Materials and methods

Cohort

Between 19th and 27th April 2020, 1170 military conscripts, 798 men (68. 2 %) and 372 women (31. 8 %) were enrolled. Median age was 20 years (range 18 – 25). As conscripts are called for military service regardless of residential area, our cohort included men and women from all over Norway.

Extended systematic infection control measures

The infection control measures performed by the Norwegian Armed Forces Joint Medical Services are visualized in Figure 1.

Pre-enrollment measures

Two weeks prior to enrollment, the conscripts were interviewed by telephone to motivate for military service and ensure compliance with infection control measures related to COVID-19. All conscripts were encouraged to conduct a self-imposed quarantine until enrollment.

Enrollment day questionnaires and examinations

Initially on the enrollment day, each conscript was asked about current respiratory symptoms, and body temperature was measured using an ear thermometer. Individuals with symptoms and/or temperature ≥ 38, 0⁰ C were further interviewed and clinically examined, and were then quarantined pending the results of the PCR virus test (Figure 1).
All conscripts had to respond to an online questionnaire, focusing on possible or confirmed COVID-19 over the past two weeks, compliance with public infection control measures and possible close contact with SARS-CoV-2 infected individuals.SARS-CoV-2 PCR specimen detection
At the enrollment day, nasopharyngeal swabs for SARS-CoV-2 PCR specimen detection were collected. PCR-tests were also performed after three and six weeks of military training. The PCR tests were analyzed at the Department of Microbiology at Oslo University Hospital using a Roche Cobas SARS-CoV-2 RNA test on the Cobas 6800 platform (Roche Diagnostics).

SARS-CoV-2 serologic analyses

Venous blood was sampled using VACUETTE® Blood Collection Tubes (Greiner bio-one) for SARS-CoV-2 serologic testing. Serologic tests were analyzed at the Department of Microbiology at Oslo University Hospital using Elecsys® anti-SARS-CoV-2 IgM/IgG assay fully automated on the Cobas e801 analyzer (Roche Diagnostics). The assay measures the combined total of IgM and IgG against the nucleocapsid (N) structural protein of SARS-CoV-2 and provides results as a cutoff index (COI) calculated based on signal sample / assay cutoff. A COI above 1, 2 was defined as a positive result.
Figure 1

SARS-COV-2 antibody rapid-test

Capillary blood was sampled for the Acro 2019-nCoV IgG/IgM rapid antibody test (Acro Biotech, Inc.) at the enrollment day only, and the test was performed on site. This rapid-test is a lateral flow chromatographic immunoassay for qualitative detection of IgG and IgM antibodies to SARS-CoV2. Individuals with a positive rapid-test went through a clinical interview and examination, and were quarantined, pending the results of the PCR virus test (Figure 1).

Ethics

Data from the extended systematic infection control are administered by the Norwegian Armed Forces Health Registry (NAFHR). The NAFHR is a central health registry with data from Norwegian Armed Forces personnel, including conscripts and civilian and military staff employed by the Armed Forces. The regulations for the Norwegian Armed Forces Health Registry authorizes the NAFHR to produce statistics and research in anonymous form 6.

Results

Enrollment day questionnaires and examinations

On the enrollment day, 17 (1. 4 %) of the 1170 conscripts reported having mild respiratory symptoms, and in addition nine conscripts (0, 8 %) had body temperature ≥ 38, 0⁰ C). None of these 26 were SARS-CoV-2 PCR positive or SARS-CoV-2 serology positive.
In the web-based questionnaire, 18 conscripts (1. 5 %) reported mild respiratory symptoms during the past 14 days prior to enrollment. Reported symptoms included runny nose and sneezing (72 %), stuffy nose (61 %), cough (55 %), sore throat (33 %), headaches (33 %), shortness of breath (16 %), fever (13 %), reduced sense of taste or smelling (11 %), sore muscles (11 %) and dizziness (6 %). None of these 18 conscripts were SARS CoV-2 PCR positive, but one had positive SARS CoV-2 serology at enrollment.
Nine conscripts (0. 8 %) answered “yes” to the question “do you think you have had COVID-19”. Among these, four conscripts were SARS-CoV-2 serology positive and one was PCR positive.

SARS-CoV-2 PCR specimen detection

Two of the 1170 conscripts had positive PCR tests at enrollment and one conscript with initial negative tests, tested positive at week six (Table 1). None of these reported any symptoms during the two weeks prior to or at enrollment, nor during the service period. However, based on the clinical interview, the two conscripts who tested positive at enrollment reported illness with symptom onset four and seven weeks prior to enrollment, respectively. The individual who tested positive in week six had not been ill, nor had he had any symptoms before or during the six-week observation period.
Table 1
Figure 2