Why Is Covid More Dangerous for Older People?

Infection fatality rate of COVID-19 based on age groups (source: [6,7])

Why coronavirus is so dangerous for older people? What are the quirks of its biology that pose a unique threat to our bodies and our lives? 

How Coronavirus enters cell and replicate

Potentially Dangerous for All People

There are at least 3 reasons that Covid-19 is deadly for all people because it:

1. Is a master of deception
• Our body's cells start releasing chemicals - called interferons - once they are being hijacked by a virus and this is a warning signal to the rest of the body and the immune system.
• But the coronavirus has an "amazing capability" of switching off this chemical warning, it does it so well you don't even know you're ill.
2. Is a 'hit and run' killer
• The virus doesn't care if you die, this is a hit and run virus.
• The virus is like a dangerous driver fleeing the scene - the virus has moved on to the next victim long before we either recover or die.
• The amount of virus in our body begins to peak the day before we begin to get sick.
• But it takes at least a week before COVID-19 progresses to the point where people need hospital treatment.
3. Does peculiar and unexpected things to the body (see Fig. 2):
   
   • COVID-19 starts off as a lung disease (even there it does strange and unusual things) and can affect the whole body
   • These whole-body effects could be due to the cellular doorway the virus strolls through to infect our cells - called the ACE2 receptor (see Fig.1). It is found throughout the body including in blood vessels, the liver and kidneys, as well as the lungs.
   • Covid does more than simply kill lung cells, it corrupts them too
   • Cells have been seen fusing together into massive and malfunctioning cells - called syncytia - that seem to stick around.
   • Blood clotting also goes strangely awry in COVID-19
   • Clotting chemicals in the blood are "200%, 300%, 400% higher" than normal in some COVID-19 patients
   • Doctors sometimes were unable to get a line into a patient because it is immediately blocked with clotted blood.
   • COVID-19 can cause runaway inflammation in some patients, making the immune system go into overdrive, with damaging consequences for the rest of the body

Localized and systemic symptoms of COVID-19

Why Coronavirus is more dangerous for older people?

Different from other coronavirus, this virus is a new one, we don't think there's much prior immunity in any groups including older people.

Building an immune defense from scratch is a real problem for older people because: 

• Their immune system is slow off the mark
• Learning to fight a new infection involves a lot of trial and error from the immune system
• They produce a less diverse pool of T-cells^[9]
• So it is harder to find ones that can defend against Coronavirus.

In addition,  hospitalized older COVID-19 patients had elevated levels of pro-inflammatory markers, which including:^[9]

• Neutrophil-lymphocyte ratio, D-dimer, C-reactive peptide and certain cytokines (i.e., IL-8 and IL-27)

Based on another publication, here are the reasons why children fare better than adults:

The expression of primary target receptor for SARS-CoV-2, i.e. angiotensin converting enzyme-2 (ACE-2), decreases with age. ACE-2 has lung protective effects by limiting angiotensin-2 mediated pulmonary capillary leak and inflammation. Severe COVID-19 disease is associated with high and persistent viral loads in adults. 

Children have strong innate immune response due to trained immunity (secondary to live-vaccines and frequent viral infections), leading to probably early control of infection at the site of entry. Adult patients show suppressed adaptive immunity and dysfunctional over-active innate immune response in severe infections, which is not seen in children. These could be related to immune-senescence in elderly.

References

1. Covid: Why is coronavirus so deadly?
2. Common cold antibodies hold clues to COVID-19 behavior
3. Diverse and functional autoantibodies revealed in COVID-19 patients
4. Diverse Functional Autoantibodies in Patients with COVID-19
5. Coronavirus Deranges the Immune System in Complex and Deadly Ways
6. Pathophysiology of COVID-19: Why Children Fare Better than Adults?
7. COVID Survival Rate For Under 20s Is 99.9987%; Stanford Epidemiology Prof Study
8. Prognostic peripheral blood biomarkers at ICU admission predict COVID-19 clinical outcomes 
9. Investigating Age Differences in COVID-19 Immune Response