SARS-CoV-2 serologic analyses

A total of eight conscripts were SARS CoV-2 serology positive during the six-week observation period, one female and seven males. Seven conscripts (0. 6 %) were SARS-CoV-2 serology positive at the day of enrollment (Table 1), including the three conscripts with PCR-positive results for SARS-CoV-2 described above. One conscript with negative PCR-tests and a negative SARS-CoV-2 serology at enrollment developed positive and rising titers at week three and six.
None of these eight conscripts had fever or reported respiratory symptoms at the enrollment day, nor did they report any respiratory symptoms during the service period. One of the eight reported mild respiratory symptoms during the two weeks prior to enrollment. However, based on the clinical interview, six of the eight conscripts with positive serology had been ill with symptom onset three to seven weeks before enrollment (Table 1).
Antibody titers increased up to almost five-fold in all but one of the seropositive conscripts during the six weeks observation period (Table 1 and Figure 2).

SARS-CoV-2 antibody rapid-test

Of the 1170 conscripts, seven were both rapid-test IgG-positive and serology-positive (Table 1), while none of them were rapid-test-negative and serology-positive. Based on the serological test at Oslo University Hospital as the gold standard, the sensitivity of the rapid- test was calculated to 100 %.Two conscripts of the 1170 tested both IgG and IgM positive on the rapid-test, 25 tested IgM positive and 29 tested IgG positive. Hence, a total of 56 conscripts tested either IgG, IgM or IgG and IgM positive on the rapid-test. Forty-nine of these 56 conscripts were serology-negative, whereas 1114 were both serology and rapid-test negative. A total of 1163 were consequently serology-negative, and the specificity of the rapid-test was calculated to be 96%.
In this cohort, with a seroprevalence of SARS-CoV-2 of 0. 6 %, the positive predictive value of the rapid-test was calculated to be 13 %.

Discussion

In this study of 1170 healthy conscripts, the sero-prevalence of SARS-CoV-2 was 0. 6 %, and 0. 2 % had a positive PCR for SARS-CoV-2 at the day of enrollment to military service. Eighteen conscripts reported mild respiratory symptoms during the 14 days prior to enrollment (all were PCR negative, one had positive serology), whereas 17 conscripts reported respiratory symptoms and nine had fever at the day of enrollment (all were PCR and serology negative). These low numbers may partly be due to the high awareness of infection control rules in the Norwegian population during the study period which was 3-4 weeks after the pandemic peaked in Norway in ultimo March/primo April 2020.

SARS-CoV-2 PCR specimen detection

Two conscripts had a positive PCR test at enrollment, and one had a positive PCR after six weeks. Whether this represents ongoing or previous sub-clinical infection is up to debate. The PCR assay detects the presence of viral RNA. Whether positive SARS-CoV-2 PCR represent viable and contagious virus or only viral remnants might vary depending on patients and stages of disease. A study found that the infectivity peaked three days after symptom onset, and it was not possible to culture virus from samples obtained more than eight days after symptom onset 7.

Seroprevalence and serodynamics

Eight of the 1170 conscripts (0. 7 %) were seropositive for SARS CoV-2 IgG during the observation period. As antibodies against SARS-CoV-2 can be detected in the middle and later stages of the COVID-19 illness8, testing may help confirm COVID-19 diagnoses in individuals who have not previously been referred to PCR testing. Systematic antibody testing may also shed light on the actual prevalence of COVID-19 in the general population, and thus their immunisation status, which is key to the overall pandemic response in most countries. Recent studies have revealed prevalence numbers in the range of 1-5% in European countries 9. Our results indicate that the prevalence of Covid-19 among young asymptomatic adults in Norway is less than 1 %, which coheres with recent mathematical estimates from the Norwegian Institute of Public Health 10.
Six of eight seropositive conscripts, among them two with a positive PCR-test, had a history of symptoms compatible with COVID-19, with symptom onset between three and seven weeks before enrollment. Titer levels increased up to nearly five-fold over the six weeks observation period. No new symptoms were reported during this period. Most people infected with SARS- CoV-2 display an antibody response between 10 and 14 days after infection, however, the antibody response may depend on disease severity 11. In some mild cases, detection of antibodies is only possible a long time after symptoms, and in a few cases, antibodies are not detected at all, at least during the time scale of the reported studies 12. The duration of antibody response is still unknown, but it is known that antibodies to other coronaviruses wane over time (range: 12 – 52 weeks from the onset of symptoms) 12. A study found that SARS-CoV-2 IgM and IgG antibody levels may remain over the course of seven weeks after onset of symptoms 13.
The symptoms and disease reported by the six seropositive conscripts were mild. COVID-19 is considered a generally mild disease in adolescents 2,3,14. One of the questions that need further study is to what extent young COVID-19-patients actually present with no symptoms at all, which may increase the risk of spreading the virus to others. Our study confirms that symptoms are generally mild among young adults, but hardly supports the idea of frequent occurrence of asymptomatic spreaders, at least not in Norway 1.

SARS-CoV-2 antibody rapid-test

The sensitivity and specificity of the rapid-test used in this study were 100 % and 96 %, respectively. The relatively large proportion of false positive rapid-tests is probably partly due to the fact that the instructions given from the manufacturer recommended that “The intensity of the color in the test line regions may vary depending on the concentration of SARS-CoV-2 antibodies present in the specimen. Therefore, any shade of color in the test line region should be considered positive” 15. This may have resulted in negative tests being interpreted as positive based on nearly invisible lines in the test field, and illustrates that adequate instruction and training of health personnel is crucial when introducing and using new rapid-tests.
The Acro rapid-test has recently been evaluated by the Norwegian Organization for Quality Improvement of Laboratory Examinations 16, and was rated “acceptable”, with a sensitivity of 0.88 and a specificity of 0.99.

Value of testing and self-reported data

In our study, asymptomatic testing did not identify any individuals suspected of active transmission, as both conscripts with a positive PCR at enrollment had been symptom-free for more than 20 days when tested. Seven of eight conscripts with a positive serology during the study period had been symptom-free for at least 14 days before enrollment.
Testing of symptomatic conscripts, on the other hand, is widely recommended 17 and used as both as a diagnostic tool and a screening method to detect cases of COVID-19 in outbreak areas. The incidence numbers in Norway have been < 5 % throughout the pandemic. Symptomatic testing in this cohort did not confirm SARS-CoV-2 in any of the 26 conscripts who reported having mild respiratory symptoms or had fever at enrollment.
Only one of the eight people with positive serology reported respiratory symptoms during the 14 days before enrollment. Given the reported time of symptom onset among the conscripts with positive serology, five of these would probably have been contagious if they had been enrolled a month earlier and three might have presented symptoms of COVID-19. Self-reported data and questionnaires therefore may have potential to be effective tools in detecting COVID-19, especially in combination with symptom-based testing. Asymptomatic testing does not seem justified given the low prevalence and corresponding low predictive value of tests in this cohort of young healthy conscripts.

Conclusion

The prevalence of SARS-CoV-2 in young adults in Norway is less than 1 %. Self-reported symptoms and questionnaires may prove useful in detecting COVID-19, especially in combination with symptom-based testing. Asymptomatic testing seems of no value in times of low COVID-19 prevalence. SARS-CoV-2 antibody titer levels increased substantially over time in young adults with relatively mild symptoms.

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