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. 2021 Apr 9;10:280. [Version 1] doi: 10.12688/f1000research.52439.1

Table 6. Main results of included studies.

Study ID Type of transmission Total number of contacts Cycle threshold Attack rates and/or secondary attack rates (SAR) Notes
Abdulrahman 2020 Community Eid Alfitr
Pre-: 71,553; Post-: 76,384
Ashura
Pre-: 97,560; Post-: 118,548
Not reported Eid Alfitr
Pre-: 2990 (4.2%); Post-: 4987 (6.7%); p <0.001
Ashura
Pre-: 3571 (3.7%); Post-: 7803 (6.6%); p <0.001
The rates of positive tests was significantly
greater after religious events
Adamik 2020 Household Unclear Not reported Unclear: 3553 (AR 26.7%)
Agergaard 2020 Household PCR: 5
Serology: 5
Not reported Index case plus 1 family member tested positive-
PCR
All 5 displayed a serological SARS-CoV-2 N/S IgG
response
Angulo-Bazán 2020 Household 52 households (n=236 people)
4.5±2.5 members per household
Not reported Serology: Amongst cohabitants, SAR was 53.0%
(125 cases): 77.6% of cases were symptomatic
Convenience sampling, no component of
temporality, selection bias
Armann 2020 Local
Household
2045 in Phase 1
1779 in Phase 2
N/A Serology: 12/2045 (0.6%)
Serology: 12/1779 (0.7%)
Arnedo-Pena 2020 Household 745 Not reported 11.1% (95% CI 9.0–13.6)
Baker 2020 Nosocomial 44 Not reported 3/44 (6.8%): 1 of these was also exposed to a
household member with COVID-19.
Recall error and bias, report is limited to
a single exposure, change in mask policy
partway through the exposure period
Baettig 2020 Local 55 Not reported Serologic attack rates: 2/55 (3.6%) Serological testing was positive for the 2
contacts 14 days after index case
Bao 2020 Community 57 index cases
1895 exposed
Not reported SAR was 3.3% at the bathing pool, 20.5% in
the colleagues’ cluster and 11.8% in the family
cluster.
Delayed detection of the activity trajectory of
the primary case, reporting bias, overlap of
close contacts
Basso 2020 Nosocomial 60 HCWs - ≥106 unique high-risk
contacts
Not reported Attack rate: 0/60 (0%)
Serology: 0/60 (0%)
Delay in diagnosing index case, recall bias
Bays 2020 Nosocomial 421 HCWs Not reported 8/421 (1.9%) In all 8 cases, the staff had close contact with
the index patients without sufficient PPE.
Hospital staff developing ILI symptoms were
tested for SARS-CoV-2, regardless of whether
they had contact with an index patient
Bi 2020 Local
Household
Community
1,296 Not reported 98/1286 (7.6%)
Blaisdell 2020 Community 1,022 Not reported 1.8% of camp attendees (10 staff members and
8 campers)
Travel was assumed to be from home state
but intermediate travel might have occurred
Böhmer 2020 Local
Household
241 Not reported 75·0% (95% CI 19·0–99·0; three of four people)
among members of a household cluster in
common isolation, 10·0% (1·2–32·0; two of 20)
among household contacts only together until
isolation of the patient, and 5·1% (2·6–8·9; 11 of
217) among non-household, high-risk contacts.
Boscolo-Rizzo 2020 Household 296 Not reported 74/296 (25.0%, 95% CI 20.2–30.3%) The prevalence of altered sense of smell or
taste was by far lower in subjects negative to
SARS-CoV-2 compared to both positives (p
< 0.001) and non-tested cases (p < 0.001).
Brown 2020 Local 21 Not reported Serologic attack rate: 2/21 (1%) Social desirability bias likely
Burke 2020 Household 445 Not reported 0.45% (95% CI = 0.12%–1.6%) among all close
contacts, and a symptomatic secondary attack
rate of 10.5% (95% CI = 2.9%–31.4%) among
household members.
2 persons who were household members
of patients with confirmed COVID-19 tested
positive for SARS-CoV-2.
Canova 2020 Nosocomial 21 Not reported 0/21 (0%)
Cariani 2020 Nosocomial Unclear 33.6 to 38.03 182 out of 1683 (10.8%) tested positive; 27 of
whom had close contact with COVID-positive
patients
Unclear how many HCWs had close contact;
likelihood of recall bias
Charlotte 2020 Community 27 Not reported 19 of 27 (70%) tested positive High risk of selection bias: The index case-
patients were not identified. A majority of
patients were not tested for SARS-CoV-2
Chaw 2020 Local
Community
1755 Not reported Close contact: 52/1755 (29.6%)
Nonprimary attack rate: 2.9% (95% CI 2.2%–
3.8%)
Potential environmental factors were not
accounted for: relative household size, time
spent at home with others, air ventilation, and
transmission from fomites.
Chen 2020 Aircraft 335 Not reported 16/335 (4.8%) Recall bias. Did not perform virus isolation
and genome sequencing of the virus, which
could have provided evidence of whether viral
transmission occurred during the flight.
Chen 2020a Local
Household
209 Not reported 0/209 (0%)
Chen 2020b Nosocomial 105 Not reported Serology: 18/105 (17.1%)
Chen 2020c Local
Community
Household
Nosocomial
2147 Not reported 110/2147 (5.12%)
Cheng 2020 Household
Nosocomial
2761 Not reported 0.70%
Chu 2020 Community 50 exposed Not reported None for antigen or antibody: 0/50 (0%) Testing was biased toward contacts who knew
the case-patient personally (office co-workers)
or provided direct care for the case-patient
(HCP).
Chu 2020a Household 526 exposed Not reported 48 (9%) (CI 7-12%) Very high risk of selection bias
Contejean 2020 Nosocomial 1344 exposed Not reported 373 (28%)
COVID-19 National Emergency Response Center 2020 Local
Household
Nosocomial
2370 Not reported 13/2370 (0.6%) There were 13 individuals who contracted
COVID-19 resulting in a secondary attack rate
of 0.55% (95% CI 0.31–0.96). There were 119
household contacts, of which 9 individuals
developed COVID-19 resulting in a secondary
attack rate of 7.56% (95% CI 3.7–14.26).
Danis 2020 Local
Household
Chalet: 16
School: 172
Not reported Attack rate: 75% in chalet
Attack rate: 0% in school
Only 73 of 172 school contacts were tested
- all tested negative
Dattner 2020 Household 3353 Not reported Attack rates: 25% in children and 44% adults (45% overall)
Serology: 9/714 (1.3%)
de Brito 2020 Household 24 exposed Not reported RT-PCR: 6/7 (86%); Seropositivity: 18/24 (75%)
Deng 2020 347 Not reported 25/347 (7.2%)
Desmet 2020 Local 84 38.8 Attack rate: 0/84 (0%) Ct reported for only one test result
Dimcheff 2020 Community
Nosocomial
Household
1476 Not reported Seroprevalence 72/1476: 4.9% (95% CI,
3.8%–6.1%)
Dong 2020 Household 259 Not reported 53/259 (20.5%)
Doung-ngern 2020 Local 211 cases plus
839 non-matched controls
Not reported
Draper 2020 Local
Household
Nosocomial
445 Not reported 4/445 (0.9%) None of the 326 aircraft passengers or
4 healthcare workers who were being
monitored close contacts became cases.
Dub 2020 Local
Household
121 Not reported Child index case: No positive cases
Adult index case: 8/51 (16%)
Serology: 6/101 (5.9%)
Expert Taskforce 2020 Local Unclear Not reported Attack rate 20.4% Attack rates were highest in 4-person cabins
(30.0%; n = 18), followed by 3-person cabins
(22.0%; n = 27), 2-person cabins (20.6%; n =
491), and 1-person cabins (8%; n = 6).
Fateh-Moghadam 2020 Community 6690 Not reported 890/6690 (13.3%)
Firestone 2020 Local Unclear Not reported 41 (80%) interviewed patients with primary
event-associated COVID-19 reported having
close contact with others during their infectious
period, with an average of 2.5 close contacts per
patient.
36 (75%) of 48 interviewed patients with primary
event-associated cases reported having close
contact with persons in their household while
infectious, and 17 (35%) reported having
other (social/workplace) close contacts while
infectious.
Fontanet 2020 Local 661 N/A Serology: 171/661 (25.9%, 95%CI 22.6-29.4)
Fontanet 2020a Local 510 N/A Serology: 45/510 (8.8%)
Gan 2020 Local
Household
Community
Unclear Not reported Not reported Family clusters accounted for 86.9% (914/1
050) of cases, followed by party dinners (1.1%)
Ghinai 2020 Community Unclear Not reported Unclear
Gong 2020 Household
Community
Unclear Not reported Unclear
Gu 2020 Local 14 Not reported RT-PCR - 3/14 (21.4%)
Serology - 2/14 (14.3%)
Hamner 2020 Local 60 Not reported Confirmed: 32/60 (53.3%)
Probable: 20/60 (33.3%)
Han 2020 Community 192 Not reported 7/192 (3.7%)
Heavey 2020 Local 1155 Not reported 0/1155 (0%)
Helsingen 2020 Local Training arm: 1,896
Nontraining arm: 1,868
Not reported 11/1896 (0.8%) vs 27/1868 (2.4%); P=0.001
Hendrix 2020 Local 139 exposed Not reported 0% Six close contacts of stylists A and B outside
of salon A were identified: four of stylist
A and two of stylist B. All four of stylist A’s
contacts later developed symptoms and had
positive PCR test results for SARS-CoV-2.
These contacts were stylist A’s cohabitating
husband and her daughter, son-in-law, and
their roommate, all of whom lived together
in another household. None of stylist B’s
contacts became symptomatic.
Hirschman 2020 Household
Community
58 Not reported 27/58 (47%)
Hobbs 2020 Local
Household
Community
397 Not reported Not reported
Hoehl 2020 Local
Community
825 children and 372 staff: 7,366 buccal
mucosa swabs and 5,907 anal swabs
Not reported 0% viral shedding in children; 2/372 (0.5%)
shedding for staff. No inapparent transmissions
were observed
Study was conducted in the summer of 2020,
when activity of other respiratory pathogens
was also low
Hong 2020 Household 431 tests Not reported 0/13 (0%) Index cases had lived with their family
members without personal protections for a
total of 258 person-days.
Hu 2020 Household
Community
15648 Not reported 471/15648 (3%)
Hua 2020 Household 835 Not reported 151/835 (18.1%)
Huang 2020 Household
Community
22 Not reported 7/22 (31.8%)
Huang 2020a Local
Household
Community
Nosocomial
3795 Not reported 32/3795 (0.84%)
Islam 2020 Household
Local
Community
Nosocomial
391 Not reported The overall secondary clinical attack rate was
4.08 (95% CI 1.95–6.20)
Jia 2020 Household Unclear Not reported Attack rate 44/583 (7.6%)
Jiang 2020 Household
Community
300 Not reported 6/300 (2%)
Jing 2020 Household Unclear Not reported Household contacts 13·2%
Non-household contacts 2·4%
The risk of household infection was
significantly higher in the older age group
(≥60 years)
Jing 2020a Household
Community
Unclear Not reported Close contacts 17.1% to 19%
Family members 46.1% to 49.6%
Jones 2020 Local 128 Not reported 6/128 (4.7%)
Kang 2020 Local 5517 Not reported 96/5517 (1.7%)
Kant 2020 Local
Community
Nosocomial
Not reported Not reported Not reported No details on number of contacts for index case
Kawasuji 2020 Nosocomial 105 Not reported 14/105 (1.33%)
Khanh 2020 Community 217 Not reported 16/217 (7.4%)
Kim 2020 Household 207 17.7 to 30 1/207 (0.5%)
Kim 2020a Household
Community
4 18.7 to 32.1 N/A
Kim 2020b Nosocomial 3,091 respiratory samples from 2,924
individuals
Not reported 3/290 (1%)
Kumar 2020 Community 822 Not reported 144/822 17.5%) Spread of infection within the state was
significantly higher from symptomatic cases,
p=0.02
Kuwelker 2020 Household 179 N/A 45% The elderly (>60 years old) had a significantly
higher attack rate (72%) than adults< 60years
old (46%, p=0·045)
Kwok 2020 Local
Household
206 Not reported 24/206 (11.7%)
Ladhani 2020 Nosocomial 254 Not reported Unclear: 53/254 (21%) tested positive. Staff working across different care homes
(14/27, 52%) had a 3.0-fold (95% CI, 1.9–4.8;
P<0.001) higher risk of SARS-CoV-2 positivity
than staff working in single care homes
(39/227, 17%).
Ladhani 2020a Nosocomial Residents: 264
Staff members: 254
Not specified Unclear: 105/264 (53%) residents tested positive Infectious virus recovery in asymptomatic
staff and residents emphasises their
likely importance as silent reservoirs and
transmitters of infection and explains the
failure of infection control measures which
have been largely based on identification of
symptomatic individuals.
Laws 2020 Household 188 Not reported 55/188 (29.3%)
Laxminarayan 2020 Local
Household
Community
575,071 Not reported 10.7% (10.5 to 10.9%) for high-risk contacts
4.7% (4.6 to 4.8%) for low-risk contacts
79.3% (52.9 to 97.0%) for high-risk travel exposure
Lee 2020 Household 12 Not reported 0/12 (0%)
Lee 2020a Household 23 Not reported 1/23 (4.4%)
Lewis 2020 Household 188 Not reported RT-PCR: 55/188 (29%)
Serology: 8/52 (15%)
Li 2020 Household 5 19.66 to 26.16 4/5 (80%)
Li 2020a Household
Nosocomial
7 Not reported 7/7 (100%) During January 14–22, the authors report that
index patient had close contact with 7 persons
Li 2020b Household 14 Not reported 14/14 (100%)
Li 2020c Household Unclear Not reported Unclear In COFs, the transmission rates of respiratory
droplets in secondary and non-infected
patients were 11.9 % and 66.7 %, respectively,
while the transmission rates of respiratory
droplets with close contacts were 88.1 % and
33.3 %, respectively. In SOFs, the proportion
of respiratory droplet and respiratory droplet
transmission with close contacts was 40 % and
60 %, respectively
Li 2020d Household 392 Not reported 64/392 (16.3%)
Liu 2020 Household 7 Not reported 4/7 (57.1%)
Liu 2020a Nosocomial 30 Not reported N/A
Liu 2020b Household
Community
Nosocomial
11580 Not reported 515/11580 (4.4%)
Liu 2020c Unclear 1150 Not reported 47/1150 (4.1%) The 16 confirmed cases who had previously
been asymptomatic accounted for 236 close
contacts, with a second attack rate of 9.7%,
while the remaining 131 asymptomatic
carriers accounted for 914 close contacts, with
a second attack rate of 2.6% (p<0.001)
López 2020 Local
Household
285 Not reported Facility SAR: 22/101 (21.8%)
Overall SAR: 38/184 (20.7%)
Variation in hygiene procedures across 3
facilities. Facility A required daily temperature
and symptom screening for the 12 staff
members and children and more frequent
cleaning and disinfection; staff members
were required to wear masks. Facility B:
temperatures of the five staff members
and children were checked daily, and more
frequent cleaning was conducted; only staff
members were required to wear masks.
Facility C: 84 staff members and children
check their temperature and monitor their
symptoms daily; masks were not required for
staff members or children.
Lopez Bernal 2020 Household
Community
472 Not reported 37% (95% CI 31–43%)
Lucey 2020 Nosocomial Not specified N/A Not reported
Luo 2020 Community 243 Not reported 12/243 (4.9%) No viral genetic sequence data were available
from these cases to prove linkage; and some
of the secondary and tertiary cases could
have been exposed to unknown infections,
especially asymptomatic ones, before or after
the bus trips.
Luo 2020a Household
Community
Nosocomial
3410 Not reported 127/3410 (3.7%)
Lyngse 2020 Household 2226 Not reported 371/2226 (16.7%)
Ma 2020 Unclear 1665 Not reported 10/1/1665 (0.6%) Only close contacts who fell ill were tested
(n=10)
Macartney 2020 Local 633 Not reported 18/633 (1.2%)
Serologic attack rates: 8/171 (4.8%)
Malheiro 2020 Household 1627 Not reported Overall AR 154/1627 (9.5%)
Maltezou 2020 Household Unclear <25 (28.1%)
25–30 (26.8%)
>30 (45.1%)
Median attack rate 40% (range: 11.1%–100%)
per family.
Maltezou 2020a Household Unclear Not reported Median attack rate: 60% (range: 33.4%-100%) Adults were more likely to develop a severe
clinical course compared to children (8.8%
versus 0%, p-value=0.021)
Mao 2020 Household
Local
Unclear Not reported 6.10% Average attack rate was 8.54% (1.02–100%)
Martinez-Fierro 2020 Unclear 81 Not reported 34/81 (42%)
Serologic attack rates: 13/87 (14.9%)
16% of contact showed positive serology after
>2 weeks
Mponponsuo 2020 Nosocomial 38 N/A 0/38 (0%)
Ng 2020 Household
Local
Community
13026 Not reported 188/7770 (2.4%)
Household: 5·9%
Work contacts: 1.3%
Social contacts: 1.3%
Serology: 44/1150 (3.8%)
Serology results were positive for 29 (5·5%)
of 524 household contacts, six (2·9%) of 207
work contacts, and nine (2·1%) of 419 social
contacts.
Ning 2020 Household
Local
Community
Unclear Not reported Imported cases: 69/3435 (0.8%)
Local cases: 31/3666 (2.0%)
Njuguna 2020 Local 98 Not reported Attack rate 57% to 82%
Ogawa 2020 Nosocomial 30 PCR/serology 33.53 to 36.83 0/15 (0%) for both PCR and serology
Paireau 2020 Household
Local
Nosocomial
6028 Not reported 248/6028 (4.1%) Family contacts, index case was 60–74, or
older than 75 years old were significantly
associated with increased odds of
transmission. The proportion of nosocomial
transmission was significantly higher than in
contact tracing (14% vs 3%, p<0.001)
Park 2020 Local
Household
Community
328 17.7 to 35 22/328 (6.7%)
Park 2020a Household
Non-household
59,073 Not reported Household contacts: 11.8% (95% CI 11.2%–
12.4%)
Non-household contacts: 1.9% (95% CI
1.8%–2.0%)
Park 2020b Local
Household
441 Not reported Attack rate 43.5% (95% CI 36.9%–50.4%)
Secondary attack rate 16.2% (95% CI 11.6%–
22.0%)
Passarelli 2020 Nosocomial 6 Not reported 2/6 (33.3%)
Patel 2020 Household 185 Not reported 79/185 (43%) Contacts not reported as tested
Pavli 2020 Aircraft 891 Not reported 5/891 (0.6%)
Phiriyasart 2020 Household 471 Not reported 27/471 (5.7%)
Poletti 2020 Unclear 2484 Not reported 2824/5484 (51.5%)
Pung 2020 Local
Community
425 Not reported 36/425 (8.5%)
Pung 2020a Household Unclear Not reported 43/875 (4.9%)
Qian 2020 Local
Household
Community
Not reported Not reported Not reported Home‐based outbreaks were the dominant
category (254 of 318 outbreaks; 79.9%),
followed by transport‐based outbreaks (108;
34.0%)
Ravindran 2020 Local Not reported Not reported Attack rate 61% to 77% All attendees participated in activities resulting
in potential exposure, such as shaking hands,
kissing, dancing, sharing
drinks and sharing shisha (smoking water pipes).
Razvi 2020 Nosocomial 2521 Not reported Serologic attack rate 19.4%
Rosenberg 2020 Household 498 Not reported 286/498 (57%)
Roxby 2020 Nosocomial 142 Not reported Attack rate in 1st round: 5/142 (3.5%) One additional positive test result was
reported for an asymptomatic resident who
had negative test results on the first round.
Sang 2020 Household 6 Not reported 4/6 (66.7%)
Schumacher 2020 Local Quarantine phase: 757 tests
Match phase: 1167 tests
Unclear Quarantine phase AR: 3.6%
Match phase AR: 4.2%
Serology: 1.1%
Schwierzeck 2020 Nosocomial 48 16.03 to 32.98 9/48 (18.8%) Ct values of symptomatic cases were
significantly lower compared to asymptomatic
cases 22.55 vs 29.94, p<0.007 (approximately
200-fold higher viral load)
Shah 2020 Household 386 Not reported 34/386 (8.8%)
Shen 2020 Household
Community
480 Not reported Close contact: 2/7 (29%)
Casual contact: 3/473 (0.6%)
Sikkema 2020 Nosocomial 1796 Not specified. WGS for Ct <32 Attack rate 96/1796 (5%) 46 (92%) of 50 sequences from health-care
workers in the study were grouped in three
clusters. Ten (100%) of 10 sequences from
patients in the study grouped into the same
three clusters:
Son 2020 Household 3223 Not reported 8.2% (95% CI, 4.7 to 12.9)
Song 2020 Household 20 Not reported 16/20 (80%)
Speake 2020 Aircraft 111 Not reported 11/111 (9.9%)
Stein-Zamir 2020 Local 1312 Not reported Attack rate 178/1312 (13.6%)
Sugano 2020 Local 72 Not reported 23/72 (31.9%)
Sun 2020 Household Unclear Not reported 34.43%
Taylor 2020 Nosocomial 600 Not reported Resident attack rate: 137/259 (52.9%) 1st round
HCW Attack rate: 114/341 (33.4%)
Teherani 2020 Household 144 Not reported 67/144 (46.5%) Of the total number of household contacts, at
least 29 (20%) had known SARS-CoV2 testing.
Child-to-adult transmission was suspected in
7/67 cases (10.5%).
Thangaraj 2020 Community 26 Not reported 17/26 (65.4%)
Torres 2020 Community 1244 N/A Overall serologic attack rate: 139/1244 (11.2%)
Tshokey 2020 Local
Community
1618 Not reported 14/1618 (0.9%) SAR: High-risk contacts was 9.0% (7/75),
and that among the primary contacts was
0.6% (7/1,095), and none (0/448) among the
secondary contacts.
van der Hoek 2020 Household 174 25.1 to 35.1 47/174 (27%)
Serology on day 3 - family members: 43/148
(29.1%)
Wang 2020 Nosocomial
Household
43 Not reported 10/43 (23.3%)
Wang 2020a Household 155 Not reported 47/155 (30%)
Wang 2020b Household 335 Not reported 77/335 (23%)
Wee 2020 Nosocomial 298 Not reported 1/298 (0.3%)
Wendt 2020 Nosocomial 254 Not reported 0/254 (0%)
Serologic attack rates 0/23 (0%)
Wolf 2020 Household 4 Not reported 3/4 (75%) 7-month old female who was breastfed, was
asymptomatic throughout the observation
period and never developed fevers or any
other symptoms, despite continuous exposure
to her parents and siblings. She remained
SARS-CoV-2 PCR-negative in repeat testing of
pharyngeal swab and stool specimens over
the entire observation period.
Wong 2020 Nosocomial 76 tests were performed on 52
contacts
Not reported 0/52 (0%) Findings suggest that SARS-CoV-2 is not
spread by an airborne route. Ct value for
throat and tracheal aspirate of index case
were 22.8 and 26.1 respectively
Wood 2020 Household Not reporred Not reported Not reported
Wu 2020 Household
Local
Community
2994 Not reported 71/2994 (2.4%)
Wu 2020a Household 148 Not reported 48/148 (32.4%)
Xie 2020 Household 56 Not reported 0/56 (0%)
Xin 2020 Household 187 Not reported 19/187 (17.9%)
Yang 2020 Household
Local
1296 Not reported 0/1296 (0%)
Serologic attack rates: 0/20 (0%)
Viral culture of 4 specimens with Ct <30 were
negative
Yau 2020 Nosocomial 330 Not reported 22/330 (6.7%)
Ye 2020 Local
Community
1293 Not reported 39/1,293 (3.02%)
Yoon 2020 Local 190 N/A 0/190 (0%)
Yousaf 2020 Household 198 Not reported 47/198 (23.7%)
Yu 2020 Household 1587 Not reported 150/1587 (9.5%)
Yung 2020 Household 213 Not reported Attack rate 6.1%
Zhang 2020 Aircraft 4492 Not reported Attack rate 161/4492 (3.6%) The authors report attack rate of 0.14% based
on 94 flights (n=14 505); however, only 4492
people were screened
Zhang 2020a Household
Local
Community
369 Not reported 12/369 (3.3%, 95% CI 1.9%–5.6%)
Zhang 2020b Household 10 Not reported 0/10 (0%)
Serologic attack rates: 0/10 (0%)
Zhang 2020c Local
Household
93 Not reported 5/93 (5.4%)
Zhang 2020d Local 8437 Not reported 25/8437 (0.3%)
Zhuang 2020 Household
Community
8363 Not reported 239/8363 (2.9%)