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. 2022 Mar;16(2):204-212.
doi: 10.1111/irv.12932. Epub 2021 Nov 9.

Trends in seroprevalence of SARS-CoV-2 and infection fatality rate in the Norwegian population through the first year of the COVID-19 pandemic

Gro Tunheim  1 , Gunnar Øyvind Isaksson Rø  1 , Trung Tran  2 , Anne-Marte Bakken Kran  1 , Jan Terje Andersen  2   3 , Eline Benno Vaage  2 , Anette Kolderup  2 , John Torgils Vaage  2 , Fridtjof Lund-Johansen  2   4 , Olav Hungnes  1
Affiliations

Trends in seroprevalence of SARS-CoV-2 and infection fatality rate in the Norwegian population through the first year of the COVID-19 pandemic

Gro Tunheim et al. Influenza Other Respir Viruses. 2022 Mar.

Abstract

Background: Infection with the novel coronavirus SARS-CoV-2 induces antibodies that can be used as a proxy for COVID-19. We present a repeated nationwide cross-sectional study assessing the seroprevalence of SARS-CoV-2, the infection fatality rate (IFR), and infection hospitalization rate (IHR) during the first year of the pandemic in Norway.

Methods: Residual serum samples were solicited in April/May 2020 (Round 1), in July/August 2020 (Round 2) and in January 2021 (Round 3). Antibodies against SARS-CoV-2 were measured using a flow cytometer-based assay. Aggregate data on confirmed cases, COVID-19-associated deaths and hospitalizations were obtained from the Emergency preparedness registry for COVID-19 (Beredt C19), and the seroprevalence estimates were used to estimate IFR and IHR.

Results: Antibodies against SARS-CoV-2 were measured in 4840 samples. The estimated seroprevalence increased from 0.8% (95% credible interval [CrI] 0.4%-1.3%) after the first wave of the pandemic (Rounds 1 and 2 combined) to 3.2% (95% CrI 2.3%-4.2%) (Round 3). The IFR and IHR were higher in the first wave than in the second wave and increased with age. The IFR was 0.2% (95% CrI 0.1%-0.3%), and IHR was 0.9% (95% CrI 0.6%-1.5%) for the second wave.

Conclusions: The seroprevalence estimates show a cumulative increase of SARS-CoV-2 infections over time in the Norwegian population and suggest some under-recording of confirmed cases. The IFR and IHR were low, corresponding to the relatively low number of COVID-19-associated deaths and hospitalizations in Norway. Most of the Norwegian population was still susceptible to SARS-CoV-2 infection after the first year of the pandemic.

Keywords: COVID-19; Norway; SARS-CoV-2; infection fatality rate; infection hospitalization rate; seroprevalence.

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Figures

FIGURE 1
FIGURE 1
Cumulative incidence of polymerase chain reaction (PCR)‐confirmed COVID‐19‐cases in Norway in 2020 and 2021 by month as reported to the Norwegian Surveillance System for Communicable Diseases (MSIS). The first collection (Round 1) of residual serum samples occurred in April/May 2020 (week numbers 17–20), the second collection (Round 2) between July and September 2020 (week numbers 30–37) and the third collection (round 3) between December 2020 and February 2021 (week numbers 53–6)
FIGURE 2
FIGURE 2
Overall seroprevalence, IFR, IHR and percentage detected infections by rounds/waves with 95% credible intervals. (A) Shows the overall estimated seroprevalence for the Norwegian population by round, (B–D) shows the overall infection fatality rate (IFR), infection hospitalization rate (IHR) and percentage of infections detected by Waves 1 and 2, respectively
FIGURE 3
FIGURE 3
Estimated percentage infected, IFR, IHR and percentage detected infections by age and waves with 95% credible intervals. (A) Shows the estimated seroprevalence for the Norwegian population by age and round, (B–D) shows the infection fatality rate (IFR), infections hospitalization rate (IHR) and percentage of infections detected by age for Waves 1 and 2, respectively
FIGURE 4
FIGURE 4
Estimated percentage infected, IFR, IHR and percentage detected infections by sex and waves with 95% credible intervals. (A) Shows the estimated seroprevalence by sex and collection round, (B–D) shows the infection fatality rate (IFR), infections hospitalization rate (IHR) and percentage of infections detected by sex for Waves 1 and 2, respectively
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