A paper published in JAMA Internal Medicine gives us insights into why it is so hard to keep the spread of this virus under control: asymptomatic infected shed just as much virus as those with symptoms.
This study was importantly based in South Korea early in the pandemic, a place where extensive testing and isolation quickly kept the virus under control. As a result, they detected and isolated a large number of individuals who had the virus but were not showing symptoms.
This study was based on 303 infected individuals at an isolation center in South Korea between March 6 and 26. Of these, 110 (36%) were asymptomatic. Of these, 21 later showed symptoms and a full 89 (29% of total) never showed symptoms. These 303 individuals were generally young adults, with a median at of 25, especially relevant given the young median age of infected during the massive surge experienced in the USA since June.
They measured viral load in the upper and lower respiratory tracts in these patients using PCR tests at various time points, with a mean of 6 measurements taken per subject. Higher viral loads in the respiratory tract would lead to more viral shedding from coughing or exhaling, the primary modes of spread for SARS-CoV-2, and so should lead to greater contagiousness.
Many would expect asymptomatic infected individuals to have less severe infections and thus have lower viral loads than symptomatic individuals. However, this study found that the asymptomatic infected had viral loads that were (1) just as high at baseline (2) remained around as long as infected individuals with symptoms. This suggests asymptomatic infected are likely just as contagious as those showing symptoms.
This is one MAJOR reason why this virus is so hard to manage -- even if we immediately test and isolate all showing symptoms the moment they show symptoms, we still may miss up to 30% of the infected individuals who can go around and infect many others without ever knowing they are infected.
Two major implications are:
It is important to test individuals who may have been exposed to the virus even if they are asymptomatic.
Everyone needs to take mask wearing and social (physical) distancing seriously, Knowing that a relatively high proportion of those who are infected and contagious are not showing any symptoms at all, it is wise to take precautions just as if you or anyone you encounter is infected. This doesn't mean living in fear and paranoia, but rather just being vigilant to take the basic precautions that are recommended for all of us. This includes mask wearing in public, maintaining distance of at least 4-6 feet when interacting with people, and avoiding crowded, enclosed areas where airborne virus could potentially accumulate.
This affirms all of the advice that we have been given, and provides additional scientific support for the point that the only way for us to keep this virus under control sans lockdowns is for us ALL to cooperate to follow the recommended guidelines and precautions.
If we can do that, we can effectively manage this virus while living our lives to the fullest extent possible while we wait for a vaccine that will hopefully become available soon that could potentially stamp the virus out for good.
One caveat of the study is that it did not contain individuals with severe disease -- it is possible that that group has even higher viral loads than those who are asymptomatic or have mild or moderate disease. However, this group is likely quarantined and hospitalized, so are not primarily responsible for the community spread. It is those with mild or no symptoms who are spreading the most, so the results and implications of this study are still relevant.