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

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.

Changing Patterns of SARS-CoV-2 Seroprevalence among Canadian Blood Donors during the Vaccine Era

“…In Canada, the seroprevalence for natural infection remained below or around 5% over the study period. This is similar to pooled seroprevalence estimates in some countries, including Spain, England, France, and Norway ( 23 , 24 ), but lower than the 20% reported in U.S. blood donors by May 2021 ( 5 ). This mirrors reported case data with 4,539 per 100,000 population in Canada ( 1 ), 5,000 in Spain ( 24 ), 5,549 in England ( 24 ), 2,504 in France ( 24 ), and 3,190 in Norway ( 23 ) but 14,089 per 100,000 population in the United States ( 25 ).…”

“…This is similar to pooled seroprevalence estimates in some countries, including Spain, England, France, and Norway ( 23 , 24 ), but lower than the 20% reported in U.S. blood donors by May 2021 ( 5 ). This mirrors reported case data with 4,539 per 100,000 population in Canada ( 1 ), 5,000 in Spain ( 24 ), 5,549 in England ( 24 ), 2,504 in France ( 24 ), and 3,190 in Norway ( 23 ) but 14,089 per 100,000 population in the United States ( 25 ). Higher prevalence of infection in racialized groups and those from neighborhoods with lower socioeconomic status in our study is consistent with higher incidence of COVID-19 morbidity and mortality in these groups in Canada and other countries ( 5 , 26 – 29 ), thought to be related to higher density living and employment with more social contacts.…”

Prevalence of antibodies against SARS‐CoV‐2 in the Norwegian population, August 2021

“…Anonymized residual sera were collected between July 19 and September 12, 2021, by 17 laboratories across Norway, as previously described. 7 , 8 Three samples were excluded due to antibody profiles indicating previous treatment with immunoglobulins. Sera lacking information on county of residence were attributed to the county of the submitting laboratory.…”

“…In children aged <12 years (unvaccinated individuals), seroprevalence of infection was estimated based on either having antibodies against RBD and spike, or against RBD and nucleocapsid as in previous studies. 7 …”

“… 7 In January 2021, at the start of the vaccine campaign, only 3.2% (95% credible interval [CrI] 2.3%–4.2%) of the Norwegian population had antibodies against SARS‐CoV‐2. 7 We wanted to study the progression of SARS‐CoV‐2 seroprevalence after several months of the national COVID‐19 vaccination campaign and a third wave of SARS‐CoV‐2 infections. We also wanted to investigate whether we could distinguish between antibodies from vaccination or infection.…”