Results
Overview of the Analysis Population
Data were collected from 16 states (Alaska, Arizona, California, Connecticut, Georgia, Hawaii, Illinois, Minnesota, Pennsylvania, Rhode Island, Tennessee, Utah, Virginia, Vermont, Washington, and Wisconsin) with 202 laboratory-confirmed COVID-19 case-patients with symptom onset during January 14–April 4, 2020. Age of COVID-19 case-patients in the sample ranged from <1 to 95 years, almost all were symptomatic (195; 97%), and 1 in 3 was hospitalized for management of COVID-19 symptoms (Appendix 2 Table 3). Of the 202 case-patients, 34 (17%) reported having diabetes mellitus and 48 (24%) reported hypertension.
Exposures
A total of 82 (41%) case-patients reported known contact with a laboratory-confirmed COVID-19 case-patient in the 14 days before symptom onset. The most commonly reported exposure setting was the household (44/82; 54%); within the household setting, the most frequently reported source of COVID-19 exposure was the spouse or partner of the COVID-19 case-patient (16/44; 36%). The second most reported exposure setting was healthcare (20/82; 24%); 14 of the 20 persons exposed in the healthcare setting were healthcare workers, 4 were seeking care for unrelated medical issues, and 2 were visitors.
Among persons reporting no known COVID-19 contact, 20/84 (24%) reported having close contact with an ill person. Persons with no known COVID-19 contact worked in a variety of industries, most commonly healthcare (10/90; 11%); professional/office settings (10/90; 11%); education (9/90; 10%); and accommodation, food, or other services (9/90; 10%) (Table 1). In comparison, 28% (20/72) of persons with known COVID-19 contact reported working in healthcare. Persons with no known COVID-19 contact were significantly less likely than those with known contact to report spending time in a healthcare setting (p = 0.004). However, they were somewhat more likely to report travel (38% vs. 26%) or attendance at a mass gathering (36% vs. 21%) and significantly more likely to report use of public transportation (44% vs. 16%), compared with persons reporting known COVID-19 contact (p = 0.005)
Of the 202 case-patients, 23 (11.3%) reported no known contact with a confirmed case-patient, no travel within 14 days before illness onset, and none of the exposure risks assessed. These persons ranged in age from 21 to 88 years and were significantly older than those reporting ≥1 possible exposure (median age 52 vs. 49 years; p<0.0001). They required hospitalization more frequently than those reporting ≥1 possible exposure (52% [12/23] vs. 30% [54/179]; p = 0.10), and were significantly more likely to report ≥1 underlying medical condition (87% [20/23] vs. 58% [104/179]; p = 0.029). They were much more likely to report having diabetes mellitus (43% [10/23] vs. 14% [24/176]; p = 0.002).
Analysis of Presumed Household Transmission
A total of 69 case-patients provided data on the symptom status of ≥1 household members and were included in our household analysis; in 48 (70%) households, the CIF subject was the first or only symptomatic person in the household (i.e., was identified as the index case-patient; Figure 1). In half (34/69; 49%) of included households, ≥1 household member, in addition to the CIF subject, was symptomatic (i.e., virus transmission was presumed). Included households ranged in size from 2 to 16 persons (median 4 persons) and comprised a variety of household types (e.g., couples, nuclear families, roommates, multigenerational); household size and members' ages, sexes, and relationships were interrelated. Presumed transmission was more frequently observed in larger households (78% of households with ≥5 members vs. 39% of households with <5 members; p = 0.005) (Figure 2). Within households with more members, a larger number of household contacts reported symptoms (Figure 2).
Figure 1.
Households included in the analysis population for study of presumed household transmission among persons with COVID-19, United States, January–April 2020. CIF, case investigation form; CIF subject, interviewed COVID-19 case-patient; COVID-19, coronavirus disease.
Figure 2.
Proportion of households with presumed severe acute respiratory syndrome coronavirus 2 transmission, by household size (including index case-patient), United States, January–April 2020. Shading indicates percentage of households with the specified number of symptomatic household contacts (i.e., excluding index case-patient); households with zero symptomatic contacts (in white) are those in which presumed household transmission did not occur. n = no. households in each stratum.
Among 201 household contacts, 193 had data on symptom status, of which 62 (32%; 95% CI 26%–39%) were symptomatic. Sensitivity analysis results showed a similar plausible range of attack rates (21%– 39%; Appendix 2 Results and Table 1). The median serial interval was 3 days (range 1–10 days).
Although our sample did not have large numbers of index case-patients at the age extremes, household contacts were more likely to be symptomatic if the index case-patient was <5 (5 households) or ≥65 years of age (9 households) (Figure 3, panel A); trends were similar, but the point estimates were significant only for index case-patients ≥45 years of age (vs. index case-patients 18–44 years of age) after adjustment for contact age, contact sex, household size, and relationship of the contact to the index case-patient (Table 2). Adult contacts were symptomatic more often than contacts <18 years of age (Figure 3, panel B), but this association was not significant in adjusted analyses (Table 2). The symptom status of household contacts was also associated with their relationship to the index case-patient (Table 2). Among the contacts of 9 index case-patients <18 years of age, 11/16 (69%) parents, 6/13 (46%) siblings, and 2/5 (40%) other household contacts later became symptomatic. Among contacts of the 60 adult index case-patients, 12/44 (27%) children (range 2–49 years of age), 12/45 (27%) spouses/partners, 7/16 (44%) parents, and 11/42 (26%) other household contacts became symptomatic. When we restricted the analysis to households in which the CIF subject was the index case-patient, overall trends were similar to those reported above, but small sample sizes precluded adjusted analyses (Appendix 2 Table 2).
Figure 3.
Symptom status of household contacts, by age group of index coronavirus disease case-patient (n = 192) and age group of household contact (n = 173), United States, January–April 2020. Age group missing for 20 contacts; age of index case-patient missing for 1 contact.
Illness severity of the index case-patient could not be assessed in multivariable models because of low sample size and correlation with age. However, among 12 household contacts of 10 index case-patients requiring hospitalization (three 18–44, five 45–64, and two index case-patients ≥65 years of age), only 2 were symptomatic.
Emerging Infectious Diseases. 2021;27(9):2323-2332. © 2021 Centers for Disease Control and Prevention (CDC)
