Investigating Risk Factors for SARS CoV 2 Household Contact Test Positivity in San Mateo County, California

Introduction: SARS CoV 2 transmission within households has been very efficient and particularly difficult to control given prolonged close contact and airborne spread from index cases to household contacts. While transmission is dependent on environmental, host and index case factors, these are variable and continue to change throughout the pandemic and post-pandemic periods. We investigated risk factors for concurrent SARS CoV 2 cases in households with an infected index case among lower-income households in San Mateo County, California. Methods: We enrolled infected SARS CoV 2 index cases diagnosed at the San Mateo Medical Center and Stanford University Hospitals and Clinics and their household contacts for inclusion in a cluster-randomized controlled trial involving an intervention of air filtration and ventilation. We interviewed index cases and household contacts about demographic information; history of SARS CoV 2 infections and vaccinations; medical comorbidities; and household characteristics, including type of dwelling; availability of isolation spaces for infected cases; and presence of HVAC systems. We tested household contacts at baseline for SARS CoV 2 using the Lucira molecular diagnostics platform. We measured index case cycle threshold values; and quantified the number of days that household contacts were exposed to a symptomatic index case. Results: From January 2023 to March 2024, we identified and attempted to contact 847 SARS CoV 2 positive index cases, 359 of whom responded including 113 interested in learning more. Of these, 44 were eligible, and 36 enrolled in the study. Among 36 households/index cases, 155 household contacts were identified, 146 of whom completed baseline testing within 2 days of the index case’s positive test; 41 out of these 146 (28%) were found to be positive on initial testing. At least one positive household contact was identified at baseline testing in 19 out of 36 (53%) households. When limiting analysis only to households with at least one positive contact, 41 out of 81 (51%) tested positive. Household contacts exposed to an index cases who reported being vaccinated had a 64% lower odds of testing positive than did household contacts exposed to index cases who reported being unvaccinated (OR=0.36, 95% CI= 0.15-0.87, p=0.02). Contacts exposed to an index case with a “low” cycle threshold value (or high viral load) had a somewhat higher positivity rate compared to those exposed to an index case with “medium” (OR=0.49, 95% CI=0.12-2.1) or “high” cycle thresholds (OR=0.67, 95% CI= 0.17-2.6). Contacts who reported being vaccinated (OR=0.56, 95% CI=0.19-1.7) and those who reported a history of prior infection (OR=0.51, 95% CI=0.2-1.3) had a lower odds of testing positive. Contacts who had 1 day of exposure to a symptomatic index case had a 5.6 fold higher odds of testing positive (OR=5.6; 95% CI=2.4-14, p<0.0001) compared to those who reported 4 days (reference) of exposure. We found a decreasing household positivity rate across three SARS CoV 2 variant time periods (January-May 2023: reference; June-October 2023: OR=0.64, 95% CI=0.2-2.0; November 2023-March 2024: OR=0.46, 95% CI=0.14-1.5). Conclusions: Household contacts living in dwellings with SARS CoV 2 positive index cases had a high rate of test-positivity when tested within 2 days of an index case’s positive test. We found low rates of vaccination among index cases; and low rates of available separate isolation space within households. Vaccination of the index case was associated with a significantly lower household contact test-positive rate. Concurrent household infections with communicable airborne pathogens such as SARS CoV 2 suggest rapid spread that is difficult to control.