I have been trying to do PEDV serum neutralizing assay When

I am working on a vaccine study and I have been trying to do PEDV serum neutralizing test (SNT)with my vaccinated and control serum samples. How ever when i performed SNT I couldn't observe any CPE effect either in control or vaccinated groups.My question is do I have to add trypsin to the post inoculation medium? I would appreciate it if you give me a little help.