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Why do some people not develop COVID-19 symptoms when they're infected?

This article was published on
August 24, 2021

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Asymptomatic COVID-19 cases (cases with no symptoms) are common, making up at least 1 out of 3 of all cases and sometimes more in some areas. The healthier and younger you are, the more likely you are to have an asymptomatic case. Other pathways are unknown but are being explored. These pathways include genetics, cross-immunity (​​past exposure to one virus providing partial protection to another virus), environment, and COVID-19 vaccination.

Asymptomatic COVID-19 cases (cases with no symptoms) are common, making up at least 1 out of 3 of all cases and sometimes more in some areas. The healthier and younger you are, the more likely you are to have an asymptomatic case. Other pathways are unknown but are being explored. These pathways include genetics, cross-immunity (​​past exposure to one virus providing partial protection to another virus), environment, and COVID-19 vaccination.

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What our experts say

An asymptomatic COVID-19 case means someone has tested positive for COVID-19 but has no symptoms while they’re infected. Asymptomatic COVID-19 cases are common around the world. They are a public health concern, because people without symptoms are more likely to be out in the community spreading the virus, because they do not feel sick or are less likely to know that they are sick. 

The exact reasons for why some people develop symptoms and others do not is unknown, but we do know that the healthier you are, the more likely you are to have an asymptomatic case. There are several pathways being explored as possible explanations for asymptomatic cases. 

Asymptomatic cases are not unique to COVID-19. In fact, asymptomatic cases are common for viral infections. For example, studies have shown that of all human papillomavirus (HPV) infections, more than 90% are asymptomatic cases.

Data from the U.S. and UK for COVID-19 shows that 1 in 3 people who have COVID-19 are asymptomatic. The amount of asymptomatic cases can differ based on context and location, however. In some African countries, for instance, up to 80% of COVID-19 cases appear to be asymptomatic.

While we do not know exactly why some COVID-19 cases cause symptoms and others do not, research is growing on this topic and there are a number of hypotheses for why asymptomatic cases occur. These include the following:

Age: Young people have been shown to have much higher amounts of asymptomatic cases than older people. A recent study by Duke University found that children aged six to thirteen have the highest number of asymptomatic cases. Asymptomatic cases were less common — but still occurred 25% of the time — in children ages 0 to 5 and teens who were 14 to 20 years old. 

Genetics: A number of studies looking at asymptomatic cases in Africa point towards genetics potentially playing a role.certain genetic variations., Ffor example, some people have variations of the ACE2 proteins (which the COVID-19 virus binds to) that make them more likely to get infected by COVID-19's as well as sicker. Those who don’t have these variations are less likely to have symptoms. genetic variations in receptors used for COVID-19 entrance into cells which could influence asymptomatic infections. BAdditionally, bBlood typegroup type and gender are two other genetic factors that might behas also been associatedindicated within asymptomatic cases, though there is conflicting dataconflicting data is present. As has gender with a study pointing towards higher rates of asymptomatic cases in women.

Cross immunity: Cross immunity is the idea that past exposure to one virus—either through infection or vaccination—could provide partial protection against another virus. Studies suggest that cross-immunity can lead to asymptomatic COVID-19 cases. One example is the Bacillus Calmette–Guérin (BCG) vaccine used to prevent tuberculosis. Recent data suggest that regions with required BCG vaccines have had lower rates of COVID-19 cases and deaths. In addition, the BCG vaccine has been shown to reduce the viral load of other viruses like the flu. This suggests that BCG vaccines could boost immunity against COVID-19 and decrease inflammation, leading to less symptoms. However, further studies are needed. 

Environment: One theory suggests that environments that expose people to a diverse range of “friendly” microbes from an early age may help protect people more against dangerous new pathogens like the virus that causes COVID-19. As a result, it’s possible that more multimicrobial environments could be resulting in more asymptomatic cases. 

COVID-19 vaccination: COVID-19 vaccines reduce both symptomatic and asymptomatic cases overall. However, among people who have gotten the vaccine and end up getting infected, they may be more likely to have an asymptomatic case because of the vaccine A growing amount of data suggests that COVID-19 vaccinations are associated with asymptomatic infections. This would be consistent with past findings with influenza and measles, in which vaccination reduces both symptomatic and asymptomatic infections.

It can seem strange that a case without symptoms can still be an infection and can still make someone contagious to other people, but it’s very common for COVID-19 and common for other viruses. Research is ongoing to figure out how transmissible asymptomatic COVID-19 cases are compared to symptomatic cases. What we do know is that asymptomatic transmission contributes significantly to overall transmission. Therefore, wearing masks and following public health measures is crucial for helping to prevent asymptomatic COVID-19 transmission.

Context and background

Studies showing asymptomatic COVID-19 cases globally have been increasing. This is a concern given that asymptomatic cases can contribute to the spread of COVID-19, often unknowingly.  The exact mechanisms for why some people develop symptoms and others do not are not known. However, research is ongoing and there is growing evidence pointing towards possible pathways for this difference.

Resources

  1. Understanding COVID-19 in Africa (Nature)
  2. SARS-CoV-2 Infections Among Children in the Biospecimens from Respiratory Virus-Exposed Kids (BRAVE Kids) Study (Clinical Infectious Diseases)
  3. What Could Explain the Lower COVID-19 Burden in Africa despite Considerable Circulation of the SARS-CoV-2 Virus? (Environmental Research and Public Health)
  4. “Factors related to asymptomatic or severe COVID-19 infection.” (Med Hypotheses)
  5. Comparison of Clinical Characteristics of Patients with Asymptomatic vs Symptomatic Coronavirus Disease 2019 in Wuhan, China (JAMA)
  6. BCG-induced trained immunity: can it offer protection against COVID-19? (Nature Reviews Immunology)

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