Purpose: Many factors might affect SARS-CoV-2 transmission, but their relevance is not well established. The objectives were to assess the secondary attack rate (SAR) and the risk factors for SARS-CoV-2 transmission from confirmed index cases to their close contacts in household and non-household settings.
Methods: This cohort study included the close contacts of SARS-CoV-2 infected cases confirmed between May and December 2020 in Navarre, Spain. Epidemiological and clinical variables of the index case and close contacts were collected. The SAR was calculated, and the independent effect of each variable on the transmission risk was evaluated by logistic regression.
Results: A total of 59,900 close contacts of 20,048 index cases were studied, and 53.6% were household contacts. SAR was 34.9% overall, 46.8% in household contacts and 21.1% in non-household contacts. The risk of transmission was higher in household setting (adjusted odds ratio (aOR) 2.96, 95% CI 2.84-3.07), from symptomatic index cases (aOR 1.50, 95% CI 1.43-1.58), immigrants (aOR 1.44, 95% CI 1.36-1.52), and increased with age. A higher susceptibility of close contacts was associated with 5-14 years of age, immigrants (aOR 1.54), very low or low-income level (aOR 1.27, and aOR, 1.17, respectively), healthcare work (aOR 1.21), and diagnosis of diabetes (aOR 1.14, 95%CI 1.03-1.25), chronic kidney disease (aOR 1.18, 95%CI 1.04-1.35), hypertension (aOR 1.11, 95% CI 1.03-1.19), and severe obesity (aOR 1.18, 95% CI 1.00-1.38). Transmission increased progressively from May to September 2020 as the B.1.177 variant became dominant.
Conclusion: The risk of SARS-CoV-2 infection was considerable among close contacts of infected persons. The higher risk associated with household contacts, immigrants, older index cases, close contacts with lower income level and comorbidities should be considered to address preventive interventions.
Keywords: Covid-19; SARS-CoV-2; close contact; household contacts; secondary attack rate; transmission; transmission risk factor.