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Spread of SARS-CoV-2 in the Icelandic Population

Our take —

This study showed 13.3% of those at the beginning of the pandemic who were tested due to suspected exposure were infected, while in the general population, the infection rate was much lower (0.8%). There were major differences in haplotypes and exposures from those reported earlier in the pandemic (beginning of March) to later (end of March). However, it was unclear how this subset of individuals was selected for further testing.

Study design

Cross-Sectional, Other

Study population and setting

The study included 9,199 people who were targeted for SARS-CoV-2 testing due to symptoms, contact with infected persons or travel to high risk areas from January 31 to March 30, 2020, 10,797 people who were part of the population screening and open invitation period from March 13 to April, 2020 and 2283 who were invited as part of a random population sample from April 1 to 4, 2020. The study included individuals across Iceland, though sample collection took place in Reykjavik.

Summary of Main Findings

Among those who received targeted testing, 13.3% tested positive for SARS-CoV-2, while for the population screening, only 0.8% had positive results. The population screening level remained stable for the 20-day period of screening. While 93% of those testing positive in the targeted-testing group showed symptoms, 57% of those testing positive in the population screening had symptoms. Viral haplotypes differed from the beginning to end of testing, with A2a3a and A2a2a being the most common haplotypes in Iceland. While most early cases were exposed via travel at the beginning in early March, the majority reported exposure from family by March 30, 2020.

Study Strengths

This study had a large number of both those for targeted testing and for the population-based sample. The inclusion of haplotyping and contact tracing data provided a large, unified dataset and robust study design.

Limitations

The major limitation was lack of clarity of the how haplotyped and contact traced subsample was selected, which may introduce bias. There may also be volunteer bias among the population sample among those who were more concerned about symptoms or potential infection than the overall population. As sample collection was limited to Reykjavik, travel to the capital may have been limited to those who were symptomatic.

Value added

This study is one of the first to report widespread population-based testing over time, as well as targeted testing earlier in the pandemic for comparison.