ABSTRACT
BackgroundHousehold transmission studies offer the opportunity to assess both secondary attack rate and persistence of SARS-CoV-2 antibodies over time. MethodsWe invited confirmed COVID-19 cases and their household members to attend up to four household visits with collection of nasopharyngeal and serum samples over 28 days after index case onset. We calculated secondary attack rates (SAR) based on the presence of SARS-CoV-2 nucleoprotein IgG antibodies (IgG Ab) and/or neutralizing antibodies (NAb) overall and per households. Three and six months later, we assessed the persistence of SARS-CoV-2 antibodies. FindingsWe recruited 39 index cases and 90 household members. Among 87 household members evaluated, SAR was 48% (n=42), including 37 symptomatic secondary cases. In total, 80/129 (62%) participants developed both IgG Ab and NAb, while three participants only developed IgG Ab. Among participants who had both IgG Ab and NAb during the initial follow-up, 68/69 (99%) and 63/70 (90%) had IgG Ab and NAb at 3 months, while at 6 months, 59/75 (79%) and 63/75 (84%) had IgG Ab and NAb, respectively. Participants who required hospital care had initially 5-fold IgG Ab concentrations compared to cases with mild symptoms and 8-fold compared to asymptomatic cases. InterpretationFollowing detection of a COVID-19 case in a household, other members had a high risk of becoming infected. Follow-up of participants showed strong persistence of antibodies in most cases. FundingThis study was supported by THL coordinated funding for COVID-19 research (Finnish Governments supplementary budget) and by the Academy of Finland (Decision number 336431). Research in contextO_ST_ABSEvidence before this studyC_ST_ABSHousehold transmission studies are pivotal to the characterization of transmission dynamics of emerging infectious diseases in a closed setting with homogenous exposure, including proportion of asymptomatic cases using serologic assessment of infection. Additionally, data on long-term persistence of immune response, including neutralizing antibodies following COVID-19 remains scarce. Our search on PubMed for articles published between January 1st 2020, and June 1st, 2021 using the search terms "household" AND "transmission" AND ("COVID-19" OR "SARS-CoV-2") retrieved 381 results including 35 relevant articles: 21 original household transmission studies, 5 reviews and 9 statistical transmission, modelling or register linkage studies. Depending on the diagnosis method and the duration of follow-up, secondary attack rates (SAR) ranged from 4.6% when household contacts were followed for 14 days and tested only in case of symptoms to close to 90%. None of the household transmission studies involved long-term convalescent follow-up. Added value of this studyThis extensive (one month) active follow-up, using RT-PCR diagnosis and serological testing for SARS-CoV-2 nucleoprotein IgG antibodies (IgG Ab) and neutralizing antibodies (NAb) showed that household transmission was high, with a 48% (42/87) SAR overall and 50% [IQR: 0-100%] at the level of the household. All but one out of 64 RT-PCR confirmed participants had developed both IgG Ab and NAb after immediate convalescence. Six months after inclusion, majority of previously seropositive (IgG and/or NAb) participants still had IgG Ab (59/75) or NAb (63/75) showing long-term persistence of humoral immunity to SARS-CoV-2. Implications of all the available evidenceThe risk of transmission of SARS-CoV-2 infections within households is considerable. Isolation of the primary case, especially from household contacts with a high risk of severe disease, e.g. due to age or comorbidities, should be considered even though viral shedding might occur before confirmed diagnosis in household contacts. Long-term persistence of antibodies following infection, even in asymptomatic and mild cases, suggests enduring natural immunity and possibly protection from severe COVID-19.
ABSTRACT
BackgroundThe role of children in SARS-CoV-2 transmission is unclear. We investigated two COVID-19 school exposure incidents in the Helsinki area. MethodsWe conducted two retrospective cohort studies after schools exposures, with a household transmission extension. We defined a case as an exposed person with either a positive RT-PCR, or positive microneutralisation testing (MNT) as confirmation of SARS-CoV-2 nucleoprotein IgG antibodies detection via fluorescent microsphere immunoassay (FMIA). We recruited close school contacts and families of school cases, calculated attack rates (AR) on school level and families, and identified transmission chains. FindingsIn incident A, the index was a pupil. Participation rate was 74% (89/121), and no cases were identified. In incident B, the index was a member of school personnel. Participation rate was 81% (51/63). AR was 16% (8/51): 6 pupils and 1 member of school personnel were MNT and FMIA positive; 1 pupil had a positive RT-PCR, but negative serology samples. We visited all school cases families (n=8). The AR among close household contacts was 42% (9/20 in 3/8 families) but other plausible sources were always reported. At three months post-exposure, 6/8 school cases were re-sampled and still MNT positive. InterpretationWhen the index was a child, no school transmission was identified, while the occurrence of an adult case led to a 16% AR. Further cases were evidenced in 3 families, but other transmission chains were plausible. It is likely that transmission from children to adults is limited. FundingThe Finnish Institute for Health and Welfare funded this study. Research in contextO_ST_ABSEvidence before the studyC_ST_ABSThe first autochthonous case of COVID-19 in Finland was identified on February 29th. Transmission of the virus has led to more than 7250 cases and over 300 deaths (As of July 12th 2020). On March 16th, assuming that children might have a role in transmission, the Finnish government ordered school closures, to the exclusion of pre-school and grades 1-3. Schools were closed from March 18 and reopened on May 14th. At the stage of closure, a very limited number of reports of school related COVID-19 clusters or exposure incidents had been published, and the potential extent of transmission in a school setting was unknown. Added value of this studyWe investigated two exposure incidents in two different schools from the Helsinki area to assess transmission among pupils, school personnel and household contacts of identified cases. In school A, contact with a COVID-19 pupil did not lead to further transmission, while in school B, out of 51 recruited contacts, eight (16%) were proved to have had COVID-19 infection, including one member of staff. Among the close household contacts of pupils who were tested positive, COVID-19 attack rate was 31% (5/16). However, in all investigated households, other sources of infections were plausible; hence household transmission following a pediatric COVID-19 case appears to be limited. Implications of all of the available evidenceIncidence of COVID-19 infections in children following school related exposure was limited, as well as secondary transmission within their household. We hope our findings will help prioritize mitigation measures as well as reduce worry among parents of school aged children as most EU countries are preparing for the start of a new school year in autumn.
