Household transmission of SARS-CoV-2 from unvaccinated asymptomatic and symptomatic household members with confirmed SARS-CoV-2 infection: an antibody-surveillance study

Maala Bhatt; Amy C. Plint; Ken Tang; Richard Malley; Anne Pham Huy; Candice McGahern; Jennifer Dawson; Martin Pelchat; Lauren Dawson; Terry Varshney; Corey Arnold; Yannick Galipeau; Michael Austin; Nisha Thampi; Fuad Alnaji; Marc-André Langlois; Roger L. Zemek

doi:10.9778/cmajo.20220026

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Figure 1:
Flow diagram showing screening and enrolment of study population. *Age less than 18 years. †Age 18 years or more. Note: RT-PCR = reverse transcription polymerase chain reaction.
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Figure 2:
Risk factors for household transmission. Note: CI = confidence interval, HC = household contact, I = index participant, OR = odds ratio.

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Table 1:

Baseline characteristics of people who tested positive for SARS-CoV-2 by reverse transcription polymerase chain reaction (index participants) and household contacts

Characteristic	Index participants, no. (%)^*			Household contacts, no. (%)^*

	Overall n = 180	Children n = 74	Adults n = 106	Overall n = 515	Children n = 266	Adults n = 249
Age, median (IQR), yr	33.9 (9.6–41.5)	8.2 (5.4–12.5)	40.5 (36.3–45.3)	17.1 (8.7–41.4)	8.9 (5.2–12.9)	41.9 (36.6–47.0)

Age group, yr

≤ 3	11 (6.1)	11 (14.9)	–	47 (9.1)	47 (17.7)	–

4–11	43 (23.9)	43 (58.1)	–	142 (27.6)	142 (53.4)	–

12–17	20 (11.1)	20 (27.0)	–	77 (15.0)	77 (28.9)	–

18–29	9 (5.0)	–	9 (8.5)	17 (3.3)	–	17 (6.8)

30–39	42 (23.3)	–	42 (39.6)	80 (15.5)	–	80 (32.1)

40–49	44 (24.4)	–	44 (41.5)	108 (21.0)	–	108 (43.4)

≥ 50	11 (6.1)	–	11 (10.4)	44 (8.5)	–	44 (17.7)

Sex

Female	81 (45.0)	24 (32.4)	57 (53.8)	275 (53.4)	139 (52.3)	136 (54.6)

Male	98 (54.4)	50 (67.6)	48 (45.3)	238 (46.2)	126 (47.4)	112 (45.0)

Missing	1 (0.6)	0 (0.0)	1 (0.9)	2 (0.4)	1 (0.4)	1 (0.4)

Indigenous

Yes	1 (0.6)	0 (0.0)	1 (0.9)	17 (3.3)	9 (3.4)	8 (3.2)

No	179 (99.4)	74 (100.0)	105 (99.0)	494 (95.9)	255 (95.9)	239 (96.0)

Missing	0 (0.0)	0 (0.0)	0 (0.0)	4 (0.8)	2 (0.8)	2 (0.8)

Race or ethnicity^†

Black	15 (8.3)	7 (9.5)	8 (7.5)	44 (8.6)	27 (10.2)	17 (6.9)

Asian	3 (1.7)	2 (2.7)	1 (0.9)	18 (3.5)	10 (3.8)	8 (3.2)

South Asian	0 (0.0)	0 (0.0)	0 (0.0)	4 (0.8)	3 (1.1)	1 (0.4)

West Asian/Arab	15 (8.3)	7 (9.5)	8 (7.5)	44 (8.6)	25 (9.5)	19 (7.7)

Latin American	5 (2.8)	5 (6.8)	0 (0.0)	8 (1.6)	4 (1.5)	4 (1.6)

White	149 (82.8)	62 (83.8)	87 (82.1)	415 (81.1)	216 (81.8)	199 (80.2)

Other^‡	2 (1.1)	1 (1.4)	1 (0.9)	3 (0.6)	2 (0.8)	1 (0.4)

Missing	0 (0.0)	0 (0.0)	0 (0.0)	3 (0.6)	2 (0.8)	1 (0.4)

Underlying comorbidity

Yes	46 (25.6)	7 (9.5)	39 (36.8)	93 (18.0)	42 (15.8)	51 (20.5)

No	134 (74.4)	67 (90.5)	67 (63.2)	419 (81.4)	222 (83.4)	197 (79.1)

Missing	0 (0.0)	0 (0.0)	0 (0.0)	3 (0.6)	2 (0.8)	1 (0.4)

Smoking or vaping	46 (25.6)	5 (6.8)	41 (38.7)	142 (27.6)	14 (5.3)	128 (51.4)

Daily/frequently	6 (13.0)	1 (20.0)	5 (12.2)	20 (14.1)	0 (0.0)	20 (15.6)

Sometimes/rarely	12 (26.1)	2 (40.0)	10 (24.4)	25 (17.6)	6 (42.8)	19 (14.8)

Past smoker/vaper	28 (60.9)	2 (40.0)	26 (63.4)	94 (66.2)	6 (42.8)	88 (68.8)

Missing	0 (0.0)	0 (0.0)	0 (0.0)	3 (2.1)	2 (14.3)	1 (0.8)

Working outside home^§

Yes	52 (28.9)	3 (4.1)	49 (46.2)	88 (17.1)	4 (1.5)	84 (33.7)

No	128 (71.1)	71 (95.9)	57 (53.8)	424 (82.3)	260 (97.7)	164 (65.9)

Missing	0 (0.0)	0 (0.0)	0 (0.0)	3 (0.6)	2 (0.8)	1 (0.4)

Student^‡

Yes	76 (42.2)	69 (93.2)	7 (6.6)	261 (50.7)	245 (92.1)	16 (6.4)

No	104 (57.8)	5 (6.8)	99 (93.4)	251 (48.7)	19 (7.1)	232 (93.2)

Missing	0 (0.0)	0 (0.0)	0 (0.0)	3 (0.6)	2 (0.8)	1 (0.4)

Symptom burden^¶

Asymptomatic	19 (10.6)	13 (17.6)	6 (5.7)	260 (50.5)	140 (52.6)	120 (48.2)

Mild	154 (85.6)	60 (81.1)	94 (88.7)	248 (48.2)	124 (46.6)	124 (49.8)

Moderate	5 (2.8)	1 (1.4)	4 (3.8)	3 (0.6)	0 (0.0)	3 (1.2)

Severe	2 (1.1)	0 (0.0)	2 (1.9)	1 (0.2)	0 (0.0)	1 (0.4)

Missing	0 (0.0)	0 (0.0)	0 (0.0)	3 (0.6)	2 (0.8)	1 (0.4)

Note: IQR = interquartile range.
↵* Except where noted otherwise.
↵† Self-reported. Participants could choose more than 1 category.
↵‡ Unspecified selection of more than 1 category.
↵§ At the time the participant or a household member tested positive.
↵¶ Mild = no hospital admission, moderate = hospital admission but not intensive care unit admission, severe = intensive care unit admission.

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Table 2:

Proportion of household contacts testing positive for SARS-CoV-2 antibodies by age of index participant

Group; age, yr	Household contact, all ages		Child household contacts		Adult household contacts		OR (95% CI)^†

	n/N^*	% (95% CI)	n/N^*	% (95% CI)	n/N^*	% (95% CI)
Index participants, all ages	239/487	49.1 (42.9–55.3)	130/246	52.8 (45.3–60.2)	109/241	45.2 (38.4–52.2)	0.8 (0.6–1.1)

Child index participants	72/194	37.1 (27.7–47.6)	23/65	35.4 (23.1–50.0)	49/129	38.0 (28.6–48.4)	1.2 (0.8–2.0)

≤ 3	6/32	18.8 (7.0–41.3)	1/11	9.1 (1.0–50.3)	5/21	23.8 (7.3–55.5)	3.2 (0.2–48.0)

4–11	47/113	41.6 (29.0–55.4)	17/40	42.5 (26.7–60.0)	30/73	41.1 (28.4–55.1)	1.0 (0.6–1.7)

12–17	19/49	38.8 (20.8–60.4)	5/14	35.7 (12.4–68.5)	14/35	40.0 (23.3–59.3)	1.6 (0.7–3.7)

Adult index participants	167/293	57.0 (49.5–64.2)	107/181	59.1 (50.3–67.4)	60/112	53.6 (44.8–62.1)	0.9 (0.6–1.3)

18–29	17/32	53.1 (32.3–72.9)	7/15	46.7 (26.0–68.6)	10/17	58.8 (34.0–79.9)	1.6 (0.8–3.3)

30–49	130/230	56.5 (47.7–64.9)	90/152	59.2 (49.4–68.3)	40/78	51.3 (40.7–61.8)	0.8 (0.6–1.2)

≥ 50	20/31	64.5 (43.9–80.9)	10/14	71.4 (34.9–92.1)	10/17	58.8 (39.0–76.1)	0.7 (0.2–2.8)

Note: CI = confidence interval, OR = odds ratio.
↵* n/N = number of household contacts with SARS-CoV-2 antibodies in households in which the index participant was in a specified age category/total number of household contacts in households in which the index participant was in specified age category.
↵† Odds that an adult versus a child in the household would become infected from index participant of a given age.

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Table 3:

Proportion of household contacts who tested positive for SARS-CoV-2 antibodies by age group and symptom status of index participant

Age group; symptom status	Household contacts, all ages		Child household contacts		Adult household contacts		OR (95% CI)^†

	n/N^*	% (95% CI)	n/N^*	% (95% CI)	n/N^*	% (95% CI)
Index participants, all ages

Asymptomatic	18/51	35.3 (18.5–56.8)	9/21	42.9 (20.4–68.7)	9/30	30.0 (14.3–52.4)	0.6 (0.3–1.4)

Symptomatic	221/436	50.7 (44.2–57.1)	121/225	53.8 (46.0–61.4)	100/211	47.4 (40.2–54.7)	0.9 (0.6–1.2)

OR (95% CI)^‡	0.6 (0.2–1.4)		0.7 (0.3–1.9)		0.5 (0.2–1.3)

Child index participants

Asymptomatic	10/34	29.4 (12.3–55.3)	4/11	36.4 (15.7–63.7)	6/23	26.1 (9.7–53.6)	0.6 (0.3–1.3)

Symptomatic	62/160	38.8 (28.3–50.4)	19/54	35.2 (21.3–52.1)	43/106	40.6 (30.0–52.1)	1.4 (0.8–2.4)

OR (95% CI)^§	0.7 (0.2–2.3)		1.3 (0.4–4.4)		0.6 (0.2–1.9)

Adult index participants

Asymptomatic	8/17	47.1 (13.1–84.0)	5/10	50.0 (10.9–89.1)	3/7	42.9 (11.0–82.0)	0.9 (0.2–4.1)

Symptomatic	159/276	57.6 (50.1–64.8)	102/171	59.6 (50.8–67.9)	57/105	54.3 (45.4–62.9)	0.9 (0.6–1.3)

OR (95% CI)^¶	0.7 (0.1–3.0)		0.7 (0.1–3.6)		0.7 (0.1–3.6)

Note: CI = confidence interval, OR = odds ratio.
↵* n/N = number of household contacts with SARS-CoV-2 antibodies in households in which the index participant was in a specified age category/total number of household contacts in households in which the index participant was in specified age category.
↵† Odds that a child versus an adult household contact would become infected from the index participant in the given category.
↵‡ Odds that a given type of household contact would become infected from an asymptomatic versus a symptomatic index participant of any age.
↵§ Odds that a given type of household contact would become infected from an asymptomatic versus a symptomatic child index participant.
↵¶ Odds that a given type of household contact would become infected from an asymptomatic versus a symptomatic adult index participant.