Household Transmission of SARS-CoV-2: A Prospective Longitudinal Study Showing Higher Viral Load and Increased Transmissibility of the Alpha Variant Compared to Previous Strains.

Cathinka Halle Julin,Marit Fodnes Killengreen,Johanna Eva Bodin,Fredrik Oftung,Olav Hungnes,Torunn Ramsem Strand,Einar Sverre Berg,Lisbeth Meyer Næss,Idunn Forland Aune,Ida Laake,Anneke Steens,Gro Tunheim,Terese Bekkevold,Kathrine Stene-Johansen,Dagny Haug Dorenberg,Anna Hayman Robertson,Rikard Rykkvin

doi:10.3390/microorganisms9112371

Cathinka Halle Julin, Marit Fodnes Killengreen + Show 15 more

Open Access

https://doi.org/10.3390/microorganisms9112371

Copy DOI

Journal: Microorganisms	Publication Date: Nov 17, 2021
Citations: 27	License type: CC BY 4.0

Affiliation: Norwegian Institute of Public Health

Abstract

We studied the secondary attack rate (SAR), risk factors, and precautionary practices of household transmission in a prospective, longitudinal study. We further compared transmission between the Alpha (B.1.1.7) variant and non-Variant of Concern (non-VOC) viruses. From May 2020 throughout April 2021, we recruited 70 confirmed COVID-19 cases with 146 household contacts. Participants donated biological samples eight times over 6 weeks and answered questionnaires. SARS-CoV-2 infection was detected by real-time RT-PCR. Whole genome sequencing and droplet digital PCR were used to establish virus variant and viral load. SARS-CoV-2 transmission occurred in 60% of the households, and the overall SAR for household contacts was 50%. The SAR was significantly higher for the Alpha variant (78%) compared with non-VOC viruses (43%) and was associated with a higher viral load. SAR was higher in household contacts aged ≥40 years (69%) than in younger contacts (40–47%), and for contacts of primary cases with loss of taste/smell. Children had lower viral loads and were more often asymptomatic than adults. Sleeping separately from the primary case reduced the risk of transmission. In conclusion, we found substantial household transmission, particularly for the Alpha variant. Precautionary practices seem to reduce SAR, but preventing household transmission may become difficult with more contagious variants, depending on vaccine use and effectiveness.

Highlights

Our overall secondary attack rate (SAR) of 49.6% is almost identical to the SAR of 49% found in a similar prospective household study from the USA performed during the same time-period [29] and in accordance with another Norwegian household study from the first wave of the pandemic, which estimated a SAR of 47% based on reverse transcription polymerase chain reaction (rRT-PCR) and seroconversion [30]
The association between taste/smell impairment and higher viral load has been found by others [48,49]. This may be dependent on variant, as we found that loss of taste/smell was more common amongst primary cases with the Alpha variant
The age span of participants was limited, with few elderly individuals and mostly adult primary cases. In this prospective longitudinal household study, we found an overall SAR for household contacts of 50%

Summary

Introduction

SARS-CoV-2, the virus that causes the respiratory disease COVID-19, was first detected in China in 2019 and spread rapidly throughout the world [1]. Health Organization (WHO) declared COVID-19 a pandemic. Households have been one of the most important sites of transmission in Norway [2], as well as in other countries [3,4]. It is important to identify risk factors for household transmission and effective precautionary practices to contain the epidemic. To this end, the WHO encouraged its member states to perform household studies. Secondary attack rate (SAR), Microorganisms 2021, 9, 2371.

Methods

Results

Discussion

Conclusion