You’re not sick anymore—so why are you still coughing?

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Have you kicked a cold, bout of flu, RSV, or COVID recently—but can’t get rid of the cough? You’re not alone. The symptom can stick around for weeks after our bodies have cleared a virus. When was the last time you walked into a public space and didn’t hear someone coughing? After three years of flinching at the sound, it can be disarming to hear so many people coughing – and embarrassing if it’s you.

But take heart in knowing that you’re not alone. A long-lasting cough following illness from an upper respiratory infection is surprisingly common. And unfortunately, with the rise in seasonal flu, COVID-19 and respiratory syncytial virus, or RVS cases in the fall of 2022 and winter of 2023, there’s been a lot of coughing lately.

Michael Shiloh, a physician specializing in infectious disease research at UT Southwestern Medical Center, says coughing patients often report that they were sick as many as eight weeks prior to seeing him. “We can’t really detect virus anymore in these individuals, and yet they’re still coughing,” he says.

According to the CDC, overall respiratory illness activity is “very high” in the U.S. Not only is there widespread incidence of these diseases, more people are coming down with severe cases. The CDC reports emergency room visits for flu and RSV are “very high,” and while visits for severe COVID are still lower, emergency cases of all three illnesses are rising. While most people will recover from flu, RSV, and COVID in around two weeks, symptoms can stretch much longer.

Scientists still aren’t sure exactly why otherwise healthy people experience this kind of stubborn cough. But research on how infections meddle with nerves in the airway is revealing new clues.

What is a cough?

A cough is a common symptom of these sorts of respiratory infections, and coughing is a complaint that leads to an estimated 30 million office visits every year. Some 40% of those end up in a pulmonologist’s office like mine.

Given how ubiquitous persistent coughing is, you might presume that the medical profession has a long list of treatments that we know work. That, unfortunately, depends on why you are coughing. But a cough following an upper respiratory infection does usually go away, in time.

Coughing is an important reflex that protects the airway from dangers like noxious fumes, water, or bits of mis-swallowed food, says pulmonologist and researcher Lorcan McGarvey of Queen’s University Belfast.

The reflex is triggered by nerves that reach into the airway. These nerves are decorated with receptor proteins that react to everything from cold air to capsaicin, the chemical makes peppers spicy. When an irritant triggers those receptors, nerves send signals up the vagus nerve to the brain that we experience as the urge to cough.

From there, the brain sends commands back down to the airway to cough—or not. This detour to the brain is why there’s a degree of conscious control in certain types of coughs.

Scientists know about many different stimuli that can trigger cough, but they still don’t agree on the specific biological mechanisms that trigger coughing while we’re sick, let alone in the weeks afterwards.

While it may seem obvious coughing is meant to clear our throats of mucus, it’s also possible that viruses trigger the reflex to help themselves spread. Many infections involve dry coughs that don’t produce phlegm at all. And if we do ultimately cough to clear out our airway during an infection, that still wouldn’t explain what exactly our nerves sense during an infection that triggers a cough.

“We don’t know,” says electrophysiologist Thomas Taylor-Clark of the University of South Florida. “But what we can say is we do know some things, one being that viral infection causes inflammation.”

How a cough works

Doctors have long wondered why the duration of cough varies so much following an upper respiratory viral or bacterial infection. The answer likely lies in differences in people, such as the presence of a condition like asthma or chronic bronchitis. I see this same variability in my office: Some patients develop a long-term cough, while others seem to kick the cough much sooner, with no clear explanation.

Coughing occurs due to a complex process that begins with an electrical impulse between the nerves within the airways, including the nose and throat. There are two types of nerves that can trigger a cough in response to external stimuli: chemical receptors and mechanical ones called mechanoreceptors. Chemical receptors respond to smells and fumes; they’re the reason people sometimes cough after breathing in spicy peppers sizzling on a hot skillet. Mechanoreceptors respond to sensations from irritants such as dust.

When these nerves are activated, the throat closes and pressure in the chest increases. This buildup of pressure leads to a burst of air and mucous out of the lungs at around 500 mph – a speed nearly twice as fast as the world’s fastest cars.

Studies show that a viral infection alters how sensitive these same nerves are. When you have a viral infection, the resulting inflammatory process produces a molecule called bradykinin that drives the urge to cough. And it’s known that the virus itself can activate genetic changes that increase the sensitivity of these nerve pathways, which leads to more coughing.

But when the acute stage of infection is over and you start to feel better, the body repairs the damages caused by inflammation in your airways and lungs. Through this process, the cough reflex also abates. And the molecular processes that were making you cough and sneeze more frequently settle down and return to a normal state – at least in most cases. Unfortunately, in some people, this process takes longer than in others.

Infections make nerves hypersensitive

When we’re sick, that inflammation jacks up the sensitivity of airway nerves, causing them to overreact, research suggests. One hypothesis is that stubborn post-infection coughs happen when nerves remain hypersensitive after the end of an illness.

“Just taking a deep breath, talking on the telephone, laughing, going out into cold air—these are things that in themselves are not harmful or noxious. But at least temporarily, they [can] send us into these bouts of coughing” even when we’re no longer sick, says McGarvey.

As early as 1990, researchers noticed that infection with a flu-like virus upped the sensitivity of airway nerves in guinea pigs, which cough like humans. Sick guinea pigs cough more when exposed to irritants like capsaicin than healthy ones, something that’s also been seen in people.

Now, scientists are linking this hypersensitivity to specific inflammatory chemicals and receptors on airway nerves.

In 2016, scientists discovered that infection with a flu-like virus caused certain nerves in the airway to make extra copies of a receptor protein called TRPV1, which reacts to capsaicin and other stimuli. Other studies, many on either guinea pigs or lab-grown human cells, have likewise reported increased copies of TRPV1 and other receptors during respiratory virus infections.

One 2017 study showed that even when a virus was “killed” with UV light, it increased copies of TRPV1 and another receptor in lab-grown human cells.

When to talk to your doctor

Post-infectious cough usually resolves in two to three weeks. But if your cough persists for more than eight weeks, Shiloh and McGarvey say that you should talk to a doctor. If the cough occurs with other symptoms like fever, shortness of breath, bloody phlegm, or weight loss, they recommend getting evaluated sooner.

Lukasz Antoniewicz, a pulmonologist who treats patients at the Medical University of Vienna’s chronic cough unit in Austria, says a few weeks of coughing after an illness is normal. Still, while eight weeks might be the official cutoff for chronic cough, in practice it can be a long time to go without relief, he says.

“I have young patients, around age 30, and they get broken ribs only due to coughing.They don’t have an underlying bone disease. So it’s a very, very strong reflex. And it hurts a lot,” says Antoniewicz. “When you cough for one month, I would say it’s maybe it would be a good reason to seek out your doctor.”

Doctors find it useful to break respiratory symptoms like cough into specific categories.

There are three main types of coughs: acute, sub-acute and chronic. An acute cough is what most people experience when they’re sick with an active viral infection. A sub-acute cough lingers for three or more weeks following an upper respiratory illness. And a chronic cough is one that persists longer than 12 weeks. Chronic cough is most commonly caused by asthma, postnasal drip and, perhaps surprisingly to some, reflux.

Postinfectious cough is one variety of sub-acute cough and is the lingering cough that many people have after they get over their respiratory infection. It can last for weeks or months and can progress into chronic cough.

Because postinfectious cough is so common, doctors have long worked to determine how many people have a cough that persists after their other symptoms go away. Those estimates vary among studies. One small study in Japan found that of people who have a sub-acute and chronic cough, 12% resulted from a respiratory tract infection.

When it comes to COVID-19, the best evidence to date shows that only 2.5% of the people who have gotten it have also developed a chronic cough after infection. That number may seem small, but it translates to a lot of people coughing, given that the U.S. has more than 280,000 new cases of COVID-19 per week, as of early February 2023. The actual number, though, is unclear because the studies that look at postinfectious cough are often small and only account for the people who got COVID-19 and showed up in their doctor’s office or in a telehealth visit for evaluation.

No simple fix

The American College of Chest Physicians and the European Respiratory Society have published guidelines to help clinicians navigate these uncertainties and the dearth of data available on the diagnosis and treatment of coughing. Although the U.S. guidelines were published in 2006, they still represent the best evidence available for clinicians and their patients.

About half of patients recover from their cough without any treatment. For those who don’t, the limited data available suggests that inhalers, steroids, narcotics and certain over-the-counter medications may provide relief for some people.

In adults, the evidence for the efficacy of various treatments is mixed and limited. In my practice, I often prescribe a non-narcotic cough suppressant called benzonatate, sold under the brand name Tessalon Perles. It works by numbing the nerves in the lungs and airways, calming the cough reflex. Data for treatments in children is equally lacking, and studies have shown that over-the-counter cough suppressants and antihistamines were no more effective than the placebo.

Home remedies can also play an important role for some patients. Many people swear by honey, and there is some limited supporting evidence behind its benefits. One trial showed that honey was more effective at soothing a cough than the placebo over a three-day period.

Resources

nationalgeographic.com, “You’re not sick anymore—so why are you still coughing?” By National Geographic Science Editors; pbs.org, “Why coughs can linger long after you recover from an illness.” By Kyle B. Enfield; pbs.org, “The connection between cold weather and catching a cold.” By John Yand & Sarah Clune Hartman;

Appendices

The connection between cold weather and catching a cold