INTRODUCTION
The COVID-19 pandemic has uniformly identified households as the highest risk setting for SARS-CoV-2 transmission 1, even when community transmission is reduced 2, 3, 4. Occupants of a household face higher risk through sharing a closed space, being in close contact without personal protective equipment, and potential crowding 2, 5. Numerous household transmission studies have identified factors which contribute to higher secondary attack rates, including a symptomatic index case, spouses compared with other household members, and that adults are more likely to transmit than children 6 4.
Transmission dynamics vary within households for reasons that are still not well understood. Clustering of infection in the household can occur, where transmission is characterised by higher secondary transmission rates, whilst in other households there may be no transmission4. SARS-CoV-2 is transmitted primarily by exposure to respiratory fluids when individuals cough or breathe, through contact and droplet or airborne transmission 7, 8. Individuals who are symptomatic often have higher nasopharyngeal viral RNA concentrations early in the course of symptomatic infection9. In addition to respiratory fluid, SARS-CoV-2 has been detected in other biological samples, such as saliva, stool and urine10, 11. Prolonged excretion has been shown to occur following negative respiratory viral testing 12. These factors may account for higher transmission in household settings and testing from multiple sample types may improve sensitivity in detection of transmission routes.
Understanding the host immune responses to SARS-CoV-2 in controlling the infection are important in determining susceptibility. The immune responses to SARS-CoV-2 differ with age; children are less likely to experience severe disease as compared to adults, and both children and adults can mount an immune response to SARS-CoV-2 without virological confirmation of infection 13, 14. Immune differences and endothelial/clotting function are proposed hypotheses for the age related severity of COVID-19 15. Emerging variants of concern (VOC) may induce different immune responses and cause varying severity of disease.
Most transmission studies have relied on SARS-CoV-2 PCR testing of nasopharyngeal swabs (NPS) and symptoms in contacts to describe secondary infection and clinical attack rates 4. However, timing of NPS, host viral load, and swab collection quality may miss the pervasive nature of the infection and underestimate transmission routes. Higher density analyses of multiple biological specimens at numerous timepoints, together with the antibody-mediated immune response following COVID-19, may provide a more comprehensive profile of SARS-CoV-2 transmission. In this study, we describe the extent of SARS-CoV-2 infection and host immune responses behind transmission dynamics with ancestral SARS-CoV-2 in households.