Background: Household transmission studies are suitable setting to examine determinants of disease by controlling many environmental variables. This study is a prospective, case-based study evaluating the household transmission and possible risk factors on the demographical and behavioral characteristics of individuals diagnosed with COVID-19 and households. Methods: Between 1 August 2021 and 1 February 2022, individuals who were RT-PCR positive and their household members were invited to the study. Swab samples were taken from contacts and RT-PCR test was performed. Secondary attack rate (SAR) was calculated to assess household transmission. Descriptive analyzes and Logistic Regression analyzes were used in the study. Statistical Package for Social Sciences program was used for analysis. Statistical significance was accepted as p<0.05. Ethical approval was obtained from Hacettepe University Ethics Committee. The research was supported by Sanofi. Results: 42 index cases and 112 contacts were involved in the study. 43% of index cases and 54% of contacts were women. The median age of index cases was 40 and contacts’ was 34. SAR was 25%. SAR was also high in contacts sleeping in the same room (%40), hugging (%39), eating together (%30), sharing goods (%38). SAR was high in contacts of index cases who were under 40 (%22), women (%39) and vaccinated (%30). Contacts living with five or more people (OR:5.54), contacts who were spouse or children of the index case (OR:3.53) had high risk of infection. Conclusions: Living in crowded house was the most important risk factor for infection transmission. In order to prevent contagion, such measures as the use of masks in the house, frequent ventilation of house, not eating in the same environment during the quarantine period should be shared with the public and policies should be developed in this direction. Keywords: SARS-CoV-2, family characteristics, close contact transmission