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In this study of all 27,473 persons who tested positive for SARS-CoV-2 from 1 September 2020 to 1 September 2021 in Oslo, registered with information on symptoms and close contacts, we found that asymptomatic cases were almost 30% less contagious than symptomatic cases. Examining 11,192 secondary cases, we also found that those infected were almost three times more likely to remain asymptomatic if their suspected infector was asymptomatic compared to symptomatic.
Main findings
The results of both analyzes were consistent with our hypotheses and previous research. There are several reasons why asymptomatic cases may be less transmissible than symptomatic cases. First, the absence of coughing, sneezing, and other respiratory symptoms can reduce the spread of respiratory droplets and make asymptomatic cases less contagious [2]. Differences in viral load and viral shedding between the two groups may also partially explain the differences in transmissibility [4]however, to date, the literature on the relationship between viral load and disease severity is inconclusive [16]. Finally, there may be differences in the behavioral patterns of symptomatic and asymptomatic individuals. Asymptomatic cases may perceive themselves and be perceived by others as non-infectious, which can lead to chains of infection that cannot be detected by contact tracing [17]. Symptomatic individuals may be more likely to have close contacts from whom they cannot be isolated (eg mother and child). While asymptomatic people (who do not know they are positive) may have closer contact with less intimacy. On the other hand, it may be that symptomatic individuals are already aware that they are ill and take precautions, while asymptomatic individuals take no action. We tried to explore this as best as possible by distinguishing between household contacts and other close contacts (most commonly students, colleagues and friends) (Additional file 1 : Supplementary figure 6). This did not change the results. Furthermore, we found no differences in the number of reported close contacts between asymptomatic and symptomatic cases.
We also found that most asymptomatic individuals were young children. At the beginning of the pandemic, children were rarely infected, and if they were infected, they had a lower prevalence than adults [18]. Previous studies have found that there was little transmission in schools during the pandemic [18, 19]. During the period of our study, schools mostly used precautionary measures such as quarantine and home schooling. At the end of the study period, strict testing criteria replaced quarantine and home schooling, which may have led to the detection of more asymptomatic cases. However, as this was not widely implemented before the autumn of 2021, it is unlikely to buy back to explain why we are seeing more young people being asymptomatic. The exposure situation may differ for young children and adults.
However, we found no differences in the proportion who were asymptomatic based on the type of close contact for the different age groups (Additional file 1 : Supplementary Table 4). The proportion of asymptomatic cases is evenly distributed among secondary cases registered as household members and other types of close contacts.
Furthermore, we found differences in transmission dynamics between asymptomatic and symptomatic for all age groups except the youngest (0–9 years). We have no evidence why there are no differences for the youngest age group, but the reasons may be that it may be difficult to distinguish between symptomatic and asymptomatic for the youngest children. For toddlers and young children, contact tracers interviewed their parents rather than the case itself, which in turn may have complicated the accuracy of reported symptoms.
Related research
Our findings are consistent with previous research. While previous studies have found that asymptomatic individuals have a relative transmissibility three to four times lower than those with symptoms [6, 7], we found this to be 0.3–0.4 times lower. One reason for this discrepancy may be that asymptomatic people are less likely to self-test. In follow-up studies, all individuals are tested regularly, while in registry studies, researchers depend on individuals physically attending testing centers to be tested. This may be more common in cases that are symptomatic compared to asymptomatic. We also found that relative transmissibility was evident for all age groups except the youngest. Data from other studies suggest that household transmission dynamics are relatively similar across age groups [20].
Similarly, we found that the mean age of asymptomatic cases (24.1) was lower than that of symptomatic cases (31.2). Similar results were found in a meta-study analyzing more than 350 studies, where the authors found that the proportion of elderly people without symptoms was significantly lower than that of children, at 19.7% compared to 46.7% [21].
As hypothesized earlier, we found that secondary cases were two to three times more likely to be asymptomatic if their putative index case was asymptomatic. To the authors’ knowledge, this is the first study to assess whether asymptomatic indices are more likely to cause asymptomatic secondary cases.
Strengths and weaknesses
A major strength of this registry study is that we could include all individuals registered with positive or negative PCR or rapid antigen tests in Oslo throughout the study period. In addition, health register data are mandatory in Norway and are generally of very high validity and reliability. A large number of observations provides more statistical power, less uncertainty, and greater generalizability of findings. Combining registry data with qualitative contact tracing data collected through interviews with trained staff and infected provides an invaluable research dynamic. During the one-year study period, Norway and Oslo had exceptional testing and contact tracing capacity, meaning that it is reasonable to assume that a large proportion of the actual number of index cases and their close contacts were identified and included in this study.
There are also some potential limitations with this study. Symptom information was collected through telephone calls to those infected and was therefore self-reported. According to our data, men are more often asymptomatic than women. We could not be sure if men were actually more often asymptomatic or if this was an example of men having a higher threshold for reporting symptoms [22]. Differences between different contact markers in what they considered asymptomatic may also have influenced the data, but probably only to a small extent, as they were all professionally trained. In addition, the national strategy for testing, isolation, contact tracing and quarantine has changed several times during the pandemic. In particular, changes in testing recommendations for close contacts could have affected the results in this study. Nevertheless, it is reassuring that the results appear to be consistent throughout the study period (Additional file 1 : Supplementary figure 2).
A second limitation is that we were unable to distinguish between asymptomatic and presymptomatic individuals. In theory, a person is asymptomatic if the test is positive but does not develop symptoms throughout the course of the disease. A presymptomatic person, on the other hand, is a person who develops symptoms at a later date [23]. Because contact markers usually had contact with positive individuals once, different times in the course of the disease may have resulted in presymptomatics being counted as asymptomatic. However, the pre-symptomatic have not yet been detected by the health system and may have continued to spread the virus.
Additionally, individuals with false-positive tests may have been classified as asymptomatic in registry data. Although PCR tests are considered very accurate and have high sensitivity and specificity, false negative and false positive tests still occur [24]. In low-prevalence diseases, small errors in specificity can lead to a large proportion of false-positive results, also known as the false-positive paradox [25, 26]. According to our data, only 13% of all SARS-CoV-2 positive cases are registered as asymptomatic, which is lower than previous estimates of 15–25% [4, 5]suggesting that the proportion of asymptomatic individuals in our population is underestimated rather than overestimated.
Finally, we did not have information on Ct values or viral load in each test and were therefore unable to say whether differences in viral load between asymptomatic and symptomatic cases could explain the lower transmissibility.
Future research
For future research, it would be interesting to investigate whether cycle threshold values (Ct) and viral load differ between symptomatic and asymptomatic cases; whether the presence of an asymptomatic course of the disease reduces the risk of developing post-acute complications, known as “long-COVID”; and whether there are differences in the likelihood of remaining asymptomatic based on different strains of virus, number of vaccine doses, or types of vaccine. Furthermore, further investigation of how these dynamics of transmission and susceptibility differ between different age groups may be important for future measures regarding school closures and measures that dramatically affect children’s daily lives.
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