Virology and immune dynamics reveal high household transmission of ancestral SARS-CoV-2 strain (preprint)

Background: Household studies are crucial for understanding the transmission of SARS-CoV-2 infection, which may be underestimated from PCR testing of respiratory samples alone. We aim to combine assessment of household mitigation measures; nasopharyngeal, saliva and stool PCR testing; along with mucosal and systemic SARS-CoV-2 specific antibodies, to comprehensively characterise SARS-CoV-2 infection and transmission in households. Methods: Between March and September 2020, we obtained samples from 92 participants in 26 households in Melbourne, Australia, in a 4-week period following onset of infection with ancestral SARS-CoV-2 variants. Results: The secondary attack rate was 36% (24/66) when using nasopharyngeal swab (NPS) PCR positivity alone. However, when respiratory and non-respiratory samples were combined with antibody responses in blood and saliva, the secondary attack rate was 76% (50/66). SARS-CoV-2 viral load of the index case and household isolation measures were key factors that determine secondary transmission. In 27% (7/26) of households, all family members tested positive by NPS for SARS-CoV-2 and were characterised by lower respiratory Ct-values than low transmission families (Median 22.62 vs 32.91; IQR 17.06 to 28.67 vs 30.37 to 34.24). High transmission families were associated with enhanced plasma antibody responses to multiple SARS-CoV-2 antigens and the presence of neutralising antibodies. Three distinguishing saliva SARS-CoV-2 antibody features were identified according to age (IgA1 to Spike 1, IgA1 to nucleocapsid protein (NP), suggesting that adults and children generate distinct mucosal antibody responses during the acute phase of infection. Conclusion: Utilising respiratory and non-respiratory PCR testing, along with measurement of SARS-CoV-2 specific local and systemic antibodies, provides a more accurate assessment of infection within households and highlights some of the immunological differences in response between children and adults.

Reduced seroconversion in children compared to adults with mild COVID-19 (preprint)

Importance The immune response in children with SARS-CoV-2 infection is not well understood. Objective To compare seroconversion in children and adults with non-hospitalized (mild) SARS-CoV-2 infection and to understand the factors that influence this. Design Participants were part of a household cohort study of SARS-CoV-2 infection. Weekly nasopharyngeal/throat swabs and blood samples were collected during the acute and convalescent period following PCR diagnosis for analysis. Setting Participants were recruited at the Royal Children’s Hospital, Melbourne, Australia between May and October 2020. Participants Those who had a SARS-CoV-2 PCR-positive nasal/throat swab. Main outcomes and measures SARS-CoV-2 antibody and cellular responses in children and adults. Seroconversion was defined by seropositivity in all three serological assays. Results Among 108 SARS-CoV-2 PCR-positive participants, 57 were children (median age: 4, IQR 2-10) and 51 were adults (median age: 37, IQR 34-45). Using three established serological assays, a lower proportion of children seroconverted compared with adults [20/54 (37.0%) vs 32/42 (76.2%); (p<0.001)]. This was not related to viral load, which was similar in children and adults [mean Ct 28.58 (SD: 6.83) vs 24.14 (SD: 8.47)]. Age and sex also did not influence seroconversion or the magnitude of antibody response within children or adults. Notably, in adults (but not children) symptomatic adults had three-fold higher antibody levels than asymptomatic adults (median 227.5 IU/mL, IQR 133.7-521.6 vs median 75.3 IU/mL, IQR 36.9-113.6). Evidence of cellular immunity was observed in adults who seroconverted but not in children who seroconverted. Conclusion and Relevance In this non-hospitalized cohort with mild COVID-19, children were less likely to seroconvert than adults despite similar viral loads. This has implications for future protection following COVID-19 infection in children and for interpretation of serosurveys that involve children. Further research to understand why children are less likely to seroconvert and develop symptoms following SARS-CoV-2 infection, and comparison with vaccine responses may be of clinical and scientific importance. Key points Question What proportion of children with non-hospitalized (mild) SARS-CoV-2 infection seroconvert compared to adults? Findings In this cohort study conducted in 2020, we found the proportion of children who seroconverted to SARS-CoV-2 was half that in adults despite similar viral load. Meaning Serology is a less reliable marker of prior SARS-CoV-2 infection in children. SARS-CoV-2-infected children who do not seroconvert may be susceptible to reinfection. Our findings support strategies to protect children against COVID-19 including vaccination.

Immune Responses in an Infant with Congenital Heart Disease and Severe COVID-19 (preprint)

Children have lower hospitalisation and mortality rates for coronavirus disease-2019 (COVID-19) than adults; however, younger children (<4 years of age)1 may develop more severe disease than older children. To date, the immune correlates of severe COVID-19 in young children have been poorly characterized. We report the kinetics of immune responses in relation to clinical and virological features in an infant with acute severe COVID-19. Systemic cellular and cytokine profiling showed initial increase in neutrophils and monocytes with depletion of lymphoid cell populations (particularly CD8+ T and NK cells) and elevated inflammatory cytokines. Expansion of memory CD4+T (but not CD8+T) cells occurred over time, with predominant Th2 bias. Marked activation of T cell populations observed during the acute infection gradually resolved as the child recovered. Significant in vitro activation of T-cell populations and robust cytokine production, in response to inactivated SARS-CoV-2 stimulation, was observed 3 months after infection indicating durable, long-lived cellular immune memory.