I don't know of any free certificates for Epi, but Coursera does offer free courses to obtain certification (limited). I have certificates but it's relevant for oral health via the American Dental Association (ADA) and American Psychiatric Association (APA) during the peak of COVID-19. I don't believe these would be relevant for you.
Maybe look into infectious disease certifications or research.
Check the link below
https://www.coursera.org/learn/disease-clusters
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Hope this helps =)