Table 3. Risk of Severe COVID-19 Infection in Nonimmune Family Members.

No. of individuals with immunity in each family	No. of nonimmune individuals at risk	Incident COVID-19 infection, No. (%)	HR (95% CI)a
Families with 2 members
0	1 081 408	803 (0.7)	1 [Reference]
1	139 896	101 (0.7)	0.88 (0.71-1.08)
Families with 3 members
0	180 537	340 (0.19)	1 [Reference]
1	61 730	61 (0.10)	0.50 (0.38-0.66)
2	19 903	7 (0.04)	0.20 (0.10-0.43)
Families with 4 members
0	33 684	164 (0.49)	1 [Reference]
1	13 290	32 (0.24)	0.45 (0.31-0.67)
2	6510	4 (0.06)	0.11 (0.04-0.30)
3	4502	1 (0.02)	0.05 (0.01-0.36)

Abbreviation: HR, hazard ratio.

^a Cox proportional hazards regression models were used to calculate HR, with the number of immune family members as exposure and COVID-19 infection that was severe enough to result in a hospital stay in nonimmune family members as the outcome. Analyses were performed separately for families with 2 to 4 members, using families with no immunity as the reference. Immunity was acquired from either a previous COVID-19 infection or full vaccination. The regression models were adjusted for age.