Table 7. Risk of Infection with SARS-CoV-2 in Close Contact Settings.

Study ID	Type of transmission	Risk of infection
Abdulrahman 2020	Community	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
Afonso 2021	Household	SAR in families that had more than one infected adult, in addition to the index case, it was 1.50 times higher than those without this feature (RR: 1.50; 95.0% CI: 1.55–4.06). SAR in symptomatic contacts was 4.87 times higher when compared to that of the nonsymptomatic group (RR: 4.87; 95.0% CI: 2.49–9.53).
Akaishi 2021	Household Community	The rate of RT-PCR test positivity was significantly higher in those with a close contact than in those with a lower risk contact (p<0.0001). Household secondary transmission rate was significantly similar lower in children aged <10 years compared to other groups (7.3% vs. 13.5%, p=0.02).
Arnedo-Pena 2020	Household	The health profession of index case was a significant protective factor (p<0.007). Older age of secondary cases, two household members, and higher age of index case were significantly associated with elevated risk of infection: p<0.001 in each case
Atherstone 2021	Community	The odds of receiving a positive test result were highest among household contacts (odds ratio = 2.7; 95% confidence interval = 1.2–6.0)
Bender 2021	Household	There was no significant difference in the SARs between household contacts of presymptomatic versus asymptomatic cases (P=0.23). Presymptomatic transmission was more frequent than symptomatic transmission.
Bernardes-Souza 2021	Household	Being a logistics worker (OR 18.0, 95%CI 8.4-38.7), living with a logistics worker (OR 6.9, 95%CI 3.3-14.5), close contact with a confirmed COVID-19 case (OR 13.4, 6.6-27.3), living with four or more people (OR 2.7, 95% CI 1.4-5.4), and being a current smoker (OR 0.2, 0.1-0.7) were significantly associated with an increased risk of SARS-CoV-2 infection.
Bhatt 2022	Household	Adults were more likely than children to transmit SARS-CoV-2 (OR 2.2, 95% CI 1.3–3.6).
Bi 2020	Local Household Community	Household contact (OR 6·3; 95% CI 1·5–26·3) and travelling together (OR 7·1; 1·4–34·9) were significantly associated with infection. Reporting contact that occurred often was also associated with increased risk of infection compared with moderate-frequency contact (OR 8·8; 95% CI 2·6–30·1)
Bi 2021	Household	The risk of being infected by a household member was the lowest among 5–9 years old and highest among those 65 years and older, with teenagers and working age adults sharing similar risks. Compared with 5–9-year-olds, 65 years and older had nearly three times the odds (OR=2.7, 95%CrI 0.9–7.9)
Bistaraki 2021	Household Community	The odds of infection [95% CI] were higher in contacts exposed within the household (1.71 [1.59–1.85] vs. other) and in cases with cough (1.17 [1.11–1.25] vs. no cough).
Bjorkman 2021	Local	Students in multiple occupancy rooms were significantly twice as likely to be infected compared to students in single rooms (19.1% vs 10.3%). Higher viral load significantly increased the risk of infection (P<0.0001)
Calvani 2021	Local Household	The probability of being positive to SARS-CoV-2 was significantly lower in children who had school contacts or who had flu symptoms compared to children who had household contacts (56.8% vs 2.5%, P<0.0001)
Carvalho 2022	Household	The odds of SARS-CoV-2 transmission when the index case was an adult were 13.98 (4.09 to 47.77) and 11.25 (1.91 to 66.4) times higher when compared to HCW and children as index cases, respectively
Cerami 2021	Household	Households with non-white index cases were significantly more likely to experience incident transmission in the household, 51% vs 19% (p=0.008)
Chen 2020b	Nosocomial	In multivariate analysis, there existed higher risk of seroconversion for close contacts with patient 2 (OR, 6.605, 95% CI, 1.123, 38.830) and doctors exposed to their patient (OR, 346.837, 95% CI 8.924, 13479.434), while the lower risk of seroconversion was closely related to direct contact with COVID-19 patients wearing face mask (OR, 0.127, 95% CI 0.017, 0.968).
Chen 2020c	Local Community Household Nosocomial	Infection rate is highest when living with the case (13.26%), followed by taking the same means of transportation (11.91%). After removing the influence factors of the "super spreader" incident, the infection rate of vehicle contact dropped to 1.80%. The infection rate (7.18%) of entertainment activities such as gatherings, meeting guests, and playing cards was also relatively high, as was short- term face-to-face unprotected conversations or doing errands (6.02%). There was a statistically significant difference in the infection rate among the four categories of life contact, transportation contact, medical contact, and other contact (p<0.005). Participation in Buddhist gatherings caused transmission. A total of 28 people were diagnosed as confirmed cases of new coronavirus pneumonia, 4 were asymptomatic infections, and the infection rate of close contacts reached 32.99% (32/97), which was much higher than the average infection rate (6.15). %), the difference is statistically significant (p<0.005).
Cheng 2020	Household Nosocomial	The overall secondary clinical attack rate was 0.7% (95% CI, 0.4%-1.0%). The attack rate was higher among the 1818 contacts whose exposure to index cases started within 5 days of symptom onset (1.0% [95% CI, 0.6%-1.6%]) compared with those who were exposed later (0 cases from 852 contacts; 95% CI, 0%-0.4%). The 299 contacts with exclusive presymptomatic exposures were also at risk (attack rate, 0.7% [95% CI, 0.2%-2.4%]). The attack rate was higher among household (4.6% [95% CI, 2.3%-9.3%]) and nonhousehold (5.3% [95% CI, 2.1%-12.8%]) family contacts than that in health care or other settings. The attack rates were higher among those aged 40 to 59 years (1.1% [95% CI, 0.6%-2.1%]) and those aged 60 years and older (0.9% [95% CI, 0.3%-2.6%]).
Chu 2021	Household	Five (10%) of 48 secondary cases compared with 130 (33%) of 398 non-case household contacts reported potential community exposures: unadjusted OR 0.24 (95%CI 0.09 to 0.62), p=0.003
Craxford 2021	Household	Cohabitees of seropositive HCW had a seropositive rate of 16%, compared to 2.5% of cohabitees without a seropositive HCW (P=0.003)
Dattner 2020	Household	PCR: 44% of adults were infected compared to 25% of the children (n=3353: 1809 children and 1544 adults) Serology: 34% of these children and 48% of the adults tested serologically positive (n=705: 417 children and 288 adults
Dimcheff 2020	Community Nosocomial Household	HCWs exposed to a known COVID-19 case outside work had a significantly higher seroprevalence at 14.8% (23 of 155) compared to those who did not 3.7% (48 of 1,296; OR, 4.53; 95% CI, 2.67–7.68; P < 0.0001)
Doung-ngern 2020	Local	Wearing masks all the time during contact was independently associated with lower risk of COVID-19 infection compared to not wearing masks (aOR 0.23, 95% CI 0.09–45 0.60), while wearing masks sometimes during contact was not (aOR 0.87, 95% CI 0.41–1.84). Maintaining at least 1m distance from a COVID patient (aOR 0.15, 95% CI 0.04–0.63) and duration of close contact ≤15 minutes versus longer (aOR 0.24, 95% CI 0.07–0.90) were significantly associated with lower risk of infection transmission
Farronato 2021	Household	Subjects tested more than 73 days after the adult negativization showed a lower probability of receiving a positive result (p = 0.059)
Fateh-Moghadam 2020	Community	Workplace exposure was associated with higher risk of becoming a case than cohabiting with a case or having a non-cohabiting family member or friend who was a case. The greatest risk of transmission to contacts was found for the 14 cases <15 years of age (22.4%); 8 of the 14, who ranged in age from <1 to 11 years) infected 11 of 49 contacts.
Fontanet 2021	Local	Infection rates were significantly lower amongst school pupils, teachers, non-teaching staff compared to pupils' parents and relatives (P<0.001)
Galow 2021	Household	The SAR of the 17 index-cases <18 years was significantly lower compared to the 126 adult index-cases: 15% vs 38% p=0.004).
Gaskell 2021	Household	The seroprevalence varied by age between 27.6% (95%CI 20.8 - 35.6%) for children aged under 5 years of age to 74% (95%CI 70.0 -77.6%) in adults
Ge 2021	Household Community	Attack rates were highest among household members of index patients (260 of 2565 [10.1%; 95% CI, 9.0%-11.4%]) and contacts exposed in multiple settings to the same index patient (3 of 44 [6.8%; 95% CI, 1.4%-18.7%])
Gonçalves 2021	Household	Mask use reduced odds of infection by 87% (OR 0.13, 95%CI 0.04–0.36). Persons who reported they were practically isolated from everyone were 59% (OR 0.41, 95% CI 0.24–0.70) less likely to become infected.
Hast 2022	Local	Participation in school sports was significantly associated with increased risks of infection: P=0.0004 and P=0.007 for elementary and middle/high school students respectively. Use of masks during sports activities was associated with significant reduction in risk of infection: P=0.005 and P=0.001 for elementary and middle/high school students respectively.
Helsingen 2020	Local	11 individuals in the training arm (0.8% of those tested) and 27 in the non-training arm (2.4% of those tested) tested positive for SARS-CoV-2 antibodies (p=0.001)
Hobbs 2020	Local Household Community	Case-patients were significantly more likely to have had close contact with a person with known COVID-19 than control participants (aOR = 3.2, 95% CI = 2.0–5.0) Case-patients were significantly more likely to have attended gatherings with persons outside their household, including social functions (aOR = 2.4, 95% CI = 1.1–5.5), activities with children (aOR = 3.3, 95% CI = 1.3–8.4), or to have had visitors at home (aOR = 1.9, 95% CI = 1.2–2.9) during the 14 days before the SARS-CoV-2 test. Parents of 64% of case-patients and 76% of control participants reported that their child and all staff members wore masks inside the facility (aOR = 0.4, 95% CI = 0.2–0.8).
Hu 2020	Household Community	Household contacts were associated with a significantly larger risk of SARS-CoV-2 infection than other types of contact (P<0.001). The transmission risk in the first generation was significantly higher than the later generations (p<0.001), possibly due to improved case isolation and contacts quarantine that deplete the number of susceptible individuals in the cluster.
Hu 2020	Local	Travelers adjacent to the index patient had the highest attack rate (3.5% [95% CI, 2.9%-4.3%]) of COVID-19 infection (RR, 18.0 [95% CI, 13.9-23.4]) among all seats.
Hu 2021	Local	There was no significant difference between the estimated upper and lower bounds of ARs (p=0.06)
Hua 2020	Household	Incidence of infection in child close contacts was significantly lower than that in adult contacts: 13.2% vs 21.2%, p=0.004
Islam 2020	Household Local Community Nosocomial	The secondary attack rate among household contacts was at the highest risk of attack (13.04%, 95% CI 9.67-16.41) followed by funeral ceremonies (8.33%, 95% CI 3.99-12.66) and family contacts (6.52%, 95% CI 4.02-9.02). The attack rate was higher in age groups 50-59 (10.89%, 95% CI 7.05-14.66) and 60-69 (9.09%, 95% CI 5.08-13.09)
Jashaninejad 2021	Household	Contacts who had more than one-hour daily contact with the index case, before the diagnosis of the disease in index cases had a higher risk of infection (adjusted OR=2.44, 95% CI: 1.52, 3.93), compared to contacts who had one-hour and less close contact
Jordan 2022	Local	Frequent hand washing was the only variable that was associated with a lower SAR, P=0.02.
Karumanagoundar 2021	Household Community	Of the 599 contacts who tested positive, more than three-fourths (78%) were household contacts. Being a household contact of a primary case with congregation exposure had a fourfold increased risk of getting COVID-19 (RR: 16.4; 95% CI: 13 to 20) than contact of primary case without congregation exposure.
Katlama 2022	Household	Independent predictors of virus transmission from index to contacts were housing surface area < 60 m2 (OR: 5.6 [1.1; 28.2] and a four-member family compared to five (OR: 3.6 [1.2; 10.3]).
Kawasuji 2020	Nosocomial	Among symptomatic patients (n =18), the estimated viral load at onset was higher in the index than in the non-index patients (median [95% confidence interval]: 6.6 [5.2–8.2] vs. 3.1 [1.5–4.8]. In adult (symptomatic and asymptomatic) patients (n = 21), median viral load at the initial sample collection was significantly higher in the index than in the non-index patients (p = 0.02)
Khanh 2020	Community	Seating proximity was strongly associated with increased infection risk (RR 7.3, 95% CI 1.2–46.2).
Kitahara 2022	Household Community	Attack rates peaked 1 day before symptom onset: 26% (95%CI 10-48)
Klompas 2021	Nosocomial	Potential contributing factors included high viral loads, nebulization, and positive pressure in the index patient's room. Risk factors for transmission to staff included presence during nebulization, caring for patients with dyspnea or cough, lack of eye protection, at least 15 minutes of exposure to case patients, and interactions with SARS-CoV-2–positive staff in clinical areas.
Koureas 2021	Household	Household size was significantly associated with the risk of infection (OR: 2.65, 95% CI: 1.00–7.07).
Kuwelker 2021	Household	The risk of household transmission was higher when the index patient had fever (aOR 3.31 [95% CI 1.52–7.24]; p = 0.003) or dyspnoea (aOR 2.25 [95% CI 1.80–4.62]; p = 0.027) during acute illness.
Laws 2020	Household	There were no significant differences in secondary infection rates between adult and paediatric contacts among all households (OR: 1.11; 95% CI: 0.56 to 2.21) or among households with children (OR: 0.99; 95% CI: 0.51 to 1.90).
Laws 2021	Household	Children of primary patients had increased odds of acquiring infection compared with children in households in which the primary patient was not their parent (OR: 17.28; 95% CI: 2.36 to 126.8).
Laxminarayan 2020	Local Household Community	Secondary attack rate estimates ranged from 1.2% (0.0 to 5.1%) in health care settings to 2.6% (1.6 to 3.9%) in the community and 9.0% (7.5 to 10.5%) in the household.
Lewis 2020	Household	Household contacts to COVID-19 patients with immunocompromised conditions and household contacts who themselves had diabetes mellitus had increased odds of infection with ORs 15.9 (95% CI, 2.4–106.9) and 7.1 (95% CI: 1.2–42.5). Household contacts of a male primary patient were more likely to have secondary infection than those of a female primary patient (SIR, 36% vs 18%; OR, 2.4; 95% CI, 1.1–5.3).
Li 2020d	Household	The secondary attack rate to children (aged <18 years) was 4% compared with 20.5% for adult members (odds ratio [OR], .18; 95% confidence interval [CI], .06–.54; P = 0.002). The secondary attack rate to the contacts in the household with index patients quarantined at home immediately since onset of symptoms was 0% compared with 18.3% for the contacts in the households without index patients quarantined during the period between initiation of symptoms and hospitalization (OR, 0; 95% CI, .00–.00; p=0.000). The secondary transmission rate for individuals who were spouses of index cases was 27.8% compared with 17.3% for other members in the households (OR, 2.27; 95% CI, 1.22–4.22; p=0.010).
Li 2021a	Household	Children and adolescents younger than 20 years of age were more likely to infect others than were adults aged 60 years or older (1·58, 1·28–1·95). Asymptomatic individuals were much less likely to infect others than were symptomatic cases (0·21, 0·14–0·31). Symptomatic cases were more likely to infect others before symptom onset than after (1·42, 1·30–1·55).
Li 2021b	Household Community	Factors associated with significantly increased SAR were living together (P<0.01), being a spouse (P<0.01), and being >60 years of age (P=0.01).
Liu 2020b	Household Community Nosocomial	Compared to young adults aged 20–29 years, the infected risk was higher in children (RR: 2.59, 95%CI: 1.79–3.76), and old people aged 60–69 years (RR: 5.29, 95%CI: 3.76–7.46). People having close relationship with index cases encountered higher infected risk (RR for spouse: 20.68, 95%CI: 14.28–29.95; RR for non-spouse family members: 9.55, 95%CI: 6.73–13.55; RR for close relatives: 5.90, 95%CI: 4.06–8.59). Moreover, contacts exposed to index case in symptomatic period (RR: 2.15, 95%CI: 1.67–2.79), with critically severe symptoms (RR: 1.61, 95%CI: 1.00–2.57)
Liu 2021	Household	SAR among paediatric household contacts was significantly lower than among adult household contacts (P = 0.04). Transmission was significantly lower in households with 4+ bedrooms compared with those with 3 or fewer [17% (95% CI: 7–36%) vs. 47% (95% CI: 32–68%), P = 0.03], for contacts where the index case was masked compared with those unmasked [17% (7–37%) vs. 48% (31–66%), P = 0.02] and with increased hand washing or use of hand sanitizer compared with those who did not report increased use [19% (9–36%) vs. 58% (36–77%), P = 0.01].
López 2021	Household	Higher risk of infection was found in the household members of domicile-isolated patients isolated and in those reporting overcrowding at home, (odds ratio [OR] 1.67, 95% confidence interval [CI] 0.89–3.12) and (OR 1.44, 95% CI 0.81; 2.56), respectively.
Lopez Bernal 2020	Household Community	Secondary attack rates were highest where the primary case was aged <18 years with a significantly higher odds of secondary infection (OR 61, 95% CI 3.3-1133). Where the primary case was admitted to hospital there was a significantly lower odds of secondary infection in the household (OR 0.5, 95% CI 0.2-0.8). Secondary attack rates were lower in larger households.
Lopez Bernal 2022	Household	There was an inverse relationship between household size and SAR, with the highest SAR in households with two people (SAR = 0.48; 95% CI: 0.36–0.60) and the lowest in households of five or more (SAR = 0.22; 95% CI: 0.11–0.33).
Luo 2020a	Household Community Nosocomial	Household contacts had a significantly higher risk for secondary infection than did persons who were exposed in health care settings (OR, 0.09, 95%CI 0.04 to 0.20) or those who were exposed on public transportation (OR, 0.01, 95%CI, 0.00 to 0.08).
Macartney 2020	Local	The rate of staff member to child transmission was lower (1·5%) than staff to staff transmission (4·4%).
Malheiro 2020	Household	Among the intervention cohort,16 of 132 close contacts tested positive during the follow-up period (attack rate:12.1%, 95% confidence interval [CI]: 7.1-18.9). In the control cohort,138 of 1495 participants tested positive (attack rate: 9.2%, 95% CI:7.8-10.8)
Martínez-Baz 2022	Household Community	The infectivity of the index case was lower in those aged 5–14 years and increased with age up to those aged 70 years or older (aOR 2.81, 95% CI 2.56–3.08). Infectivity was higher from immigrants (aOR 1.44, 95% CI 1.36–1.52) and from symptomatic index cases (aOR 1.50, 95% CI 1.43–1.58).
McLean 2022	Household	Compared to when the primary case was age 18 to 49 years, SIR in household contacts was significantly lower when the primary case was age 12 to 17 years (RR, 0.42; 95% CI, 0.19–0.91)
Mercado-Reyes 2022	Household	The number of rooms per household and the number of people per household were significantly associated with risks of seropositivity
Metlay 2021	Household	Independent factors significantly associated with higher transmission risk included age greater than 18 years (eg, adjusted odds ratio [OR] for those aged 50-64 years, 3.66; 95% CI, 2.92-3.66; P<0.001) and multiple comorbid conditions (eg, adjusted OR for individuals with hypertension, 1.93; 95% CI, 1.58-2.44; P<0.001)
Miller 2021	Household	SARs from index cases with respiratory or systemic symptoms were significantly higher than in those without such symptoms.
Montecucco 2021	Local Household Community	Wearing respiratory protections by both the case and the close contact resulted an effective measure compared with no use (IRR = 0.08; 95% CI: 0.03-0.2; P<0.0001). Fatigue (IRR= 17.1; 95% CI: 5.2-55.8; P<0.001), gastrointestinal symptoms (IRR= 6.6; 95% CI: 2.9-15.2; P<0.001) and cough (IRR= 8.2; 95% CI: 3.7-18.2; P<0.001) were found to be significantly associated with transmission of infection.
Musa 2021	Household	Contacts were at a significantly higher risk for infection if the primary case had both cough and runny nose (OR 4.31, 95% CI 1.60–11.63), if the contact was aged 18–49 years (OR 4.67, 95% CI 1.83–11.93), if the contact kissed the primary case (OR 3.16, 95% CI 1.19–8.43), or if the contact shared a meal with the primary case (OR 3.10, 95% CI 1.17–8.27).
Ng 2021	Household	Independent risk factors that were significantly associated with higher transmission risk in the household included an index case who was symptomatic (aOR 1.5; 95% CI 1.1–2.2), and household index aged greater than 18 years (aOR 7.0; 95% CI 4.4–11.3)
Ogata 2021	Household	Spouses of index patients were significantly more likely to be infected compared to other household contacts: OR 2.85 (95%CI 1.25–6.5)
Park 2020a	Household Non- household	With index patients 30–39 years of age as reference, detection of COVID-19 contacts was significantly higher for index patients >40 years of age in nonhousehold settings.
Petersen 2021	Household	The risk for seropositivity was significantly higher for household contacts compared with other contacts (adjusted odds ratio [aOR] 5.4, 95% CI 1.9–15.2).
Pett 2021	Household Community	SAR among household contacts (15.9%, 95% CI 6.6%–30.1%) was more than 6 times higher compared to high-risk contacts (2.5%, 95% CI 0.9%–5.4%)
Phiriyasart 2020	Household	Locally religious and household contacts of confirmed cases had significantly higher risks of SARS-CoV-2 infection than other community members.
Poletti 2020	Unclear	Individuals younger than 70 years were at a significantly lower risk of death after infection than older patients (p<0.001). The risk of death was 62% lower (95% CI: 31–80%; p<0.001) during the second phase of the epidemic.
Ratovoson 2022	Household	In both the univariate and multivariate analyses, there was a relationship between the age of contacts and SAR, with the highest SAR in contacts aged 35 years old or more.
Razvi 2020	Nosocomial	HCWs in patient facing roles had a significantly higher frequency of positive COVID-19 antibody tests (295/1302 [22.7%]) than those in non-patient facing roles (88/669 [13.2%]), p<0.0001)
Reukers 2021	Household	Being a child was strongly associated with decreased probability of infection (P=0.006)
Robles Pellitero 2021	Household	Wearing a mask during quarantine was significantly associated with reduced risk of infection: 30.5% vs 45.7% P<0.001).
Rosenberg 2020	Household	Prevalence significantly increased with age, ranging from 23% among those aged <5 years to 68% among those 65 years or older (p<0.0001)
Satter 2022	Household Community	People living in high-density areas with high SES had significantly higher levels of SARS-CoV-2-specific IgG antibodies on both study day 1 (P=0.011) and study day 28 (P=0.005) compared to the people with low SES.
Schoeps 2021	Local	Teacher index cases caused on average more secondary cases (169/157, risk=1·08) than students/children (145/591, risk=0·25; IRR 4·39, p<0·001). The average number of student/child-to-teacher transmission was 0·04 (corresponding to about one teacher secondary case in 25 student/child index cases) compared to 0.56 for teacher-to-teacher transmission (one teacher secondary case in 2 teacher index cases, IRR 13·25, p<0.0001).
Shah 2021	Household	The family size of the index cases causing secondary infection was comparatively larger than index cases without secondary household infection (6.75 ± 2.3 versus 4.9 ± 1.9; P=0.03).
Sordo 2022	Household	The odds of secondary transmission were lower in primary cases who were asymptomatic at diagnosis than in symptomatic cases (odds ratio, OR: 0.13; 95% 0.04-0.48); and higher in primary cases aged 60 years and over than in those aged 19-39 years (OR: 3.45; 95%CI: 1.53- 7.75). Being a spouse of the primary case was also associated with increased transmission compared to non-spouses (OR: 1.93; 95% CI: 1.24-3.02).
Soriano-Arandes 2021	Household	The SAR was significantly lower in households with COVID-19 paediatric index cases during the school period relative to summer (P=0.02) and compared to adults (P=0.006).
Speake 2020	Aircraft	The risk for secondary infections among passengers seated in the mid cabin was significantly greater than for those seated in the aft cabin (p<0.005). The SAR among mid-cabin passengers in window seats was significantly greater than among those not in window seats (RR 5.2; 95% CI 1.6–16.4; p<0.007).
Stich 2021	Household	The overall secondary attack rate was was significantly higher in exposed adults (37.5%) than in children (24.6%-29.2%; P<0.015). The risk of infection was also significantly higher when the index case-patient was >60 years of age (72.9%; P=0.04)
Sun 2020	Household	The family recurrence rate of spouses who introduced cases from the family was 63.87%, which was higher than the recurrence rate of children (30.53%), parents (28.37%) and other family members (20.93%), and the difference was statistically significant ( P <0.001) .
Sundar 2021	Household Community	The risk of infection was significantly higher in household contacts compared to open environmental work contacts (RR 30.9, 95%CI 9.7-98.3, P<0.001), or closed environmental work contacts (RR1.68, 95%CI 1.15-2.44, P=0.006). The risk was significantly higher among closed environmental work contacts compared to open environmental work contacts (RR 18.3, 95%CI 5.8-58.2, P<0.001).
Tadesse 2021	Household	Persons aged 41–65 years were significantly more likely to be infected than people above the age of 65 years: OR 2.5 (95%CI 1.1- 5.5). Employed population groups have increased risk for infection by compared to unemployed groups: OR 1.3 (95% CI 1.0-1.6).
Tanaka 2021	Household	SARS-CoV-2 Alpha variant had an approximately 1.9–2.3-fold higher transmissibility than the pre-existing virus (P<0.001)
Tanaka 2022	Household	Fewer children were symptomatic compared with adults [91 (51.4%) vs. 142 (65.7%), P=0.004]. Children index cases were associated with periods of lower community case rates while adult index cases were associated with periods of high community transmission and rapid incidence rise of COVID-19 cases (P=0.006).
Torres 2020	Community	Antibody positivity rates were 9.9% (95%CI: 8.2-11.8) for 1,009 students and 16.6% (95%CI: 12.1-21.9) for 235 staff. Among students, positivity was significantly associated with history of contact with a confirmed case (p<0.0001). The greater the number of contacts, the greater the probability that a child was antibody positive (p=0.05).
Tsang 2022	Household Community	Compared to within households, the odds of infection was much lower during air travel, OR= 0.08 (95% CrI: 0.01, 0.34), and in other settings, OR= 0.04 (95% CrI: 0.01, 0.09).
van der Hoek 2020	Household	In families of a confirmed COVID-19 patient, children between 1 and 11 years were less often positive in PCR and serology than older children and adults.
Vičar 2021	Household	There was no significantly higher SAR in families with an adult primary case compared to those with children (77.1% vs. 65.8%, P=0.05).
Wang 2020b	Household	Face mask use by the primary case and family contacts before the primary case developed symptoms was 79% effective in reducing transmission (OR=0.21, 95% CI 0.06 to 0.79). Daily use of chlorine or ethanol-based disinfectant in households was 77% effective (OR=0.23, 95% CI 0.07 to 0.84). Wearing a mask after illness onset of the primary case was not significantly protective. The risk of household transmission was 18 times higher with frequent daily close contact with the primary case (OR=18.26, 95% CI 3.93 to 84.79), and four times higher if the primary case had diarrhoea (OR=4.10, 95% CI 1.08 to 15.60). Household crowding was not significant.
White 2022b	Local	The SAR in flights of ≥5 h duration was significantly higher than shorter flights (P=0.008)
Wiens 2021	Household	The risk of seropositivity was lowest among participants 20 to 49 years old
Wood 2020	Household	Households without children had a significantly lower rate of COVID-19: HR per child 0.89; 95% CI 0.84-0.95. Households with children had higher rates of COVID-19 tests (9.2% vs 6.1%) Compared to those in households without children, the risk of COVID-19 requiring hospitalisation was lower in those with one child and lower still in those with two or more children: HR 0.72 per child (95% CI 0.60-0.85, p<0.001); adjusted for age - HR 0.83 per child (95% CI 0.70-0.99)
Wu 2020	Household Local Community	Contacts living in the same household as the index case had significantly higher risk of infection vs those who had only had brief contact with the index case: RR 41.7 (17.7–98.5), p<0.001). Contacts who had visited, or had contact with the index case in a medical institution had significantly higher risk of acquiring infection vs brief contact with the index case: RR 3.6 (1.42–8.98), p=0.004. Family members who had contact with an index case had significantly higher risk of infection vs healthcare providers or other patients who had been exposed to an index case: RR 31.6 (7.69–130.01), p<0.001. Those who had contact with the index case through work, through study, or in a place of entertainment had a significantly higher risk of infection vs those who had contact with the index case in a medical institution: RR 6.7 (1.34–33.25), p=0.01. Those who had contact with the index case in or near his/her home had a significantly higher risk of infection vs those who had contact with the index case in a medical institution: RR 17.3 (4.20–70.77), p<0.001. The incidence rate among those who wore face masks was significantly lower than that among those who did not use protective measures (0.3% vs. 4.7%, respectively, p<0.001). The incidence rate of contacts with data collected by field investigation was significantly higher than that of contacts with data collected by big data (5.35% versus 0.07%, p<0.001).
Wu 2020a	Household	Contacts with >72 hours of exposure (SIR, 41.7%; [95% CI: 26.8%–58.3%]) had a higher SIR compared with those without (SIR, 23.2%; [95% CI: 11.4%–41.5%]). One household-level factor was significantly associated with SIR: household members without protective measures after illness onset of the index patient (odds ratio [OR], 4.43; [95% CI: 1.37–14.34]).
Wu 2021	Local Household Community	The SARs among close contacts of symptomatic and asymptomatic index cases were 4.1% (128 of 3136) and 1.1% (12 of 1078), respectively, corresponding to a significantly higher transmission risk from symptomatic cases (OR 3.79; 95%CI 2.06-6.95).
Xie 2021	Household	Handwashing ≥ 5 times/day was associated with significantly reduced infection risk (52.8% vs. 76.9%, P=0.04).
Xin 2020	Household	Increasing risk of infection among household contacts with female index patients (adjusted hazard ratio [aHR] = 3.84, 95% CI = 1.07–13.78), critical disease index patients (aHR = 7.58, 95% CI = 1.66–34.66), effective contact duration with index patients > 2 days (aHR = 4.21, 95% CI = 1.29–13.73), and effective contact duration > 11 days (aHR = 17.88, 95% CI = 3.26–98.01)
Yi 2021	Household	The frequency of exposure to positively SARS-CoV-2 cases was significantly higher in index patients (20.7% vs. 6.8%, P=0.01).
Yu 2020	Household	Family members, colleagues/classmates/travel companions, and doctors-patients accounted for 88.1% (1398), 10.7% (170), and 0.3% (5), respectively. Following this order, the infection rate was 10.2%, 1.8% and 40.0%, respectively.
Yung 2020	Household	Young children <5 years old were at lowest risk of infection (1.3%). Children were most likely to be infected if the household index case was the mother.
Zhang 2020a	Household Local Community	SAR among household contacts was 16.1% vs 1.1% for social contacts, and 0% for workplace contacts. Older close contacts had the highest SAR compared with other age groups; 8.0% in persons >60 years of age compared with 1.4%–5.6% in persons <60 years of age. Close contacts that lived with an index case-patient had 12 times the risk for infection and those who had frequent contact with an index case-patient, >5 contacts during 2 days before the index case was confirmed, had 29 times the risk for infection.
Zhuang 2020	Household Community	The main sources of secondary infection were family exposure (74.5%, 178 cases), transportation exposure accounted for 8.4% (20 cases), friend/colleague meal exposure accounted for 5.9% (14 cases). Shopping malls, markets, pharmacies, and other public place exposure accounted for 5.0% (12 cases), workplace exposure accounted for 3.8% (9 cases), and community exposure accounted for 2.5% (6 cases).