Laboratory-based surveillance of COVID-19 in the Greater Helsinki area, Finland, February-June 2020

Abstract

Objectives: The aim was to characterise age- and sex-specific severe acute respiratory syndrome coronavirus disease-2 (SARS-CoV-2) RT-PCR sampling frequency and positivity rate in Greater Helsinki area in Finland during February-June 2020. We also describe the laboratory capacity building for these diagnostics.

Methods: Laboratory registry data for altogether 80,791 specimens from 70,517 individuals was analysed. The data included the date of sampling, sex, age and the SARS-CoV-2 RT-PCR test result on specimens collected between 1 February and 15 June 2020.

Results: Altogether, 4057/80,791 (5.0%) of the specimens were positive and 3915/70,517 (5.6%) of the individuals were found positive. In all, 37% of specimens were from male and 67% from female subjects. While the number of positive cases was similar in male and female subjects, the positivity rate was significantly higher in male subjects: 7.5% of male and 4.4% of female subjects tested positive. The highest incidence/100,000 was observed in those aged ≥80 years. The proportion of young adults in positive cases increased in late May 2020. Large dips in testing frequency were observed during every weekend and also during public holidays.

Conclusions: Our data suggest that men pursue SARS-CoV-2 testing less frequently than women. Consequently, a subset of coronavirus disease-2019 infections in men may have gone undetected. People sought testing less frequently on weekends and public holidays, and this may also lead to missing of positive cases. The proportion of young adults in positive cases increased towards the end of the study period, which may suggest their returning back to social behaviour with an increased risk of infection.

Keywords: COVID-19; Nucleic acid amplification; Real-time RT-PCR; SARS-CoV-2; Surveillance.

Figure 1

Number of SARS-CoV-2 RT-PCR tests and positive cases. (A) The number of SARS-CoV-2 RT-PCR tests conducted in HUSLAB by date of sampling and positivity rate, between 1 March and 15 June 2020. (B) The number of SARS-CoV-2 positive cases (columns) with seven-day central moving average (line) by the date of sampling, between 1 March and 15 June 2020.

Figure 2

Age characteristics of SARS-CoV-2 RT-PCR tested cases. (A) The number of individuals tested for SARS-CoV-2 RT-PCR by age group (columns) and the proportion of SARS-CoV-2 positive cases (line). (B) The number of SARS-CoV-2 positive cases by age group and the age-specific incidence/100,000 population of SARS-CoV-2 in the Greater Helsinki area (line), data from the National Infectious Diseases Registry. (C) Age stratification of SARS-CoV-2 positive cases by calendar week of 2020. The proportion (%) of each age group is shown.

Figure 3

Age-specific number of SARS-CoV-2 RT-PCR tests and positive cases according to calendar week. (A–C) The age-specific number of individuals tested for SARS-CoV-2 RT-PCR by calendar week of 2020. (D–F) The age-specific number of SARS-CoV-2-positive cases by calendar week of 2020. (G–I) The age-specific proportion (%) of SARS-CoV-2-positive cases by calendar week of 2020. The age range for all tested male subjects was 1 d–102 years (mean 43.1 and median 41 years) and for positive male subjects 2 months–97 years (mean 43.5 and median 41 years). The age range for all tested female subjects was 0 d–106 years (mean 45.5 and median 43 years) and for positive female subjects was 1 month–100 years (mean 47.7 and median 45 years).

Figure 4

Sex-specific number of SARS-CoV-2 RT-PCR tests and positive cases according to calendar week. (A) The sex-specific number of SARS-CoV-2 individuals tested for SARS-CoV RT-PCR by calendar week of 2020. (B) The sex-specific number of SARS-CoV-2 positive cases by calendar week of 2020. (C) The sex-specific proportion (%) of SARS-CoV-2 positive cases by calendar week of 2020.