A randomised, triple-blinded, placebo-controlled study with BioGaia's probiotic Protectis was shown to increase SARS-CoV-2 antibody titres in healthy volunteers when 28 days or more had elapsed from vaccination. It suggests that probiotic supplementation may enhance the long-term protection against breakthrough infections. The study included 159 healthy adults without prior SARS-CoV-2 infection or COVID-19 vaccination.

The participants consumed either BioGaia Protectis (Limosilactobacillus reuteri DSM 17938) + 10 microgram vitamin D3, or a placebo product that contained only vitamin D3 for 6 months. During the intervention period 12 subjects were infected, and 17 were fully vaccinated and sampled 28 days or more postvaccination. The purpose of the study was to investigate the effect of probiotic supplementation of BioGaia Protectis on SARS-CoV-2 specific antibody responses after natural infection or vaccination.

Even though the primary outcome, based on infected subjects, failed to reach a statistically significant difference between the intervention groups, vaccinated individuals who consumed BioGaia Protectis showed significantly increased IgA antibody levels when 28 days or more had elapsed from vaccination. The study was conducted at the Nutrition-Gut-Brain Interactions Research Centre, Örebro University, Sweden. The results suggest that BioGaia Protectis may improve long-term protection against breakthrough infections after vaccination.

The same type of antibodies that are increased by probiotics in this study have previously been shown to be protective against breakthrough infections.