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. 2022 Nov 8;20(1):427.
doi: 10.1186/s12916-022-02642-4.

Lower transmissibility of SARS-CoV-2 among asymptomatic cases: evidence from contact tracing data in Oslo, Norway

Fredrik Methi  1 , Elisabeth Henie Madslien  2
Affiliations

Lower transmissibility of SARS-CoV-2 among asymptomatic cases: evidence from contact tracing data in Oslo, Norway

Fredrik Methi et al. BMC Med. .

Abstract

Background: Asymptomatic COVID-19 cases have complicated the surveillance and tracking of the pandemic. Previous studies have estimated that 15-25% of all infectees remain asymptomatic.

Methods: Based on contact tracing data from Oslo, Norway, we estimated transmission and susceptibility dynamics among symptomatic and asymptomatic cases and their contacts as identified by manual contact tracing between September 1, 2020, and September 1, 2021.

Results: Among 27,473 indexes and 164,153 registered contacts, the secondary attack rate (SAR-14) was estimated to be 28% lower through asymptomatic exposure (13%) compared to symptomatic exposure (18%). Furthermore, those infected by asymptomatic cases were almost three times more likely to be asymptomatic compared to those infected by symptomatic cases.

Conclusions: Symptomatic cases spread the virus to a greater extent than asymptomatic, and infectees are more likely to be asymptomatic if their assumed infector was asymptomatic.

Keywords: Asymptomatic; COVID-19; SARS-CoV-2; Secondary attack rate; Transmission.

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Conflict of interest statement

The authors declare that they have no competing interests.

Figures

Fig. 1
Fig. 1
Flowchart of included index persons and close contacts. “Other exclusion criteria” includes index persons with more than 100 close contacts, index cases where all close contacts were missing in the population registry, or index cases where all close contacts had previously tested positive in the last 6 months
Fig. 2
Fig. 2
Probability of testing positive. Figure shows the share of close contacts testing positive within 14 days after index case tested positive. Shaded areas show the 95% confidence interval. Note that this is not conditional on being tested—all close contacts are included, tested or not. Additional file 1: Supplementary Fig. 4 presents the same figure only including those with at least one registered test within the 14-day time frame. And Additional file 1: Supplementary Fig. 5 presents the same figure stratified by household contacts and other contacts
See this image and copyright information in PMC

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