Table 2

Outcome measures and schedule of intervention and assessments

	Period	Enrolment	Allocation or baseline	Treatment period				Follow-up period
	Time points	−1 week	0 weeks	5 weeks	10 weeks	15 weeks	20 weeks	24 weeks
Enrolment	Eligibility screening	x
	Informed consents	x
	Characteristic	x
	Medical history	x
	Laboratory examination		x
	Biological specimens		x
	Random allocation		x
Intervention	Probiotics
	Placebo
Assessment	BMI for age z-scores will be analysed using WHO AnthroPlus		x	x	x	x	x	x
	IgG will be assessed by electrochemiluminescence immunoassay		x		x		x
	α and β-gut microbiota diversity will be assessed by NGS of 16S ribosomal RNA		x				x
	Monoclonal antibody affinity against SARS-CoV-2 will be assessed by competitive ELISA		x		x		x
	sIgA specific to SARS-CoV-2 titre concentrations assessed by competitive ELISA		x		x		x
	Dietary quality will be assessed by the Healthy Eating Index 2015 from 24-hour recall data		x	x	x	x	x	x