During the current worldwide pandemic, coronavirus disease 2019 (Covid-19) was first diagnosed in Iceland at the end of February. However, data are limited on how SARS-CoV-2, the virus that causes Covid-19, enters and spreads in a population.
We targeted testing to persons living in Iceland who were at high risk for infection (mainly those who were symptomatic, had recently traveled to high-risk countries, or had contact with infected persons). We also carried out population screening using two strategies: issuing an open invitation to 10,797 persons and sending random invitations to 2283 persons. We sequenced SARS-CoV-2 from 643 samples.
As of April 4, a total of 1221 of 9199 persons (13.3%) who were recruited for targeted testing had positive results for infection with SARS-CoV-2. Of those tested in the general population, 87 (0.8%) in the open-invitation screening and 13 (0.6%) in the random-population screening tested positive for the virus. In total, 6% of the population was screened. Most persons in the targeted-testing group who received positive tests early in the study had recently traveled internationally, in contrast to those who tested positive later in the study. Children under 10 years of age were less likely to receive a positive result than were persons 10 years of age or older, with percentages of 6.7% and 13.7%, respectively, for targeted testing; in the population screening, no child under 10 years of age had a positive result, as compared with 0.8% of those 10 years of age or older. Fewer females than males received positive results both in targeted testing (11.0% vs. 16.7%) and in population screening (0.6% vs. 0.9%). The haplotypes of the sequenced SARS-CoV-2 viruses were diverse and changed over time. The percentage of infected participants that was determined through population screening remained stable for the 20-day duration of screening.
In a population-based study in Iceland, children under 10 years of age and females had a lower incidence of SARS-CoV-2 infection than adolescents or adults and males. The proportion of infected persons identified through population screening did not change substantially during the screening period, which was consistent with a beneficial effect of containment efforts
In Iceland, the prevalence of SARS-CoV-2 infection among persons at high risk for infection and the stability of the infection rate in the general population provide grist for both assurance and alarm. The percentage of participants who tested positive in population screening remained stable (0.8%) over the course of 20 days, and the infection rates in the two screening groups (recruited through open invitation and through random sampling) were not substantially different. These results were consistent with a slow spread of SARS-CoV-2 through the Icelandic population.
The lack of increase in the incidence of infection over time may be due to containment efforts by the Icelandic health care authorities and their nimble response to the outbreak abroad. Testing of exposed persons with symptoms had been carried out for 1 month before the first SARS-CoV-2 case was identified in Iceland. Self-isolation, quarantining, and other social-distancing measures may also have helped to prevent an increase in the infection rate.
Although we asked participants who had respiratory symptoms that they described as more than mild not to participate in population screening, close to half the participants reported symptoms, most commonly rhinorrhea and coughing. Thus, a weakness in the design of the population-screening component of the study was that persons who were concerned about potential infection may have been more likely to participate than others. Symptoms were common both in participants who tested positive and in those who tested negative for SARS-CoV-2 in the overall population-screening group. Notably, 43% of the participants who tested positive reported having no symptoms, although symptoms almost certainly developed later in some of them. During the study, the prevalence of symptoms decreased considerably in both testing groups (despite the stability of the SARS-CoV-2 infection rate), probably owing to a general decrease in other respiratory infections, which in turn may have been brought about through measures implemented to decrease the spread of SARS-CoV-2.
Young children and females were less likely to test positive for SARS-CoV-2 than adolescents or adults and males. Whether the lower incidence of positive results in these two groups resulted from less exposure to the virus or from biologic resistance is not known. In other studies, investigators have found that infected children and females were less likely to have severe disease than adults and males, respectively.11,12
The haplotype composition of the viruses from persons who were identified through population screening was different from that of viruses infecting persons who tested positive in the early phase of targeted testing, so we conclude that the haplotypes of the virus that were propagating in the general population came from a different source (as compared with those infecting high-risk persons in the early phase of targeted testing), perhaps brought into Iceland by persons arriving from countries that had not yet been designated as high-risk areas.
Thus, the frequency of the SARS-CoV-2 infection in the overall Icelandic population was stable from March 13 to April 1, a finding that appears to indicate that the containment measures had been working. However, the virus has spread to the extent that unless we continue to test and isolate, track contacts, and quarantine, we are likely to fail in our efforts to contain the virus.
Reference & source information: https://www.nejm.org/
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