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. 2021 Nov 11;16(11):e0259908.
doi: 10.1371/journal.pone.0259908. eCollection 2021.

Evaluation of the SARS-CoV-2 positivity ratio and upper respiratory tract viral load among asymptomatic individuals screened before hospitalization or surgery in Flanders, Belgium

Laura Heireman  1 , Steven Abrams  2   3 , Peggy Bruynseels  1 , Reinoud Cartuyvels  4 , Lize Cuypers  5 , Pieter De Schouwer  1 , Wim Laffut  6 , Katrien Lagrou  5 , Niel Hens  2   7 , Erwin Ho  8 , Elizaveta Padalko  9 , Marijke Reynders  10 , Sarah Vandamme  11 , Nathalie Van der Moeren  1 , Walter Verstrepen  1 , Philippe Willems  12 , Reinout Naesens  1
Affiliations

Evaluation of the SARS-CoV-2 positivity ratio and upper respiratory tract viral load among asymptomatic individuals screened before hospitalization or surgery in Flanders, Belgium

Laura Heireman et al. PLoS One. .

Abstract

Introduction: The incidence of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) infections in the Belgian community is mainly estimated based on test results of patients with coronavirus disease (COVID-19)-like symptoms. The aim of this study was to investigate the evolution of the SARS-CoV-2 reverse transcriptase polymerase chain reaction (RT-PCR) positivity ratio and distribution of viral loads within a cohort of asymptomatic patients screened prior hospitalization or surgery, stratified by age category.

Materials/methods: We retrospectively studied data on SARS-CoV-2 real-time RT-PCR detection in respiratory tract samples of asymptomatic patients screened pre-hospitalization or pre-surgery in nine Belgian hospitals located in Flanders over a 12-month period (1 April 2020-31 March 2021).

Results: In total, 255925 SARS-CoV-2 RT-PCR test results and 2421 positive results for which a viral load was reported, were included in this study. An unweighted overall SARS-CoV-2 real-time RT-PCR positivity ratio of 1.27% was observed with strong spatiotemporal differences. SARS-CoV-2 circulated predominantly in 80+ year old individuals across all time periods except between the first and second COVID-19 wave and in 20-30 year old individuals before the second COVID-19 wave. In contrast to the first wave, a significantly higher positivity ratio was observed for the 20-40 age group in addition to the 80+ age group compared to the other age groups during the second wave. The median viral load follows a similar temporal evolution as the positivity rate with an increase ahead of the second wave and highest viral loads observed for 80+ year old individuals.

Conclusion: There was a high SARS-CoV-2 circulation among asymptomatic patients with a predominance and highest viral loads observed in the elderly. Moreover, ahead of the second COVID-19 wave an increase in median viral load was noted with the highest overall positivity ratio observed in 20-30 year old individuals, indicating they could have been the hidden drivers of this wave.

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Conflict of interest statement

The authors have declared that no competing interests exist.

Figures

Fig 1
Fig 1
Evolution of the monthly SARS-CoV-2 real-time RT-PCR positivity ratio in the Flemish population (black solid line) and in a-/pre-symptomatic patients screened pre-hospitalization/pre-surgery by province and overall (black dots and dashed line) (in Flanders) together with the daily number of SARS-CoV-2 hospitalizations in Flanders (pink shaded area) as a function of time (from 1 April 2020 to 31 March 2021). The size of the dots is proportional to the number of observations that are available. The Oxford stringency index (red solid line) is presented together with the time at which important mitigation measures (gray) were put into place.
Fig 2
Fig 2. Evolution of the monthly SARS-CoV-2 real-time RT-PCR positivity ratio in a-/pre-symptomatic patients screened pre-hospitalization/pre-surgery by ten-year age group as a function of time (from 1 April 2020 to 31 March 2021).
The black solid line shows the age-weighted overall SARS-CoV-2 positivity ratio based on the age distribution in Belgium anno 2020. Error bars represent 95% exact Clopper-Pearson confidence limits for the age-specific positivity ratios.
Fig 3
Fig 3. Boxplots showing the evolution of the (distribution of the) log-transformed viral load (in copies/mL) for SARS-CoV-2 positive a-/pre-symptomatic patients by month (from 1 April 2020 to 31 March 2021) and the evolution of the percentage of strongly positive patients among the SARS-CoV-2 positive a-/pre-symptomatic patients by month (red dots) with pointwise 95% exact Clopper-Pearson confidence intervals.
Fig 4
Fig 4. Boxplots of the log-transformed viral load (in copies/mL) for SARS-CoV-2 positive a-/pre-symptomatic patients by age (upper panel) and province (lower panel) with reported number of observations per group within the boxplots and data collected between 1 April 2020 to 31 March 2021.
Fig 5
Fig 5. Estimated evolution of the SARS-CoV-2 real-time RT-PCR positivity ratio in a-/pre-symptomatic patients screened pre-hospitalization/pre-surgery by province for individuals aged 54 years (median age of screened patients) together with pointwise 95% confidence bounds (shaded areas).
Fig 6
Fig 6
Estimated evolution of the SARS-CoV-2 real-time RT-PCR positivity ratio in a-/pre-symptomatic patients screened pre-hospitalization/pre-surgery by age (individuals aged 19 years, 33 years, 54 years, 69 years and 80 years) in Limburg (upper left panel, A), East Flanders (upper right panel, B), West Flanders (middle left panel, C), Flemish Brabant (middle right panel, D), Antwerp (lower panel, E) together with pointwise 95% confidence bounds (shaded areas).
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