SHARE

This post has been updated. It was originally published on March 10, 2021.

Click here to see all of PopSci’s COVID-19 coverage.

Anywhere from one-fifth to one half of SARS-CoV-2 carriers are asymptomatic. In the rest, the virus can manifest as an unwitting selection from a buffet of coronavirus symptoms—coughing, fever, loss of taste, loss of appetite, vomiting, diarrhea, brain fog, fatigue—that’s made the coronavirus wildly unpredictable. That leaves many wondering whether these are coronavirus symptoms or just due to a passing bug. The progression of coronavirus symptoms tends start with a fever, but there’s no set pattern (and some people don’t even get a fever!).

There are, however, patterns in how the virus manifests itself. Using data that American, British, and Swedish patients entered into a tracking app, researchers with the COVID Symptom Study have found that coronavirus symptoms tend to fall in one of six clusters.

Coronavirus symptoms

Cluster 1: Flu-like, with no fever

  • Headache
  • Loss of smell
  • Cough
  • Sore throat
  • Muscle pains
  • Chest pain

You might intrinsically associate the coronavirus with a fever, especially because quick temperature checks have become ubiquitous in many public spaces. In reality a great many COVID cases—potentially as many as half—don’t involve a fever at all.

Cluster 2: Flu-like, with fever

  • Headache
  • Loss of smell
  • Cough
  • Sore throat
  • Hoarseness
  • Fever
  • Loss of appetite

This cluster unites what some call “the classic triad,” three ailments characteristic to COVID-19: fever, loss of smell, and a persistent cough. In fact, the British NHS requires one of these three for most COVID tests. But research suggests that a narrow-minded focus on the classic triad misses a serious number of cases.

Cluster 3: Gastrointestinal

  • Headache
  • Loss of smell
  • Sore throat
  • Loss of appetite
  • Diarrhea
  • Chest pain

Despite COVID-19′s reputation as a respiratory illness, some patients may only face digestive symptoms such as diarrhea, vomiting, and abdominal pains. In fact, that may have helped initially spread the virus before the link was well-understood.

Cluster 4: Severe level one (fatigue)

  • Headache
  • Loss of smell
  • Cough
  • Hoarseness
  • Fever
  • Chest pain
  • Fatigue

Less than 2% of patients in cluster one needed emergency breathing help, but that percentage more than quadruples here. It’s not uncommon for viral infections to come with a side of fatigue, and COVID-19 is no exception. Some patients can feel lethargic, exhausted, and cripplingly low-energy for months after they’ve recovered from their other symptoms.

Cluster 5: Severe level two (confusion)

  • Headache
  • Loss of smell
  • Cough
  • Sore throat
  • Hoarseness
  • Fever
  • Loss of appetite
  • Chest pain
  • Fatigue
  • Confusion
  • Muscle pain

Some patients find themselves blanketed in a “brain fog,” like a miasma that slows thoughts and blurs focus, making it hard to concentrate on even the most everyday tasks. It’s one of the least-understood aspects of the disease, and just like Cluster 4′s fatigue, this brain fog can linger for months.

Cluster 6: Severe level three (abdominal and respiratory)

  • Headache
  • Loss of smell
  • Loss of appetite
  • Cough
  • Fever
  • Hoarseness
  • Sore throat
  • Chest pain
  • Fatigue
  • Confusion
  • Muscle pain
  • Shortness of breath
  • Diarrhea
  • Abdominal pain

This is the most extreme cluster, combining the worst of respiratory problems with gastrointestinal symptoms like diarrhea, not to mention fatigue and brain fog. The cluster’s dire name is well-earned: nearly a fifth of patients within this cluster needed a respirator. And a patient in this group was three times more likely to visit a hospital than a patient in cluster one.

[Related: Here’s where all the COVID-19 vaccine candidates currently stand]

Researchers didn’t make these clusters just to sort COVID symptoms into bins; they’ve found a few patterns in who tends to develop different versions of the disease. Broadly, patients who are in the latter three clusters are more likely to be overweight, elderly, or have a condition like COPD or diabetes. In other words, patients with COVID-19 risk factors were more likely to end up at the severe end of the cluster list. Patients from cluster six were also more likely to face the slog of so-called long COVID. Most COVID-19 patients recover, more or less, and carry on with their lives. But a fraction can suffer months of fatigue, headaches, recurring fevers, concentration troubles, and debilitating hair loss. The link between coronavirus and hair loss has come in focus, but public health experts suspect it’s not a viral symptom so much as it is a side effect of COVID-induced stress.

These clusters aren’t the end-all be-all of COVID-19. Since researchers formulated those clusters in September 2020, we’ve learned about another common symptom: skin rashes. In fact, some public health experts even suggest that they ought to be a fourth member of the “classic triad.” COVID-related skin rashes can take on a few forms: hives, chickenpox-esque red bumps, or the beet-colored sores dubbed “Covid fingers” or “Covid toes.”

More recently, “Covid tongue” has emerged. It’s not common—it’s estimated less than 1 in 500 patients have something like it—but a few have reported painful mouth ulcers or something furry, yellow, and white coating their tongue.

It’s likely we’ll never get a complete list of every COVID-19 symptom. The coronavirus is a continually evolving beast, and every patient’s experience is unique. But studying the patterns has taught public health experts how different coronavirus symptoms are interconnected. And as the virus will no doubt continue infecting the world for years to come, that can help save lives.

Symptoms of breakthrough COVID cases

As more people get vaccinated, we’ve learned that it’s possible for some inoculated people to contract the virus. However, the infection and its symptoms, including from the Delta variant, tend to be much less severe. As such, those who develop breakthrough cases are less likely to require hospitalization; vaccinated people are eight times less likely to be infected and 25 times less likely to be hospitalized or die.

But since symptoms for breakthrough cases are generally mild, it’s important to be on the lookout for any atypical feelings. Even congestion, headaches, and coughing should be taken seriously, especially if you don’t normally experience allergies. A quick trip to your local drug store for a COVID test, whether the PCR type or the rapid variety, will help you sort out the answer.

Right now, vigilance and vaccines are the best options for continuing to navigate the pandemic.

MORE TO READ