SARS-CoV-2 Secondary Attack Rates in Vaccinated and Unvaccinated Household Contacts during Replacement of Delta with Omicron Variant, Spain

• DOI: 10.3201/eid2810.220494
• Publication Date: 2022-09-23T14:39:10Z
• Authors: Israel López-Muñoz; Ariadna Torrella; Olga Pérez-Quílez; Amaia Castillo-Zuza; Elisa Martró; Antoni E. Bordoy; Verónica Saludes; Ignacio Blanco; Laura Soldevila; Oriol Estrada; Lluís Valerio; Sílvia Roure; Xavier Vallès
• Keywords: SARS; SARS-CoV-2; Research; Incidence; R; COVID-19; Infectious and parasitic diseases; RC109-216; 3. Good health; respiratory infections; 03 medical and health sciences; Cross-Sectional Studies; 0302 clinical medicine; coronavirus disease; Spain; Medicine; Humans; Prospective Studies; severe acute respiratory syndrome coronavirus 2

We performed a prospective, cross-sectional study of household contacts of symptomatic index case-patients with SARS-CoV-2 infection during the shift from Delta- to Omicron-dominant variants in Spain. We included 466 household contacts from 227 index cases. The secondary attack rate was 58.2% (95% CI 49.1%-62.6%) during the Delta-dominant period and 80.9% (95% CI 75.0%-86.9%) during the Omicron-dominant period. During the Delta-dominant period, unvaccinated contacts had higher probability of infection than vaccinated contacts (odds ratio 5.42, 95% CI 1.6-18.6), but this effect disappeared at ≈20 weeks after vaccination. Contacts showed a higher relative risk of infection (9.16, 95% CI 3.4-25.0) in the Omicron-dominant than Delta-dominant period when vaccinated within the previous 20 weeks. Our data suggest vaccine evasion might be a cause of rapid spread of the Omicron variant. We recommend a focus on developing vaccines with long-lasting protection against severe disease, rather than only against infectivity.