The answer to the question depends on whether it refers to naïve sows vaccinated against PRRS for the first time or to animals that have received multiple vaccinations with a MLV PRRS vaccine.
Naïve sows vaccinated with MLV PRRS vaccine for the first time
When naïve sows are vaccinated for the first time (modified live PRRS vaccine) about half of them seroconvert between 7-10 days after vaccination and by day 21 after vaccination all sows develop antibodies against PRRSV.
ELISA values (optical densities or S/P ratios) may vary very much between animals.
In contrast, when sows are multiple vaccinated with PRRS vaccines, a variable proportion of them may test negative by ELISA.
Seroconversion of multivaccinated sows
Moreover, in many animals already seropositive, re-vaccination with a PRRS vaccine does not produce a clear seroconversion in ELISA.
It is worth to note, that by using different ELISA kits, the individual animals testing negative may be different. In other words, the ELISA used has influence in the result.
In our experience, when animals are examined using several tests simultaneously, overall less than 5% of the vaccinated sows can be catalogued as real low or non-responders. Since ELISA results are not correlated to protection, the real significance of those sows testing negative is difficult to say.
In summary, ELISA testing is adequate to assess PRRS seroconversion after initial vaccination or to check previous contact with the virus but is not reliable as a mean to monitor PRRS vaccination programs in multiple vaccinated sows.
If you want to learn more about the monitoring diagnostic techniques and the interpretation of each situation, visit chapter 9 “Monitoring” of this site.
Centre for Research on Animal Health (CReSA), Barcelona University (UAB) – Spain