High transmission rates persist despite high seroprevalence

A Swedish study published in the journal Emerging Infectious Diseases observes a high prevalence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) omicron infection despite more than 80% seroprevalence in Sweden.

Study: High Prevalence of SARS-CoV-2 Omicron Infection Despite High Seroprevalence, Sweden, 2022. Image Credit: ibrahimbeyy / Shutterstock

Background

The coronavirus disease 2019 (COVID-19) pandemic caused by SARS-CoV-2 has created tremendous healthcare and socioeconomic damage worldwide, with more than 765 million infections and over 6.9 million deaths registered to the World Health Organization (WHO).

The most recently emerged omicron variant of SARS-CoV-2 has shown significantly higher transmissibility and infectivity compared to other variants of concern (VOCs) with improved viral fitness that have emerged during the pandemic.

Because of the high percentage of mutations in the spike receptor binding domain (RBD), omicron has gained high immune evasion potency, causing breakthrough infections and reinfections worldwide despite preexisting anti-SARS-CoV-2 immunity developed by COVID-19 vaccination or natural SARS-CoV-2 infection.

In Sweden, general polymerase chain reaction (PCR) testing for COVID-19 has been suspended in early 2022. Currently, the country largely depends on point prevalence surveys that are conducted to detect acute SARS-CoV-2 infections by PCR and previous infections by serological testing.

In the current study, scientists have conducted two cross-sectional surveys during 2022 to estimate COVID-19 point-prevalence and overall seroprevalence in Sweden.

Study design

Two surveys covering 21 and 11 regions in Sweden were conducted in March and September, respectively. The participants representing 64% of the Swedish population, were provided with appropriate kits and related instructions for self-sampling at home.

In the March survey, a total of 2,906 individuals (age range: 2 – 96 years) were enrolled. In the September survey, a total of 1,774 individuals (age range: 2 – 94 years) were enrolled.

Important observations

The point prevalence of COVID-19 in the Swedish population was estimated to be 1.4% at the end of March. All detected infections were caused by omicron subvariants except one that was caused by delta variant.

Among participants tested for ongoing infection, 24% had previously reported SARS-CoV-2 infection and 79% had received at least three doses of COVID-19 vaccine. Among participants who tested positive for SARS-CoV-2, about 8% had previously reported SARS-CoV-2 infection.  

The analysis of anti-spike antibody levels in participants revealed a seroprevalence of 93% in the Swedish population at the end of March. Among participants aged below 11 years and above 11 years, the estimated seroprevalences were 80% and 94 – 98%, respectively.

In the September survey, the point prevalence was estimated to be 1.5%. All detected infections were caused by omicron subvariants.

Among participants tested for ongoing infection, 29% had previously reported SARS-CoV-2 infection and 85% had received at least three doses of COVID-19 vaccine. Among participants who tested positive for SARS-CoV-2, about 71% had previously reported SARS-CoV-2 infection. 

The seroprevalence in the Swedish population was estimated to be 93% at the end of September. Among participants aged below 11 years and above 11 years, the estimated seroprevalences were 84% and 84 – 100%, respectively.

Regarding symptoms, about 65% and 67% of the participants from the March and September surveys, respectively, reported more than one symptom within two weeks before sampling.

Study significance

The study reveals that approximately 1 in every 66 individuals in Sweden was infected with SARS-CoV-2 by March and September 2022. Almost all of the detected infections were caused by omicron subvariants. Moreover, the study reveals that the estimated seroprevalence in the Swedish population was more than 80% during the same period.

Overall, the study findings indicate that current COVID-19 vaccines provide only limited, short-term protection against omicron breakthrough infections. Similarly, a high level of reinfections observed in the study indicates that omicron subvariants are capable of evading not only vaccine-induced immunity but also infection-induced immunity.

Considering the high transmissibility of omicron subvariants despite high vaccine coverage, the scientists highlight the need for continuous monitoring of the general population for the detection of newly emerging and possibly more pathogenic viral variants.