A PRRS positive farm that reached stability has suffered the disease again. What should be done?

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

"A PRRS positive Site I farm, that had reached stability, has suffered the disease again. What management should be carried out: close the farm and infect the sows again as it was done on the first occasion? At this moment, PRRS vaccine is not used"

See Enric Mateu's answer below...