Table 1.

Effectiveness of the CoronaVac COVID-19 vaccine in preventing symptomatic COVID-19 outcomes in children 3–5 years of age in the study cohort according to immunization status, 6 December 2021 through 26 February 2022^a

Immunization status		Cases		Vaccine effectiveness (95% CI)
	Person-days	No.	Incidence rate per 1,000 person-days	Weighted, standard adjustmentb	Weighted, stratified analysisb
Symptomatic COVID-19
Unvaccinated	29,404,535	7,555	0.2569	–	–
CoronaVac	18,499,492	4,562	0.2466	37.9	38.2
(≥14 days after second dose)	(36.1; 39.6)	(36.5; 39.9)
Hospitalization
Unvaccinated	29,579,595	62	0.0021	–	–
CoronaVac	18,990,209	23	0.0012	65.2	64.6
(≥14 days after second dose)	(50.4; 75.6)	(49.6; 75.2)
Admission to ICU
Unvaccinated	29,580,825	9	0.0003	–	–
CoronaVac	18,993,888	3	0.0002	68.8	69.0
(≥14 days after second dose)	(18.0; 88.1)	(18.6; 88.2)

^aWe classified participants’ status into two categories during the study period: unvaccinated and fully immunized (≥14 days after receiving the second dose of CoronaVac). The days between the first dose vaccine administration and the full immunization were excluded from the at-risk person-time. We provide the results for the standard and stratified versions of the Cox hazard models using inverse probability of treatment weighting.

^bThe analyses were adjusted for age, sex, region of residence, health insurance category (a proxy of household income), nationality and whether the patient had underlying conditions that have been associated with severe COVID-19 in children, coded as described in Supplementary Table 1. The standard and stratified versions of the extended Cox proportional hazard models were fit to test the robustness of the estimates to model assumptions.