Safety - Spread of the Porcilis PRRS vaccinal virus

One of the most important safety concerns with live PRRSV vaccines is the ability of the vaccinal virus to spread. The following laboratory and field trials were done to determine the ability of the Porcilis PRRS vaccinal virus to spread.

Determining the spread of vaccinal virus

A critical parameter to determine spread is the reproduction ratio of a virus (Ro). Ro represents the number of animals that will be infected by one infected animal.

• Ro > 1- a virus will spread through a population, eventually infecting all the animals.
• Ro < 1- an infection will die out after a few rounds of replication.

Experiment 1

Method
Part 1
4 groups of 10 pigs each were used. In each group:

• 4 pigs - vaccinated with Porcilis PRRS
• 6 pigs left as unvaccinated sentinels

Part 2
Method as in Part 1. Vaccination with with live modified vaccine based on the American serotype.

Results

Results Porcilis PRRS

Group	Seroconverted sentinels	Ro
A	2	0.37
B	0	0
C	0	0
D	0	0
Total	0	0.11

Results American Serotype vaccine

Group	Seroconverted sentinels	Ro
A	3	0.53
B	3	0.53
C	1	0.2
D	1	0.2
Total	8	0.38

The R0 value observed in other trials with Porcilis PRRS has been below 0.1 (0.03-0.06)

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Field Study

In this field study, performed in a small PRRS virus negative herd in Denmark, the ability of Porcilis PRRS to spread from vaccinated to unvaccinated fully susceptible pigs and sows were assessed (Astrup, P. et al., 2002).

Method

• 78 pigs in 9 pens were vaccinated with Porcilis PRRS
• 17 pigs in 2 pens situated between the vaccinated pens were left unvaccinated.
• 25 pregnant sows were left unvaccinated and housed in the same room in rows opposite to the vaccinated pigs.

Results

• Almost all vaccinated pigs seroconverted (Table 3)
• 4 sows and one sentinel pig seroconverted (Table 3)
• Virus was detected from 23 vaccinated pigs and 1 sow at 3 weeks and from 1 vaccinated pig at 6 weeks after the vaccination only
• The R 0 value was calculated to 0.06

Incidence of seroconversion

Sample time	Vaccinated	Sentinel sows	Sentinel pigs
Start	0	0	0
Week 3	73	0	0
Week 6	2	4	0
Week 9	1	0	0
Week 12	0	0	1
Week 15	-	-	0
Week 18	-	-	0

The study showed that spreading of Porcilis PRRS vaccine virus was very limited to only a few animals and ceased spontaneously.

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Conclusion

1. The virus will rarely spread to pigs when there is no nose-to-nose contact.
2. Only limited spread will occur when susceptible pigs are mixed with vaccinated pigs in direct nose-to-nose contact.
3. The reproduction ratio when vaccinated and susceptible pigs are mixed is 0.1.
4. The chance of the reproduction ratio being 1 or greater is 1 out of 100,000.