Covid-19 and asymptomatic transmission

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Since the first cluster of cases of the novel coronavirus was reported in China last December, scientists have been racing to get a better understanding of the Covid-19, and how to stop its spread. It has been proved that the pathogen of Covid-19 is severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which has high homology with SARS-CoV. Similar to SARS-CoV, the dominant SARS-CoV-2 transmission mode is human-to-human transmission. Differently from SARS, the reproductive number (R0) is higher. Furthermore, the transmission of SARS occurs during the symptomatic period. For Covid-19, numerous asymptomatic infections were found among close contacts of confirmed patients. While asymptomatic transmission does occur, no one knows for sure how frequently it happens. Studies and models have suggested that many of infected persons never show symptoms. However, it remains an open question whether they are a large force driving transmission. The messages that have come from the scientific community have been sometimes confused and conflicting.

What does “to be asymptomatic” mean?

Like many aspects of this pandemic, the answer to what to be asymptomatic means is complicated. Asymptomatic means a person who has been infected by the virus, but does not feel sick or develop any symptoms. However, asymptomatic infections may be associated with subclinical lung abnormalities, as detected by computed tomography (CT). In studies from Japan and South Korea, the lungs of asymptomatic people were found to have abnormalities, revealed as hazy regions on their CT scans — the distinctive “ground-glass opacities” that have become a well-known sign of Covid-19. Moreover, persons who are currently categorized as having asymptomatic Covid-19 infections appear on a spectrum ranging from those who are truly asymptomatic to those who have mild symptoms but don’t seek medical support.  The latter group is sometimes referred to as “pauci-symptomatic” or “subclinical” because they are below the threshold of detection by the healthcare system. These people rarely alter their daily activity, so no one knows they are infected. Pre-symptomatic means a person who does not show the symptoms in the early stages of the illness but will developsthem later on. The difficulty of distinguishing asymptomatic persons from those who are merely pre-symptomatic is a stumbling block. To be clear, the asymptomatic individual is infected with SARS-CoV-2 but will never develop symptoms of Covid-19. In contrast, the pre-symptomatic individual is similarly infected but eventually will develop symptoms. It is well known that for those who are not asymptomatic, the time between infection and the onset of symptoms can range from one to 14 days. Most infected persons show symptoms within five to six days.

How many persons with Covid-19 are without symptoms?

Since February 2020, there have been reports of persons who were infected with SARS-CoV-2 but did not develop symptoms of Covid-19. In a study published on June 3 in the Annals of Internal Medicine, researchers at the Scripps Research Translational Institute reviewed data from 16 different groups of Covid-19 patients from around the world to get a better idea of how many cases of coronavirus can likely be traced to persons who spread the virus without ever knowing they were infected. On the basis of the 3 cohorts with representative samples—Iceland and Indiana, with data gathered through random selection of participants, and Vo’ (Italy), with data for nearly all residents—the asymptomatic infection rate was as high as 40% to 45%. A conservative estimate was 30% or higher to account for the pre-symptomatic admixture that had thus far not been adequately quantified.

Can asymptomatic persons spread SARS-CoV-2?

The general consensus is that  asymptomatic persons can spread SARS-CoV-2. Recent data, mainly derived from observational studies, confirm the potential transmissibility of Covid-19 from asymptomatic people. The asymptomatic transmission of the Covid-19 is an important topic in understanding the transmissivity of Covid-19. Comprehensive studies on transmission from asymptomatic persons are difficult to conduct, as they require testing of large population cohorts and more data are needed to better understand and quantified the transmissibility of SARS-CoV-2. Although, in a recent cohort study on the cruise ship Diamond Princess, the majority of asymptomatically infected persons remained asymptomatic throughout the course of the infection, the risk of transmission from pre-symptomatic or pauci-symptomatic patients should be always considered because of the difficulty of distinguishing them from asymptomatic persons.

Are asymptomatic Covid-19 carriers less contagious?

Whether asymptomatic Covid-19 carriers are less contagious remains unclear and more research is required. Based on current evidence, it seems that asymptomatically infected people can transmit Covid-19, however they may be less likely to transmit the virus than those who develop symptoms. Global research on Covid-19 continues to be conducted, including how the severe acute respiratory syndrome SARS-CoV-2 is transmitted. Available data suggest that the relative transmissibility of asymptomatic cases may be smaller than that of the symptomatic patients and for shorter periods of time, however asymptomatics can transmit the virus. We stress that how efficient of the transmissibility of asymptomatic cases comparing to symptomatic cases warrants more studies. 

Why it is important to know if asymptomatic persons can spread SARS-CoV-2

To know if asymptomatic persons can spread SARS-CoV-2 is very important. On an individual level, we should take confidence that wearing a mask outside the home remains essential because of the significant possibility that we might not have symptoms even if infected. To protect others, we always should behave as though we are infected. On a societal level, to know that asymptomatic infection is widespread means coronavirus testing should be significantly ramped up. 

These factors support the case for the general public to use face masks, in crowded outdoor or indoor spaces, as recently pointed out.