Background: It's unclear whether vaccination individuals against SARS-CoV-2 protects family members in the same way.
Objective: The assessment of risk factors for SARS-CoV-2 transmission from index cases with COVID-19 breakthrough infection in completely vaccinated patients to fully vaccinated family members.
Methods: From February 1 to November 30, 2021, an observational, longitudinal, and prospective research of families with one primary case of COVID-19 breakthrough infection was done in a general medicine practice in Toledo, Spain.
Results: Thirteen primary cases of COVID-19 breakthrough infection in 13 families with at least one other family member were included, of which 9 were positive secondary cases (sick) and 8 were negative partners (healthy) properly vaccinated. Being a woman, being over 45 years old, being a social-health professional, being an ethnic minority, and having chronic conditions all increased the likelihood of developing COVID-19 in fully vaccinated contacts and main cases. Although vaccination 2ChAdOx1 nCoV-19 had a higher effectiveness than BNT162b2 mRNA, none of these risk or protective factors were statistically significant.
Conclusion: Young women, social health workers, ethnic minority groups, and people with chronic diseases are the completely vaccinated contacts with the highest risk of having COVID-19, after primary cases also vaccinated in the family, in Toledo (Spain), when the delta variant became dominant but before the rise of omicron.