from the influenza virus strains present in the commercial vaccine. Two days post challenge, one infected seeder pig (confirmed by nasal swab RRT-PCR) was moved into each replicate of the treatment groups and transmission evaluated by collecting nasal swabs from all pigs on a daily basis and up to 14 days post contact (dpc).
All the contact pigs (100%) in the control group were found RRT-PCR positive at 5 dpc, while in the vaccinated groups, 15 out of 40 pigs (37.5%) in the heterologous group and none of the pigs (0%) in the homologous group were positive by RRT-PCR for the duration of the study. Although flu virus transmission was identified in all replicates from the heterologous group, not all pigs became positive and transmission was delayed in comparison to transmission in the control group. In the control group, the estimated number of secondary cases per infectious animal ranged from 3.8 to 4.6. In contrast, the Ro values for the heterologous group were above 1 in two of the four replicates and below 1 in the other two replicates, and 0 in all the replicates from the homologous group. A significantly reduction in the Ro value was observed in vaccinated pigs (P value < 0.001). Among vaccinated pigs, transmission in the homologous vaccinated pigs could not be detected for any of the replicates. Transmission in the heterologous group was observed in all replicates, and R was above 1 in two of the four replicates. However, statistical differences could not be found between the mean values of R between vaccinated groups.
Results from this study indicated differences in the reproduction ratio for influenza virus in populations of vaccinated and non-vaccinated pigs. Moreover differences, though not statistically significant, were also observed between populations of pigs vaccinated with either a homologous or heterologous vaccine. Flu spread rapidly in non-immune populations but its transmission was prevented in homologous vaccinated populations. Interestingly, transmission in the heterologous vaccinated populations occurred, but was quite variable.
A. Romagosa, M. Gramer, H. Soo Joo, J. Deen, M. Torremorell. Transmission patterns of influenza virus in vaccinated and non-vaccinated pig populations. 2011 AASV Annual Meeting; 39
