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.