SARS-CoV-2 Secondary Attack Rates in Vaccinated and Unvaccinated Household Contacts during Replacement of Delta with Omicron Variant, Spain
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
Abstract
T he mass vaccination against SARS-CoV-2 that be- gan at the end of 2020 reduced COVID-19-related mortality and severity in countries where substantial vaccine coverage was achieved (1,2). The vaccines also had a protective effect against the most recent variants (3,4). However, expectations that vaccines would stop community transmission of SARS-CoV-2 through herd immunity were quickly dampened by the early observation of infection and re-infection among vaccinated persons; waning vaccine effectiveness against transmission (VET) over time was observed (1,3) and confirmed in a large systematic literature review (5). Despite these results, protective effects of vaccination against infection among contacts have been reported (6). The vaccination status of index case-patients was also shown to play a role (6), underscoring the importance of vaccination for reducing the circulation of SARS-CoV-2. Nonetheless, the emergence of new variants of concern (VOC) with increased infectivity is an ongoing challenge for VET of currently licensed vaccines; early reports have shown a substantially lower VET for the Delta variant (B.1.617.2) compared with previous VOCs (7). Furthermore, rapid replacement of the Delta variant by Omicron (B.1.1.529) began in late 2021; the Omicron variant showed a transmission advantage because of its shorter generation time (S. Abbott et al., unpub. data, https://www.medrxiv.org/content/10.1101/2 022.01.08.22268920v1).