Transmission of SARS-CoV-2 infection and risk factors in a cohort of close contacts

ABSTRACT

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
• transmission
• transmission risk factor
• secondary attack rate
• close contact
• household contacts

Acknowledgments

The authors thank the other members of The Working Group for the Study of COVID-19 in Navarra: Miguel Fernández-Huerta, Carmen Martín, Ana Miqueleiz, Ana Navascués, Isabel Polo, María Eugenia Portillo, Carmen Ezpeleta, Amaya Bacaicoa, Judith Chamorro, Juana Vidán (Hospital Universitario de Navarra); Ingrid Estévez, Igberto Tordoya, Delia Quílez (Hospital Reina Sofía de Tudela); Francisco Lameiro, Ana Isabel Álvaro (Hospital García Orcoyen de Estella); Esther Albéniz, Aitziber Echeverría, Paula López Moreno, Javier Gorricho (Servicio Navarro de Salud-Osasunbidea); Eva Ardanaz, Maite Arriazu, Fernando Baigorria, Aurelio Barricarte, Enrique de la Cruz, Jorge Díaz, Nerea Egüés, Manuel García Cenoz, Marcela Guevara, Nerea Iriarte, Kenya Nekotxea, Conchi Moreno-Iribas, Carmen Sayón-Orea, Maitane Tellechea, Marian Nuín (Instituto de Salud Pública y Laboral de Navarra).

Disclosure of financial/other conflicts of interest

The authors have no relevant conflicts of interest to disclose. Peer reviewers on this manuscript have no relevant financial or other relationships to disclose.

Additional information

Funding

This study was supported by the Horizon 2020 program of the European Commission (I-MOVE-COVID-19, grant agreement No 101003673), and by the Carlos III Institute of Health with the European Regional Development Fund (COV20/00542 and CM19/00154).