It's been five years since the World Health Organization (WHO) officially labeled COVID-19 a pandemic. Ever since, scientists have been busy figuring out how the virus impacts the body months and even years after the initial bout is over. With hundreds of millions of cases globally, they're finally beginning to get a better idea of the long-term effects of the virus on the body — even if you feel like you've recovered completely.

What long-term effects can COVID-19 trigger?

When we first heard about COVID-19, we knew how risky it was, particularly for individuals who already have other health conditions like diabetes and cardiovascular disease. Yet some symptoms never go away when the virus vanishes — they last for several months or years. This phenomenon is called long-term COVID-19, in which symptoms last for a period of at least three months since infection.

It's estimated that roughly 400 million people globally have suffered from some kind of long COVID. Symptoms can vary from brain fog and fatigue to lung and heart complications, and even gastrointestinal issues. But now scientists think the solution to many of these persistent issues may lie in inflammation — the body's natural reaction to combating infection.

Dr. Braden Kuo, a neuro gastroenterologist at Massachusetts General Hospital, describes how inflammation is generally temporary but may become too strong or prolonged and harm various components of the body.

The following is what researchers have learned thus far about the impact of COVID-19 on major body systems:

Lungs

COVID-19 primarily targets the lungs, hence the symptoms of coughing and shortness of breath. However, for other individuals, lung complications do not resolve after healing.

Why is this the case?

The virus infects the cells that line the airways, leading to inflammation that destroys lung tissue. When the lungs attempt to heal, they can create scar tissue. The scar tissue complicates the lungs' ability to expand and breathe in oxygen, causing long-term respiratory issues.

Small studies indicate that more than 10% of individuals who were hospitalized due to COVID-19 continued to have lung scarring and other complications two years after the fact.

Dr. Ziyad Al-Aly, a clinical epidemiologist at Washington University in St. Louis, describes how damaged lungs become rigid, making breathing more difficult and leading to chronic coughing and shortness of breath.

Gut

Most individuals with COVID-19 have gastrointestinal symptoms such as nausea, vomiting, and diarrhea. But for others, the issues persist.

Why is this?

Scientists don't know precisely why COVID-19 impacts the gut so much, but they do have some theories.

The virus can interfere with the gut's natural balance of good and bad bacteria, causing chronic stomach issues.

Loss of good bacteria makes it more difficult to regulate inflammation, while bad bacteria can drive it up.

Infection-induced inflammation may harm the intestinal lining, releasing toxins and food particles into the bloodstream, causing an immune response and resulting in food intolerances.

According to a 2024 study, 10% of individuals with COVID-19 experienced persistent abdominal pain, while 13% continued to experience gastrointestinal symptoms after one year of recovery.

Dr. Kuo explains that inflammation may also damage the nerves that help control digestion, which could explain why some people develop chronic constipation or diarrhea.

Brain

Many people with COVID-19 complain about headaches, confusion, and trouble concentrating during the infection — but for some, these problems don’t go away.

Why does this happen?

Studies indicate that between 20% and 30% of those who had COVID-19 are still experiencing brain fog three months later.

Experts suspect that inflammation may be killing off neurons (brain cells) and disrupting the brain's mechanism for making new connections between synapses, something necessary for thought and memory.

Dr. Kuo indicates that some regions of the brain used for thinking and emotions appear particularly susceptible to inflammation, which may account for the fact that COVID-19 is also capable of causing mental illnesses such as depression and anxiety.

Another hypothesis is that the virus can break down the blood-brain barrier — a defensive cover on the brain — so toxic substances become more likely to get in and destroy brain tissue. Fragments of the virus may also linger in the brain years after the initial illness and be a cause of long-term cognitive symptoms, according to some researchers.

Heart

COVID-19 not only damages the lungs — it also strains the heart. A major study discovered that contracting COVID-19 doubled the risk of heart attacks, strokes, and other cardiovascular events for as long as three years.

Why is this?

When you have a fever or inflammation when you have COVID-19, the heart works harder.

If the person already has plaque in the arteries, this added stress can cause a heart attack or arrhythmia.

The virus can also destroy the cells that line the blood vessels, which could result in blood clots and risk sudden cardiac issues.

Individuals hospitalized for COVID-19 are at greatest risk of cardiac complications. Interestingly, studies indicate that individuals with non-O blood groups (A, B, or AB) could be at greater risk of developing cardiac problems post-COVID-19.

Blood and circulatory system

Certain individuals with long COVID struggle with blood circulation. Their bodies cannot move blood from the abdomen and legs back to the heart, resulting in tiredness and the inability to exercise.

Why does it occur?

Scientists believe that inflammation could be hurting some nerve fibers that assist blood vessels in compressing and pumping blood toward the heart.

Dr. David Systrom, a pulmonary and critical care specialist at Brigham and Women's Hospital, describes how decreased blood flow causes the heart to circulate less oxygenated blood throughout the body, resulting in fatigue and shortness of breath.

In others, the muscles even find it hard to take oxygen from the blood and the mitochondria (the energy factories of the cells) fail to function normally. This might be the reason that even light exercise becomes tiring for others with long COVID.

Although scientists know quite a bit about the long-term implications of COVID-19, many questions still surround it. Researchers continue to figure out why some individuals recover fully and others develop long-term illness.

Dr. Kuo and other scientists feel that learning more about inflammation can help create treatments for it to lessen its harmful effects. Until then, healthcare professionals are concerned with treating symptoms and allowing individuals to recuperate.

If you’re experiencing long-lasting symptoms after a COVID-19 infection, it’s important to seek medical advice. Understanding these lingering effects is the first step toward recovery.

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