Aim The objective of this observational study was to investigate the association between SARS-CoV-2 transmission risk, RT-PCR Cycle threshold (Ct) values, and age of infected cases in Danish households. Background The Covid-19 pandemic is one of the most serious global public health threats in recent times. Understanding transmission of SARS-CoV-2 is of utmost importance to be able to respond to outbreaks and take action against the spread of the disease. Viral load is generally thought to correlate with transmission risk. Methods We used comprehensive administrative register data from Denmark, comprising the full population and all SARS-CoV-2 tests (August 25, 2020 to February 10, 2021), to estimate household transmission risk. Results We found that the transmission risk was negatively associated—approximately linear—with the Ct values of the tested primary cases. Also, we found that even for relatively high Ct values, the risk of transmission was not negligible; e.g., for primary cases with a Ct value of 38, we found a transmission risk of 8%. This implies that there is no obvious cut-off for Ct values for risk of transmission. We estimated the transmission risk according to age and found an almost linearly increasing transmission risk with the age of the primary cases for adults (≥20 years) and negatively for children (<20 years). Age had a higher impact than Ct value on the risk of transmission. Conclusions Lower Ct values (indicating higher viral load) are associated with higher risk of SARS-CoV-2 transmission. However, even at high Ct values, transmission occurs. In addition, we found a strong association between age and transmission risk, and this dominated the Ct value association.