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.