Vaccinated COVID-19 Index Cases Are Less Likely to Transmit SARS-CoV-2 to Their Household Contacts: A Cohort Study

• Published in: Vaccines (Basel) 2024-02, Vol.12 (3), p.240
• Main Authors: Godoy, Pere, Martínez-Baz, Iván, Parron, Ignasi, García-Cenoz, Manuel, Ferras, Joaquim, Carol, Mònica, Bes, Nuria, Guillaumes, Montserrat, Godoy, Sofia, Toledo, Diana, Follia, Núria, Miret, Carme, Pardos, Jessica, Alsedà, Miquel, Plans-Rubio, Pedro, Sanz, Inma, Sala, Maria-Rosa, Caylà, Joan A, Mendioroz, Jacobo, Muñoz-Almagro, Carmen, Castilla, Jesús, Domínguez, Ángela, On Behalf Of The Sars-CoV-Transmission To Household Contacts Working Group
• Format: Article
• Language: English
• Subjects: Cohabitation; Cohort analysis; COVID-19; COVID-19 vaccines; Disease prevention; Disease transmission; Epidemiology; Estimates; Evaluation; Health surveillance; household contact; Households; Hypotheses; Immunization; incidence rate; Independent variables; Infections; Primary care; Public health; Public health administration; Questionnaires; Regression analysis; Safety and security measures; SARS-CoV-2; secondary attack rate; Severe acute respiratory syndrome coronavirus 2; Vaccination; vaccine
• ISSN: 2076-393X; 2076-393X
• DOI: 10.3390/vaccines12030240

The aim of this study was to evaluate the impact of index case vaccination on SARS-CoV-2 transmission to household contacts. In our epidemiological cohort study (May 2022-November 2023), we surveyed registered index case vaccination status and test results for contacts (testing on day 0, and on day 7 for negative contacts) and calculated the secondary attack rate (SAR), i.e., newly infected contacts/susceptible included contacts. The association of the independent variable, index case COVID-19 vaccination (yes/no), with household contact infection was determined using the adjusted odds ratio (aOR) and its 95% confidence interval (CI). We recorded 181 index cases and 314 contacts, of whom 250 agreed to participate; 16 contacts were excluded upon testing positive on day 0. Of the 234 included contacts, 49.1% were women, and the mean (SD) age was 51.9 (19.8) years. The overall SAR of 37.2% (87/234) was lower in the contacts of both vaccinated index cases (34.9% vs. 63.2%; = 0.014) and index cases with a previous SARS-CoV-2 infection history (27.0% vs. 46.3%; = 0.002). Index case vaccination showed a protective effect against infection for their household contacts (aOR = 0.21; 95% CI: 0.07, 0.67). The household SAR was high when the Omicron variant circulated. Vaccinated index cases were less likely to transmit SARS-CoV-2 to their contacts.