One month after vaccinating sows and piglets with PRRSv MLV (VR2332 strain), is it normal and safe to detect the vaccine strain in over 20% of the serum samples?

Published controlled studies have estimated the length of viremia after PRRSv vaccination with MLV vaccines to be 29 days on average (range 10–42 days).

However, PRRSv viral RNA has been detected in tonsil scrapings up to 90 days post vaccination.

We recently performed a challenge study where we followed the pigs for 62 days after vaccination also with the VR2332 strain and found positive pigs up to 42-62 days after PRRSv vaccination (Kristensen et al., 2018).

So, the answer is YES.

Extended viremia after vaccination

It’s not unusual to see extended viremia after vaccination in 20% of the animals.

In the same study, we found viral shedding (by PCR) up to 14 days after vaccination.

Thus, even though some animals are PRRSv positive beyond 2 weeks after vaccination, the risk that these animals will transmit the virus horizontally to other vaccinated animals is probably low, but the persistence of vaccine virus for two-three months after vaccination emphasize the importance of keeping vaccinated animals isolated from non-vaccinated animals for a sustained time period prior to mingling.

Which is the risk of having over 20% PRRSv positive serum samples 1 month after vaccination?

In respect to risks in the herd, if these PRRSv vaccine viraemic animals are housed in a stable seropositive sow herd the consequence will probably be low.

However, if they excrete virus when housed in the farrowing unit they may transmit the virus to piglets in the farrowing section and by that lead to weaning of PRRSv positive piglets.

In my opinion, mass vaccination of herds, including vaccination of sows in the last trimester, will lead to some incidences of viral transfer to fetuses and then birth of viraemic pigs, but there is a lack of control studies on the negative impact following PRRSv mass-vaccination.

The situation is probably very herd specific and also depend on the vaccination strategy and the vaccine used.

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