It starts with a sneeze, maybe a sniffle.

You think to yourself, it's just mild allergies or a minor cold. You're not worried about COVID-19, because the symptoms don't match up with the distinct, often severe indicators of COVID-19: joint aches, violent coughing, a fever or chill, and the dreaded loss of ability to taste or smell.

But with the omicron variant now the dominant strain in the United States infecting the unvaccinated and fully inoculated alike, health experts warn the symptoms that previously helped people to gauge whether they had a cold, flu or COVID-19 are no longer the useful marker they once were.

Making risks assessments on whether to travel, gather with others or get tested for the coronavirus based on symptoms "is not going to work anymore," said Emily Landon, the chief infectious-disease epidemiologist at University of Chicago Medicine.

Complicating matters is that pandemic-fatigued populations must once again revise what they thought they knew about the coronavirus. Health experts said grids and infographics that suggest certain symptoms are particular to one virus or another, which are popular on social media, may have at one point been helpful but are now outdated with the rise of omicron.

"The problem with grids like that is that people look at them and kind of see what they want to see. It's like looking at your horoscope and saying 'that does apply to me,'" said Landon. Symptom grids and graphics can easily lead people to erroneous conclusions, like believing their symptoms only fit the category of a regular cold, not the coronavirus.

"It's confirmation bias," Landon said.

In this omicron-dominant season, symptoms of cold, flu or COVID-19 are overlapping to a large degree (with the exception of the losing a sense of taste or smell, which remains specific to COVID-19).

"The symptoms are now like a Venn diagram where all the circles are overlapping," said Landon.

Anecdotally, Landon has heard a growing number of patients who have sneezy, stuffy heads and scratchy throats - "things that seem more associated with the common cold" - only to have test results confirm it's the omicron variant of COVID-19.

There are a handful of factors contributing to the blurring of symptoms among viruses, including omicron's characteristics, the way symptoms present in vaccinated people and the rise in cold and flu cases compared with last year's pandemic winter.

Unlike earlier variants of concern, like delta, omicron has a higher affinity for the upper respiratory epithelium, said Landon: "It's more likely to make people sniffle more, sneeze more or be congested."

The mildness of omicron's symptoms for vaccinated people in particular may give them a false sense of confidence that they have a cold rather than a highly-transmissible variant of COVID-19.

"Those who are vaccinated and have minor cold-like symptoms are way less likely to stay home now, and they can spread it to people who can get pretty sick with it," Landon said. "The risk differentiation seems pretty big right now."

Preliminary real-world data from the United Kingdom found people infected with the omicron variant were almost 60% less likely to be hospitalized than those infected with delta. Even though early data indicates omicron may not be as dangerous as delta for unvaccinated individuals, the ease with which omicron spreads still poses a significant threat to the most vulnerable populations.

"So many people are going to get sick that you'll have a lot of people in hospitals, with long COVID and with long-term consequences," Landon said.

Instances of the common cold and flu are cropping up more this winter compared with last year, making it more difficult to know what you've been infected with.

The most recent influenza data from the Centers for Disease Control and Prevention shows moderate-to-high levels in flu activity throughout roughly the country.

"Seasonal influenza activity in the United States is increasing, including indicators that track hospitalizations, according to the CDCs Influenza Surveillance Report ending the week of Dec. 11. Flu activity is notably on the rise in eastern and central parts of the United States.

"I think people are circulating more than they were a year ago," said Philip J. Landrigan, a pediatrician, public health physician and epidemiologist at Boston College, on why the cold and flu are resurging to pre-pandemic levels.

If people want to be conscientious about not bringing the virus home or spreading it to vulnerable people, get tested regardless of symptoms, Landrigan said.

"The reason to get tested now is for your own peace of mind, but an even more important reason is if you're going to a gathering and there are vulnerable people," he said. "If you're going to be going into that kind of gathering, it's not about yourself, it's about protecting the people around you."

As for test type, Landrigan said a nasal swab remains the gold standard for determining if you have the cold, flu or COVID-19.

"When in doubt, test it out."