June 21, 2024

Why do you test negative on an antigen test with COVID-19 symptoms? Do you have flu? Not necessarily — here’s why


COVID-19 isn’t the flu or common cold, yet their symptoms can be strikingly similar

The CDC says a loss or change in taste or smell is more common in COVID-19, but it’s not a precondition for having omicron variant

You have a blistering headache. Your body aches. You’ve developed a cough. You’re sneezing. But you test negative on an antigen test. Do you have COVID-19, the disease caused by the virus SARS-CoV-2? Or the flu? Or “flurona” — that is, both?

It’s a stressful dilemma and an increasingly common question. Flu season is upon us. Omicron is upon us. If you have a flu vaccine and/or a COVID-19 vaccine, your immune system will kick in faster to fight the virus. And then you start to feel sick.

You isolate for at least 5 full days — day zero being the first day of symptoms — because that’s the responsible thing to do. Yet you still have your sense of taste and smell, but that doesn’t mean you don’t have COVID-19.

“Get vaccinated, get boosted, wear a mask in public indoor settings in areas of substantial and high community transmission,” says Rochelle Walensky, director of the U.S. Centers for Disease Control & Prevention. “Take a test before you gather.”

As the omicron variant sweeps across the world, and flu season takes hold, how can you tell the difference between COVID-19 and the flu? Until you finally get that positive COVID-19 antigen test, the truth is it’s almost impossible to tell.

COVID-19 and flu symptoms are practically the same

Omicron and delta pose a greater threat for those who are not vaccinated, but a persistent dry cough and shortness of breath are two symptoms that could point to COVID-19, especially if you were exposed to someone who tested positive.

However, both COVID-19 and flu may result in fever, coughing, shortness of breath/difficulty breathing, fatigue, sore throat, runny or stuffy nose, body aches, headache, vomiting, diarrhea, change in or loss of taste or smell.

The CDC says a loss or change in taste or smell is more common in COVID-19, but it’s not a precondition for having omicron, the highly transmittable variant that’s blazing a trail across schools, workplaces and family gatherings.

Do you see the Catch 22? The flu and COVID-19 have near identical symptoms. Despite all the guidelines helping people to distinguish between the two, a positive rapid antigen test is still the only surefire way of knowing which illness you have.

What’s more, tests are not one and done. Someone with COVID-19 and suffering from symptoms may still test negative on antigen tests for 1-3 days before the virus grows. Their immune system, supported by a vaccine, may actually defeat the virus.

Repeatedly testing negative with COVID-19 or flu symptoms

Therein lies the silver lining to feeling ill. Early in the pandemic, our immune systems fought the coronavirus and we felt ill. Without any preexisting immunity we likely had a large amount of virus in our systems by the time we felt rotten.

Vaccines were a game changer. Michael Mina, an epidemiologist, says feeling sick can be a good thing. Vaccines help our immune system to more quickly recognize and fight the virus, and that makes us feel sick — not necessarily the virus load.

Mina said you can be symptomatic and pre-infectious, infectious, post-infectious or never infectious. “Symptoms don’t = contagious virus,” he wrote on Twitter. “This is literally a reflection of the fact that vaccines are doing their job!”

That’s why we feel horrible even when we’re negative. “We recognize the virus quickly after it lands in us, we develop symptoms, we kind of fight it off, then it often eventually wins, and grows fast AFTER immunity/symptoms started,” he added.

“Thus, if you are symptomatic and negative — although it means you’re probably not contagious at that moment, be very very cautious,” Mina adds. “Quarantine even if possible and test the next morning or that night.”

Isolate and test with symptoms. But when do you exit isolation?

How do you interpret those rapid tests? One red line is negative, while two red lines means positive. A stronger second red line likely means you’re at your most infectious. A lighter red line means you’re at the beginning or end of your infection.

When and how you exit isolation has just gotten trickier. The CDC is maintaining its position on its isolation policy, instructing people who had symptoms and who have access to a test to take one before exiting isolation. But it does not require it.

Even if you are less contagious after your symptoms have alleviated, the CDC’s critics argue that this creates another gray area of confusion, and puts the onus on the individual to make decisions that are best left up to public-health officials.

The CDC’s explanation: “The majority of SARS-CoV-2 transmission occurs early in the course of illness, generally in the 1-2 days prior to onset of symptoms and the 2-3 days after.” That, however, was before omicron spread like wildfire.

This stance has divided public opinion, and frustrated some epidemiologists. Many public-health advocates say it’s safer to get that negative antigen test before going out into the world. For his part, Mina called the CDC guidelines “reckless.”

By Quentin Fottrell

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