Asymptomatic diseases are where a disease or infection does not lead to any symptoms. Some people will remain asymptomatic throughout their disease course, whereas others will eventually become symptomatic later on.
Image Credit: ALDECA studio/Shutterstock.com
Diseases and infections in humans can lead to a specific set of symptoms to occur, or a broad range of generic symptoms which overlap with many other diseases. The severity of symptoms can vary considerably depending on the disease itself, age, comorbidities, and immune status e.g., prior vaccinations or being immunocompromised.
Asymptomatic disease is where a person is infected with a disease (or develops a disease; diagnosed) but fails to display any noticeable symptoms. These are also referred to as subclinical diseases or infections. However, if symptoms do eventually develop after a period of being asymptomatic, then that phase is known as pre-symptomatic.
Asymptomatic until symptomatic – silent diseases
Many diseases and infections can be asymptomatic, including those that may be potentially fatal in some people. These include (but are not limited to): tuberculosis, breast cancer, endometriosis, HIV/AIDS, herpes, hepatitis, chlamydia, hypertension, common colds/flu, and type-2 diabetes mellitus. Many of these conditions remain largely asymptomatic until very advanced disease stages when they suddenly become symptomatic. Others can remain more or less asymptomatic throughout their disease course.
For example, hypertension – or high blood pressure – may not be symptomatically apparent until you measure blood pressure to reveal hypertension. Furthermore, the effects of hypertension on the body may not manifest clinically for decades. Often, acute target organ injury results in hospital admission when hypertension is severe and neglected. Thus, despite the absence of symptoms in hypertensive patients, it is important to manage or reduce blood pressure gradually to ensure such acute events do not take place.
Similarly, type-2 diabetes mellitus is often asymptomatic for many years before being diagnosed. Even after diagnosis, there may not be any overt symptoms that occur. Just like hypertension, only routine check-ups may pick up both diseases and early intervention is key for both. Untreated or poorly managed diabetes and hypertension can accelerate other pathologies such as atherosclerosis that can have negative consequences on health. Even then, atherosclerosis itself is asymptomatic for decades until target organ failure or pathology occurs – typically coronary (heart), renal (kidney), or in the brain.
Infectious diseases can also be completely asymptomatic (with no symptoms ever manifesting), particularly in younger and healthier individuals. For example, hepatitis (hepatitis C) infections can take up to 6 months to develop, and even then, approximately 80% of infected individuals may not experience any symptoms. Other examples include cholera, herpes, measles, and rubella which can be completely asymptomatic. The best protection from any of these diseases is a full course of vaccination which can prevent symptomatic disease, and most importantly, death or hospitalization.
A significant proportion of individuals testing positive for COVID-19 are asymptomatic (around 20%). However, this does not mean that they are not contagious as they are still able to transmit the virus to others around them as the viral load can be similar compared to symptomatic patients, though recent studies have shown asymptomatic individuals to be less contagious than symptomatic (but still contagious). This is why routine rapid antigen testing (lateral flow tests) are able to pick up the presence of SARS-CoV-2 even if asymptomatic.
The issue, however (in asymptomatic cases), is that having a positive LFT, or PCR is not proof of infectiousness as the virus may not be actively infecting (live) and that can only be determined by cell culture. As such, the true degree of infectiousness of asymptomatic individuals compared to symptomatic individuals is still debated (though typically lower, especially after the first few days of infection).
However, in the global pandemic, self-isolating if positive (asymptomatic or symptomatic or if a close contact of a positive case), mask-wearing, hand-hygiene and increasing ventilation irrespective of being negative or positive (with the unknown virus live status) is beneficial as there may be a chance the virus could be infectious and shedding in asymptomatic people who; unless tested regularly, will be unaware of their status.
In summary, asymptomatic disease refers to diseases and infections which do not lead to any symptoms in patients (subclinical) for the whole disease course or until they develop symptoms in which the asymptomatic phase is referred to as pre-symptomatic.
In many respiratory infections including COVID-19, asymptomatic disease is common and may be a source of transmission within the community, though more research is needed to establish the exact contribution asymptomatic transmission has on the community rates of infection.