Transmission of SARS-CoV-2 within households: a prospective cohort study in the Netherlands and Belgium – Interim results

Abstract

Background: Household transmission studies are useful to obtain granular data on SARS-CoV-2 transmission dynamics and to gain insight into the main determinants. In this interim report we investigated secondary attack rates (SAR) by household and subject characteristics in the Netherlands and Belgium. Methods: Households with a real-time reverse transcription polymerase chain reaction (RT-PCR) confirmed SARS-CoV-2 index case were enrolled <48 hours following report of the positive test result. Daily symptom follow-up, standardized nose-throat sampling at enrollment and at new-onset acute respiratory illness (ARI) and paired dried blood spots (DBS) were collected from each participant. Children 0-2 years of age were additionally requested to collect a stool sample 7 days after enrollment and at new-onset of ARI. Swabs and stool samples were tested by RT-PCR for virus detection and DBS by multiplex protein microarray for detection of SARS-CoV-2 antibodies. The SAR was calculated 1) per-household as the proportion of households with ≥1 secondary SARS-CoV-2 case and 2) per-person as the probability of infection in household members at risk. We explored differences in SARs by household and subject characteristics. Results: This analysis includes 117 households that completed follow-up between April-December 2020. Among 382 subjects, 74 secondary infections were detected, of which 13 (17.6%) were asymptomatic and 20 (27.0%) infections were detected by seroconversion only. Of cases detected by RT-PCR, 50 (67.6%) were found at enrollment. The household SAR was 44.4% (95%-CI: 35.4-53.9%) and was higher for index cases meeting the ARI case definition (52.3%; 95%-CI 41.4-62.9%) compared to mildly symptomatic (22.2%; 95%-CI: 9.4-42.7%) and asymptomatic index cases (0.0%; 95%-CI: 0.0-80.2%). The per-person SAR was 27.9% (95%-CI: 22.7-33.8%). Transmission was lowest from child to parent (9.1%; 95%-CI: 2.4-25.5%) and highest from parent to child (28.1%; 95%-CI: 19.7-38.4%) and in children 6-12 years (34.2%; 95%-CI: 20.1-51.4%). Among 141 subjects with RT-PCR confirmed SARS-CoV-2 infections, seroconversion was detected in 111 (78.7%). Conclusion: We found a high household SAR, with the large majority of transmissions detected early after identification of the index case. Our findings confirm differential SAR by symptom status of the index. In almost a quarter of RT-PCR positive cases, no antibodies were detected. Other factors influencing transmission will be further explored as more data accumulate.

Factors Associated With Household Transmission of SARS-CoV-2

“…Compared with the SAR for 28 studies 12,14,17,27,94 from January to February 2020 (13.4%; 95% CI, 10.7%-16.7%), the SAR was significantly higher for 30 studies 6,7,15,16,19,22,25,26,28,30,[68][69][70][71][72][73][74][75][76][77][78][79][80][81][82][83][84][85][86]93 from March to April 2020 (19.4%; 95% CI, 15.2%-24.5%; P = .03) and 15 studies 5,8,10,18,20,21,23,24,29,31,32,35,37,38,40 from July 2020 to March 2021 (31.1%; 95% CI, 22.6%-41.1%; P < .001) but not significantly different from the SAR for 14 studies…”

“…To elucidate factors associated with differences in SAR, we explored attributes of studies from the periods with the lowest and highest household SARs. Among 28 studies 12,14,17,27,94 from January to February 2020 and 15 studies 5,8,10,18,20,21,23,24,29,31,32,35,37,38,40 from July 2020 to March 2021, 6 studies 12,46,54,57,59,62 (21.4%) and 4 studies 8,10,20,23 (25.0%), respectively, reported testing contacts at least twice, 1 study 46 (3.6%) and 3 studies 5,8,23 (18.8%) reported following contacts for longer than 14 days, 1 study 45 (3.6%) and 6 studies 8,23,24,29,37,40 (33.3%) were published as preprints, 21 studies 12,14,27,46,[48][49][50]…”

“…Characteristics of Studies Included in Analysis of Household Secondary Attack Rates for SARS-CoV-2 (continued) ,23,24,29,37,40,45,79,[88][89][90]92 21.0(13.8-30.6) No 75[4][5][6][7][9][10][11][12][13][14][15][16][17][18][19][20][21][22][25][26][27][28][30][31][32][33][34][35][36][37][38][39][74][75][76][77][78][80][81][82][83][84]…”

Evaluation of a throat spray with lactobacilli in COVID-19 outpatients in a randomized, double-blind, placebo-controlled trial for symptom and viral load reduction

“…Our finding of reduced viral loads also suggests some to-be-validated potential to reduce transmission to household members and other high-risk contacts. In a prospective cohort study conducted in the Netherlands and Belgium, it was shown that secondary transmission within households occurred in 44.4% of the households, mostly very early after the index patient was positive [17]. Moreover, it might be useful for future studies to stratify potential responders and non-responders based on the microbiome (e.g.…”

“…Patients were asked to use the verum spray or placebo for 14 days, with one week of follow-up, and filled in an online diary via Qualtrics (Qualtrics, Provo, UT, USA) (detailed description in Figure S1). Ten common COVID-19 symptoms were monitored, according to [17]. Different symptom summary scores were compared between the verum and placebo groups, and time to improvement was evaluated based on the timepoint when participants reached their symptomatic tipping point (supplementary methods).…”