Statistical analyses
infection
REGRESSION, INFECTION
To study the effects of probiotic supplementation on SARS-CoV-2 specific antibody response after infection or vaccination, only participants who seroconverted during the study were included in the statistical analyses. Seroconversion was determined by having serum anti-S IgG concentrations above 34 BAU/ml (Örebro University Hospital’s cutoff). Participants who were seropositive at baseline were excluded from the analyses. Participants with serum anti-N IgG titers above 11.8 BAU/ml (manufacturer’s cutoff) in addition to anti-S IgG concentrations > 34 BAU/ml were considered infected. For statistical analyses, participants were divided into three separate sub-groups based on their antibody status and information regarding vaccinations: Infected (serum anti-N IgG > 11.8 BAU/ml, and no reported vaccinations before the visit), partially vaccinated (serum anti-N IgG < 11.8 BAU/ml, serum anti-S IgG > 34 BAU/ml, and reported to have received one vaccine dose before the visit), and fully vaccinated (serum anti-N IgG < 11.8 BAU/ml, serum anti-S IgG > 34 BAU/ml, and reported to have received two vaccine doses before the visit). Antibody values from the study visit in which the participant first fulfilled one of these categories were used in all analyses. To test for group-wise differences in virus-specific antibody levels, the individual antibody values were tested for normality using the D’Agostino-Pearson normality test. If the antibody values failed the normality test, the values were log-transformed and tested again for normality. Based on the results of the normality tests, differences in serum antibody titers between the two study arms were tested with independent t-test using the log-transformed values except the nAB titers which were analyzed with Mann-Whitney test using the raw values. Multivariate linear regression analysis was performed to adjust for age, body mass index (BMI), time between vaccination and sampling, as well as time between the vaccine doses. Correlation between serum IgG and IgA antibody levels and the number of days between sampling and vaccination was analyzed with Pearson’s correlation test. Correlations between nAB titers and IgG and IgA antibody levels, as well as nAB titers and the number of days between sampling and vaccination were analyzed using Spearman’s correlation test. Intention-to-treat (ITT) population was used as the primary analysis. For the per-protocol analysis, participants with major protocol deviations during the study period were excluded. Major protocol deviations were compliance below 70% throughout the trial or running out of study product before the final visit, continuous and prolonged use of anti-inflammatory medication not related to infection symptoms during the study period, continuous and prolonged use of other probiotic or prebiotic supplements during the study period, use of antibiotics during the study period, or any other possible deviations the principal investigator considered as a major breach of the study protocol. All values are reported as median (minimum-maximum) unless otherwise stated. All statistical analyses were performed using GraphPad Prism 9 and group-wise differences were deemed significant if 