On March 9 the first patient with covid-19 was diagnosed in Norrbotten, the northernmost county in Sweden, a sparsely populated area by the Arctic Circle with four major cities by the Bay of Bothnia. The first fatality was noted March 25. As of now (2020-06-15) 693 patients have been diagnosed by PCR, a cumulative incidence of 277/100 000 inhabitants. 242 have been treated in hospital, including those who neededintensive care. 59 patients have died, a cumulative mortality of 23,6/100 000. Presently 12 patients are treated in general wardsand 7 in intensive care units.

In order to monitor the epidemic situation of covid-19 in Norrbotten we invited 500 randomly selected men and women,20 to 80 years of age (out of an adult population of 182 828) to blood sampling and a symptom survey. We performed sampling during week 22 and 23 (May 25-June 5).

We analysed antibodies with a validated assay (Abbott SARS-CoV-2 IgG kit) and positive samples were confirmed by asecond assay (Euroimmun Anti-SARS-CoV-2 ELISA (IgG).

425 participated (85 %). Non-participation was greatest among the youngest. 8 participants had positive serology (1.9 %) (95 % CI (0,8; 3,7) %). Positive serology was most common among the younger participants and lower among the elderly 65-80 years.

1.8 % of men and 2.0 % of women had antibodies against covid-19. None of those seropositive reported being unsymptomatic whereas 33 % of those seronegative had been unsymptomatic.

Conclusion

More than two months after the first case of covid-19 in Norrbotten, 1.9 % of the general population in theage span of 20-80 years old had antibodies. Expressed as aconfidence interval, this means that we estimate the number of persons in the county infected before week 18 to be between 1460 and 6760. 

Read the full report here (Swedish)