A Clearer Picture of Covid's Lasting Effects on the Body

New York Times

10-03-2025

Five years — and hundreds of millions of cases — after the World Health Organization declared the Covid-19 pandemic, scientists are getting a clearer picture of how the virus can affect the body long after an infection seems to pass.
Some of Covid's effects became apparent soon after the virus began spreading. We quickly understood how deadly an infection could be, especially for those with underlying conditions like diabetes and heart disease. But it has taken years of research to start making sense of how a bout of Covid can lead to lasting, sometimes invisible changes in different parts of the body.
Some of these effects, such as chronic fatigue and brain fog, are considered long Covid, defined as symptoms from an infection that persist for at least three months. By some estimates, 400 million people worldwide have been diagnosed with some form of long Covid. But an infection can also lead to other issues, including lung and heart damage and microbiome changes in the gut, that may not always be recognized as long Covid but can still have a lasting effect on our health.
We now have a better sense of what might be behind those changes, including the role of the widespread inflammation that Covid can cause. For most people, inflammation will subside once the virus clears. But for some, if it 'rages' too intensely or persists as a 'slow burn' for too long, it can wreak havoc around the body, said Dr. Braden Kuo, a neurogastroenterologist at Massachusetts General Hospital.
Here's what scientists have learned so far about inflammation and the other factors driving these effects.
The Lungs
Covid irritates the lungs and can cause long-term issues, like persistent shortness of breath and coughing. In rare cases, Covid can lead patients to develop pneumonia and leave scarring and small masses of tissue, called nodules, throughout the lungs. Those scars can make it harder to breathe. Small studies have suggested that over 10 percent of people hospitalized with a Covid infection had lung scarring and other issues two years later.
The cause: The virus invades cells along the airways, causing inflammation that can attack and sometimes destroy healthy lung tissue. This can impair the lungs' ability to deliver oxygen throughout the body, said Dr. Ziyad Al-Aly, a senior clinical epidemiologist at Washington University in St. Louis.
As the lungs try to recover and repair, they form scars. But scar tissue itself can stiffen the lungs and reduce lung capacity, leading to lasting symptoms such as a cough and shortness of breath.
The Gut
Covid can cause short-term symptoms like nausea, vomiting and diarrhea.
But in some people, Covid can lead to chronic gastrointestinal problems, like reflux, constipation, diarrhea and abdominal pain. These issues can last for months or even years. In a 2024 study, researchers estimated that bouts with Covid had left as many as 10 percent of people with lasting abdominal pain and 13 percent with gastrointestinal issues a year later.
The cause: Scientists don't know why, exactly, Covid can throw off normal gut function so much — but they're getting a better understanding of what might be at play. For example, it's now clear that the virus can disrupt the gut microbiome, reducing beneficial microbes and increasing the numbers of harmful ones. 'Good' microbes can help tamp down inflammation while 'bad' ones can increase it.
Inflammation from the infection itself, as well as from the altered gut microbes, might harm the lining of the intestine. This can allow toxins and the broken-down components of food to escape from the gut into other tissues of the body. Immune cells might then mount an allergy-like response to certain foods, leading to food intolerances.
Inflammation may also 'chew away' at the nerves that signal pain in the gut or that control the intestinal contractions that keep food moving along, Dr. Kuo said. This can cause stomach or intestinal pain or make food move too fast or too slow through the digestive tract, resulting in symptoms like diarrhea or constipation.
The Brain
At the height of an infection, patients often develop headaches and can feel dizzy and confused. They sometimes struggle to find the right words, have difficulty concentrating or following a conversation or find they have gaps in their memory.
These symptoms can linger: Studies have found that roughly 20 to 30 percent of people infected with Covid experienced brain fog at least three months after an initial infection. Research also shows that Covid can lead to conditions like anxiety or depression, or exacerbate existing mental health issues.
The cause: Scientists are still working to identify all the factors that contribute to lasting neurological issues after Covid. But one culprit seems clear: persistent inflammation, which damages neurons and inhibits the creation of key connections between synapses. All of this may cause symptoms like those described above. Some researchers also think that areas of the brain involved in cognition and emotion are particularly vulnerable to inflammation, which helps explain why an infection can induce or worsen mental health issues.
Another theory is that the virus disrupts the blood-brain barrier, which protects brain tissue and is vital for cognitive function.
Fragments of the virus may also linger in the brain long-term, which could explain why some cognitive symptoms last beyond the initial infection.
The Heart
A Covid-19 infection increases the risk of heart problems, including heart attacks, strokes, damage to the heart muscle and an irregular heart beat, known as an arrhythmia. One large study found that having Covid doubled the risk of a major cardiovascular event for up to three years.
The cause: When you have an acute Covid infection, the stresses of fever and inflammation can place excess demand on the heart. In someone who already has plaque buildup in the arteries or heart muscle that's started to become stiff, that demand can lead to an irregular heartbeat or heart attack.
But more common, scientists think, is that the virus provokes inflammation that injures the heart muscle. The virus may also damage the cells lining the blood vessels, leading to inflammation there. This could cause a new clot to form or make existing plaque break off and clog a blood vessel. That type of blockage can cause sudden death from a heart attack or lead to downstream damage to heart muscles and other tissue, which can result in heart failure or an arrhythmia.
People who have been hospitalized for Covid have the highest short- and long-term risk of heart complications. Some research suggests that people with a non-O blood type — A, B or AB — have a particularly increased risk, perhaps because blood type may be linked to how the blood clots.
The Circulatory System
Studies of patients with long Covid show that their bodies have trouble moving blood out of the legs and abdomen and back up to the heart. That can reduce the amount of blood the heart pumps out, causing fatigue, shortness of breath and a feeling of being unwell after exercise.
The cause: It's not clear why these circulatory problems happen, but scientists hypothesize that in some patients, inflammation harms certain nerve fibers outside the brain and spinal cord that regulate the squeezing ability of blood vessels. This could result in impaired blood flow, said Dr. David Systrom, a pulmonary and critical care physician at Brigham and Women's Hospital in Boston.
In some patients with long Covid, it also appears that the muscles are less able to extract oxygen from the blood than normal, hampering their ability to keep up with exercise, Dr. Systrom said. In addition, the mitochondria — the energy-producing powerhouses of cells — may not work properly or at full capacity, delivering another hit to muscle tissue.