2.3. Study Visits
At the baseline visit, sociodemographic information, clinical details, and history of COVID-19 vaccination were obtained from study subjects or extracted from their medical notes. Faecal specimens were collected for microbiota analysis. They were provided with study packages during the visit. Study participants were followed up in a research clinic with faecal specimen collection for metagenomic sequencing at 1 month and 3 months after the second dose of the vaccine. Stool specimens were tested for microbial analysis by metagenomic sequencing. We documented any hospital admission in each follow-up visit. Compliance with taking the SIM01/placebo was assessed by returned sachet pack counts at 1 and 3 months. No probiotic or prebiotic preparations were permitted during the study period. The use of antibiotics should be avoided unless there was a clinical need. Study participants with an intake of prohibited medication or antibiotics during the study period remained in the study and the outcome was assessed. The reported intake of prohibited medication or antibiotics was recorded. We documented the number and reasons for early withdrawals.