PRRS mass vaccination programme: Can we minimise the stress on sows due to vaccination?

A user from Italy asked the following question through the section "Updates on PRRS" of PRRSControl site:

"On a PRRS unstable farm with a mass vaccination (4 times/year) programme for PRRS and 3 times/year vaccination programme for Aujeszky´s disease, both for sows, we want to minimise stress due to vaccination, what do you recommend?"

See Enric Mateu's answer below...