Most of the viral infections that we experience throughout our lives are mild. However, some leave sequelae and damage to our body that we suffer long after we have been cured.
In the case of Covid-19, these sequelae are included under the generic term of persistent Covid or prolonged Covid. And now that the SARS-CoV-2 virus has been with us for nearly three years and we can study them, we are in a position to say that, for the moment, the outlook is not very rosy.
It is estimated that 3% of those who have had Covid with symptoms suffer damage that emerges in the long term. In USA it is even worse, since it is estimated that the percentage rises to 8% of the active population. And a report released a few days ago estimated that persistent Covid was responsible for at least 3,544 deaths in the country during the first 30 months of the pandemic.
There is a long list of symptoms associated with persistent Covid, too extensive to cover in an article of this type. For this reason, we will focus on those that are not so noticeable in the short term but pose a danger in the medium and long term. It deals with the sequelae in the endothelium and in the brain.
The endothelium It is the tissue that lines the inside of the blood vessels, from the heart to the capillaries. In the latter, it constitutes a single layer of cells that facilitates the exchange of substances between the blood and the tissues. As if it were a very sophisticated filter that allows some substances to pass but not others.
In addition, the endothelium fulfills three other important functions:
- Secretes substances that prevent blood coagulation, preventing the formation of clots;
- It alerts the cells of the immune system, the lymphocytes, to promote the inflammatory response in case of infection. This occurs mainly in cases of “sepsis” or generalized infection caused by pathogenic bacteria entering the bloodstream;
- It controls the dilation and constriction of blood vessels, regulating blood pressure.
Endothelial cells express the ACE2 receptor, through which SARS-CoV-2 enters. That is why when the virus reaches the lungs it comes into contact with the blood capillaries and begins to damage this tissue. There are other viruses that affect the endothelium, such as dengue, Ebola or cytomegalovirus.
In a Article A recent study described the general effects of SARS-CoV-2 on endothelial tissue. One of them is the increase in blood pressure caused by the death of endothelial cells that the virus attacks. The death of this layer of cells leaves us without a “guardian” and the pressure is out of control. Another general effect is inflammation due to damaged endothelial cells sending out danger signals, perhaps thinking that we are suffering from sepsis.
The lung suffers immediately, the heart suffers in the long term
Regarding the effects on specific organs, the one that suffers the most in the short term (as we have already mentioned) is the lung. Upon entering the capillaries of the alveoli, the virus attacks endothelial cells. The “filter” that forms this single layer of cells is lost, and permeability between the lung and the blood occurs. As a consequence, cells of the immune system (leukocytes) enter the lung and inflammation increases.
The liver is not going well either. The virus attacks a type of endothelial cells called sinusoidal cells. The result, again, is inflammation and liver damage. In the kidney, which contains a vast capillarization, high levels of ACE2 are also expressed and the process of kidney damage is similar to that of the lung: excessive inflammation and, in some aspects, very similar to that developed by sepsis.
In other organs, post-covid-19 damage occurs in a more subtle way, less obvious in the short term. This is the case of coronary disease. A high number of complications related to myocarditis, myocardial infarction and atherosclerosis have been observed once the infection has been overcome. It seems that when endothelial tissue is damaged, aggregates of cells involved in tissue repair (including leukocytes) are formed, contributing to the formation of atherosclerotic plaque and increased inflammation.
All this damage to individual tissues and organs has not come free at the population level. So, a study carried out in the US with more than half a million people who have had moderate or mild covid reveals an increase in blood pressure of about 2 mm Hg. And it is not transitory, but it is maintained. About heart disease, in the US alone there were 12,000 strokes and 44,000 myocardial infarctions in 2020. Many more than the average of previous years.
In 2021, these numbers rose to 18,000 strokes and 66,000 heart attacks. This represents an 8% increase in strokes and 2% in heart attacks. In addition, a 247% increase has been observed in the number of myocarditis suffered by people who have passed Covid. It is quite obvious that endothelial damage from Covid may have been a major factor in the recent rise in coronary heart disease.
Mental fatigue, memory loss, difficulty concentrating… these are neurological symptoms that, together with anxiety and depression, persist in up to 20% of people who have overcome covid-19.
The impact of Covid on the central nervous system has been analyzed in a recent study where they propose the possible mechanisms causing these symptoms.
According to the authors, inflammation of the respiratory system in turn causes inflammation in the nervous system due to the release of the famous cytokines (signaling molecules) and an increase in the microglia. These cells are especially reactive to immunological signals, causing deregulation of the rest of the nerve cells (demyelination of neurons, increased reactivity in astrocytes) that could trigger dysfunction in neuronal circuits. This is, ultimately, what would cause the neurological symptoms.
But what is the brain damage due to? There are several possible explanations:
1. Autoimmunity. Autoimmune processes could also be involved. Reactive antibodies against neurons have been located in Covid patients that directly attack the nervous system as if it were something strange and dangerous.
2. Infection in the brain. Direct invasion of the virus into the central nervous system occurs in some cases. However, the absence of the virus in the brain at autopsy and in patients with neurological symptoms makes this mechanism less likely.
3. Unity is strength: coinfection. It has been described that latent virus reactivation can occur (Epstein-Barr virus is found in 90% of the population) when another infection occurs. In this case, the production of viral particles (even if they are from another virus) can trigger the inflammatory response and its effects.
4. Malfunction of the blood-brain barrier and the neurovascular system. It is closely related to the endothelial damage that we have seen previously (thrombosis). It results in the release of pro-inflammatory molecules that can cause neuronal damage. Hypoxia (lack of oxygen) and the metabolic disorders it produces are also directly related to neuronal damage.
When we began to learn more about the virus and the disease that was coming upon us, we knew that, inevitably, we were going to get (almost) all of Covid. Vaccines have dramatically decreased deaths and disease and, according to the data, can also to prevent partially the persistent Covid.
However, with the advent of such a transmissible variant as omicron and the abandonment of many of the measures that reduce the transmission of the virus, it is important to know what footprint the virus can leave as it passes through our bodies. In this way we can decide on an individual level the degree of risk of getting infected that we are willing to assume.