Vaccination of influenza a virus decreases transmission rates in pigs

Abstract

Limited information is available on the transmission and spread of influenza virus in pig populations with differing immune statuses. In this study we assessed differences in transmission patterns and quantified the spread of a triple reassortant H1N1 influenza virus in naïve and vaccinated pig populations by estimating the reproduction ratio (R) of infection (i.e. the number of secondary infections caused by an infectious individual) using a deterministic Susceptible-Infectious-Recovered (SIR) model, fitted on experimental data. One hundred and ten pigs were distributed in ten isolated rooms as follows: (i) non-vaccinated (NV), (ii) vaccinated with a heterologous vaccine (HE), and (iii) vaccinated with a homologous inactivated vaccine (HO). The study was run with multiple replicates and for each replicate, an infected non-vaccinated pig was placed with 10 contact pigs for two weeks and transmission of influenza evaluated daily by analyzing individual nasal swabs by RT-PCR. A statistically significant difference between R estimates was observed between vaccinated and non-vaccinated pigs (p < 0.05). A statistically significant reduction in transmission was observed in the vaccinated groups where R (95%CI) was 1 (0.39-2.09) and 0 for the HE and the HO groups respectively, compared to an Ro value of 10.66 (6.57-16.46) in NV pigs (p < 0.05). Transmission in the HE group was delayed and variable when compared to the NV group and transmission could not be detected in the HO group. Results from this study indicate that influenza vaccines can be used to decrease susceptibility to influenza infection and decrease influenza transmission.

Figure 1

Paired influenza A Multiscreen ELISA s/n values against the nucleoprotein of influenza virus two weeks after vaccination (prior to exposure) and at necropsy (bars represent the mean of s/n values (± SD) to influenza ■ after vaccination, □ at necropsy. Positive s/n < 0.673; negative s/n > 0.673. ^{a,b and z,y} Statistically significant differences between paired samples for the NV and HE groups before exposure and at necropsy (p < 0.05).

Figure 2

Time to infection curves for the three treatment groups. ^a,bStatistically significant differences (p < 0.05) Difference between vaccine groups (a) is statistically significant at 90% confidence level (p = 0.101) ● Censored data.

Figure 3

Number of new cases represented as the proportion of 10 000 simulations from the stochastic SIR model with initial values of (S = 10, I = 1, R = 0) for each NV replicate.

Figure 4

Cumulative proportion of the number of new cases of 10 000 simulations from the stochastic SIR model with initial values of (S = 10, I = 1, R = 0) for each NV and HE replicate.

Figure 5

Number of new cases represented as the proportion of 10 000 simulations from the stochastic SIR model with initial values of (S = 10, I = 1, R = 0) for each HE replicate.