On October 30, 2020, this report was posted online as an MMWR Early Release.
Improved understanding of transmission of SARS-CoV-2, the virus that causes coronavirus
disease 2019 (COVID-19), within households could aid control measures. However, few
studies have systematically characterized the transmission of SARS-CoV-2 in U.S. households
(
1
). Previously reported transmission rates vary widely, and data on transmission rates
from children are limited. To assess household transmission, a case-ascertained study
was conducted in Nashville, Tennessee, and Marshfield, Wisconsin, commencing in April
2020. In this study, index patients were defined as the first household members with
COVID-19–compatible symptoms who received a positive SARS-CoV-2 reverse transcription–polymerase
chain reaction (RT-PCR) test result, and who lived with at least one other household
member. After enrollment, index patients and household members were trained remotely
by study staff members to complete symptom diaries and obtain self-collected specimens,
nasal swabs only or nasal swabs and saliva samples, daily for 14 days. For this analysis,
specimens from the first 7 days were tested for SARS-CoV-2 using CDC RT-PCR protocols.
†
A total of 191 enrolled household contacts of 101 index patients reported having no
symptoms on the day of the associated index patient’s illness onset, and among these
191 contacts, 102 had SARS-CoV-2 detected in either nasal or saliva specimens during
follow-up, for a secondary infection rate of 53% (95% confidence interval [CI] = 46%–60%).
Among fourteen households in which the index patient was aged <18 years, the secondary
infection rate from index patients aged <12 years was 53% (95% CI = 31%–74%) and from
index patients aged 12–17 years was 38% (95% CI = 23%–56%). Approximately 75% of secondary
infections were identified within 5 days of the index patient’s illness onset, and
substantial transmission occurred whether the index patient was an adult or a child.
Because household transmission of SARS-CoV-2 is common and can occur rapidly after
the index patient’s illness onset, persons should self-isolate immediately at the
onset of COVID-like symptoms, at the time of testing as a result of a high risk exposure,
or at the time of a positive test result, whichever comes first. Concurrent to isolation,
all members of the household should wear a mask when in shared spaces in the household.
§
The data presented in this report are from an ongoing, CDC-supported study of household
transmission of SARS-CoV-2 in Nashville, Tennessee and Marshfield, Wisconsin, and
include households enrolled during April–September 2020.
¶
Households were eligible if the index patient had symptom onset <7 days before household
enrollment and the household included at least one other person who was not symptomatic
at the time of the index patient’s illness onset and was thus deemed to be at risk.
Characteristics of the index patients, household members, and their interactions were
ascertained using Research Electronic Data Capture (REDCap),** an online application
for data collection, or through paper-based surveys. The 7-day secondary infection
rate was calculated by dividing the number of laboratory-confirmed SARS-CoV-2 infections
identified during the 7-day follow-up period by the number of household members at
risk per 100 population.
††
Because saliva samples are considered an emerging diagnostic approach but are not
yet standard for SARS-CoV-2 detection (
2
), secondary infection rates were also calculated using positive test results from
nasal swab specimens only. To account for household members possibly having been infected
when the index case became ill, secondary infections rates were also conservatively
calculated excluding household members who had positive test results at enrollment.
The study protocol was reviewed and approved by the Vanderbilt University Medical
Center’s and Marshfield Clinic Research Institute’s Institutional Review Board, and
was conducted consistent with applicable federal law and CDC policy.
§§
For this analysis, 101 households (including 101 index patients and 191 household
members) were enrolled and completed ≥7 days of follow-up. The median index patient
age was 32 years (range = 4–76 years; interquartile range [IQR] = 24–48 years); 14
(14%) index patients were aged <18 years, including five aged <12 years and nine aged
12–17 years. Among index patients, 75 (74%) were non-Hispanic White, eight (8%) were
non-Hispanic persons of other races, and 18 (18%) were Hispanic or Latino (Table 1).
Index patients received testing for SARS-CoV-2 a median of 1 day (IQR = 1–2) after
illness onset and were enrolled in the study a median of 4 days (IQR = 2–4) after
illness onset.
TABLE 1
Characteristics of index patients with laboratory-confirmed SARS-CoV-2 infection and
household members enrolled in a prospective study of SARS-CoV-2 household transmission
— Tennessee and Wisconsin, April–September 2020
Characteristic
No. (%)*
Index patients
(n = 101)
Household members
(n = 191)
Median age, yrs (IQR)
32 (24–48)
28 (14–46)
Age group, yrs
<12
5 (5)
32 (17)
12–17
9 (9)
30 (16)
18–49
65 (64)
92 (48)
≥50
22 (22)
37 (19)
Male
41 (41)
88 (46)
Race/Ethnicity
White, non-Hispanic
75 (74)
127 (67)
Other race, non-Hispanic
8 (8)
24 (13)
Hispanic or Latino
18 (18)
40 (21)
Underlying medical condition
Any
22 (22)
37 (19)
Asthma
12 (12)
24 (13)
Other chronic lung disease
0 (0)
2 (1)
Cardiovascular disease
4 (4)
7 (4)
Diabetes
4 (4)
7 (4)
Chronic renal disease
0 (0)
2 (1)
Immunocompromising condition
2 (2)
3 (2)
Smoking/Vaping†
2 (2)
4 (2)
Abbreviation: IQR = interquartile range.
* Percentages might not sum to 100% because of rounding.
† Data available for 98 index cases and 166 household members.
The median number of household members per bedroom was one (IQR = 0.8–1.3). Seventy
(69%) index patients reported spending >4 hours in the same room with one or more
household members the day before and 40 (40%) the day after illness onset. Similarly,
40 (40%) of index patients reported sleeping in the same room with one or more household
members before illness onset and 30 (30%) after illness onset.
Among all household members, 102 had nasal swabs or saliva specimens in which SARS-CoV-2
was detected by RT-PCR during the first 7 days of follow-up, for a secondary infection
rate of 53% (95% CI = 46%–60%) (Table 2). Secondary infection rates based only on
nasal swab specimens yielded similar results (47%, 95% CI = 40%–54%). Excluding 54
household members who had SARS-CoV-2 detected in specimens taken at enrollment, the
secondary infection rate was 35% (95% CI = 28%–43%).
TABLE 2
Rates of secondary laboratory-confirmed SARS-CoV-2 infections among household members
enrolled in a prospective study of SARS-CoV-2 household transmission — Tennessee and
Wisconsin, April–September 2020
Characteristic
Laboratory-confirmed SARS-CoV-2 infections/Household members at risk
Secondary infection rate
% (95% CI)*
All household members
102/191
53 (46–60)
Nasal swab–positive tests only
89/191
47 (40–54)
RT-PCR–negative at enrollment
48/137
35 (28–43)
Index patient age group, yrs
<12
9/17
53 (31–74)
12–17
11/29
38 (23–56)
18–49
64/116
55 (46–64)
≥50
18/29
62 (44–77)
Index patient sex
Female
66/108
61 (52–70)
Male
36/83
43 (33–54)
Index patient race/ethnicity
White, non-Hispanic
71/139
51 (43–59)
Other race, non-Hispanic
9/17
53 (31–74)
Hispanic or Latino
22/35
63 (46–77)
Household member age group, yrs
<12
18/32
57 (39–72)
12–17
14/30
47 (30–64)
18–49
54/92
59 (48–68)
≥50
16/37
43 (29–59)
Household member sex
Female
52/103
50 (41–60)
Male
50/88
57 (46–67)
Household member race/ethnicity
White, non-Hispanic
67/127
53 (44–61)
Other race, non-Hispanic
9/24
38 (21–57)
Hispanic or Latino
26/40
65 (50–78)
Household size, no. of persons
2
26/38
68 (53–81)
3
25/41
61 (46–74)
4
18/40
45 (31–60)
≥5
33/72
46 (35–57)
Abbreviations: CI = confidence interval; RT-PCR = reverse transcription–polymerase
chain reaction.
* Secondary infection rate, and 95% CI, estimated over 7 days of follow-up. Enrolled
household members who did not report symptoms at time of illness onset in the index
case-patient were considered at risk.
Forty percent (41 of 102) of infected household members reported symptoms at the time
SARS-CoV-2 was first detected by RT-PCR. During 7 days of follow-up, 67% (68 of 102)
of infected household members reported symptoms, which began a median of 4 days (IQR = 3–5)
after the index patient’s illness onset. The rates of symptomatic and asymptomatic
laboratory-confirmed SARS-CoV-2 infection among household members was 36% (95% CI = 29%–43%)
and 18% (95% CI = 13%–24%), respectively.
Discussion
In this ongoing prospective study that includes systematic and daily follow-up, transmission
of SARS-CoV-2 among household members was common, and secondary infection rates were
higher than have been previously reported (
1
,
3
–
7
). Secondary infections occurred rapidly, with approximately 75% of infections identified
within 5 days of the index patient’s illness onset. Secondary infection rates were
high across all racial/ethnic groups. Substantial transmission occurred whether the
index patient was an adult or a child.
Several studies have reported estimates of household transmission, largely from contact
tracing activities, with limited follow-up and testing of household members or delayed
enrollment relative to index patient identification (
3
–
5
,
7
). These different approaches to ascertain infections could explain the higher secondary
infection rates observed in this study relative to other estimates. In addition, other
studies, particularly those conducted abroad, might have found lower secondary infection
rates because of rapid isolation of patients in facilities outside households or different
adoption of control measures, such as mask use, in the home (
3
–
5
,
7
,
8
).
Because prompt isolation of persons with COVID-19 can reduce household transmission,
persons who suspect that they might have COVID-19 should isolate, stay at home, and
use a separate bedroom and bathroom if feasible. Isolation should begin before seeking
testing and before test results become available because delaying isolation until
confirmation of infection could miss an opportunity to reduce transmission to others.
Concurrently, all household members, including the index patient, should start wearing
a mask in the home, particularly in shared spaces where appropriate distancing is
not possible. Close household contacts of the index patient should also self-quarantine,
to the extent possible, particularly staying away from those at higher risk of getting
severe COVID-19. To complement these measures within the household, a potential approach
to reduce SARS-CoV-2 transmission at the community level would involve detecting infections
before onset of clinical manifestations; this would require frequent and systematic
testing in the community with rapidly available results to enable prompt adoption
of preventive measures. The feasibility and practicality of this approach is undergoing
extensive discussion (
9
) and study. This ongoing household transmission study will provide critical data
regarding the recommended timing and frequency of testing.
An important finding of this study is that fewer than one half of household members
with confirmed SARS-CoV-2 infections reported symptoms at the time infection was first
detected, and many reported no symptoms throughout 7 days of follow-up, underscoring
the potential for transmission from asymptomatic secondary contacts and the importance
of quarantine. Persons aware of recent close contact with an infected person, such
as a household member, should quarantine in their homes and get tested for SARS-CoV-2.
¶¶
The findings in this study are subject to at least four limitations. First, the initial
household member who experienced symptoms was considered the index patient, but it
is possible that other household members were infected concurrently but developed
symptoms at different times or remained asymptomatic. Although households were enrolled
rapidly, several infections among household members were already detectable at enrollment,
underscoring the rapid spread of infections within households and the challenge inherent
in conclusively reconstructing the transmission sequence. Second, although living
in the same household might impart a high risk of acquiring infection, some infections
might have originated outside the household, leading to higher apparent secondary
infection rates. Third, respiratory samples were self-collected; although this might
have reduced the sensitivity of detections, studies have reported performance comparable
to clinician-collected samples (
10
). Finally, the families in the study might not be representative of the general U.S.
population.
These findings suggest that transmission of SARS-CoV-2 within households is high,
occurs quickly, and can originate from both children and adults. Prompt adoption of
disease control measures, including self-isolating at home, appropriate self-quarantine
of household contacts, and all household members wearing a mask in shared spaces,
can reduce the probability of household transmission.
Summary
What is already known about this topic?
Transmission of SARS-CoV-2 occurs within households; however, transmission estimates
vary widely and the data on transmission from children are limited.
What is added by this report?
Findings from a prospective household study with intensive daily observation for ≥7
consecutive days indicate that transmission of SARS-CoV-2 among household members
was frequent from either children or adults.
What are the implications for public health practice?
Household transmission of SARS-CoV-2 is common and occurs early after illness onset.
Persons should self-isolate immediately at the onset of COVID-like symptoms, at the
time of testing as a result of a high risk exposure, or at time of a positive test
result, whichever comes first. All household members, including the index case, should
wear masks within shared spaces in the household.