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The One Sure Sign You May Have Already Had COVID

The #1 symptom is the same for vaccinated and unvaccinated.

There aren't too many "sure things" when it comes to COVID-19—the virus is unpredictable and its symptoms can be vague (and some people who are infected never show symptoms at all). But researchers say one symptom, in particular, is becoming increasingly common with the Delta variant—and it's probably not what you think. Read on to find out more—and to ensure your health and the health of others, don't miss these Sure Signs You've Already Had COVID.


The Most Common Delta Symptom

Exhausted young tattooed business woman keeping eyes closed, touching head and suffering from the headache while sitting at her working place in the modern office

Researchers with the COVID Symptom Study are tracking symptoms associated with new COVID cases via an app. They're separating those new cases into three groups—the unvaccinated, the fully vaccinated, and the partially vaccinated. The scientists found that headache is now the #1 symptom reported by all three groups.

"Even though headaches are a less well-known symptom of COVID-19, they are one of the earliest signs of the disease and more common than the 'classic' symptoms of cough, fever and loss of smell (anosmia)," the researchers write. But it's not just any kind of headache. Keep reading to see what these headaches feel like.

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What These Headaches Feel Like

Tired woman holding her head with her hands.

The COVID Symptom Study researchers say these headaches tend to:

  • Feel moderately to severely painful
  • Create a pulsing, pressing or stabbing sensation
  • Involve both sides of the head, not just one area
  • Last for more than three days
  • Be resistant to regular painkillers

It's not clear why COVID is causing headaches. The virus might produce inflammation in the brain, or the aching could be a result of dehydration. A headache isn't the only telling sign of Delta. Next, discover the other symptoms.

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Common Cold-Like Symptoms Are Common

Sick woman with flu at home

In the first waves of the pandemic, cough, shortness of breath, and the loss of taste or smell were the telltale symptoms of COVID-19 infection. The Delta variant seems to have changed the equation slightly. "It seems like cough and loss of smell are less common," Dr. Inci Yildirim, a Yale Medicine infectious diseases specialist, said last month. 

"The symptoms we are seeing now are much more commonly identified with the common cold," Dr. Andrew T. Chan, an epidemiologist and one of the COVID Symptom Study's lead investigators, told The New York Times. "We are still seeing people presenting with a cough, but we are also seeing a higher prevalence of things like runny nose and sneezing." 

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But So Are Traditional COVID Symptoms

That said, doctors say the more well-known symptoms of COVID are still common. According to the CDC, these include:

  • Fever or chills
  • Cough
  • Shortness of breath or difficulty breathing
  • Fatigue
  • Muscle or body aches
  • New loss of taste or smell
  • Sore throat
  • Congestion or runny nose
  • Nausea or vomiting
  • Diarrhea

If you experience any of the symptoms, get tested for COVID-19 ASAP, even if you've been fully vaccinated.

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Why It's Important to Get Vaccinated

Woman with face mask getting vaccinated, coronavirus, covid-19 and vaccination concept.

If you haven't been vaccinated against COVID-19, it's important to do so. Studies show that vaccination 

  • reduces your chance of catching COVID, including the Delta variant
  • nearly eliminates the chances of being hospitalized or dying from COVID, if you do contract a breakthrough infection
  • and reduces the chance of developing "long COVID"—a chronic syndrome of debilitating symptoms like fatigue and muscle aches—by about 50%.

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How to Stay Safe Out There

Brunette woman wearing a KN95 FPP2 mask.

Follow public health guidelines and help end this pandemic, no matter where you live. Get vaccinated ASAP. If you live in an area with low vaccination rates, wear a face mask that fits snugly and is double layered. Don't travel. Practice social distancing, avoid large crowds, practice good hand hygiene, and to protect your life and the lives of others, don't visit any of these 35 Places You're Most Likely to Catch COVID.

Michael Martin
Michael Martin is a New York City-based writer and editor. Read more about Michael