Virology and immune dynamics reveal high household transmission of
ancestral SARS-CoV-2 strain
Abstract
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.