COVID-19 symptoms are overlapping again with colds, flu, and seasonal allergies. Here's how to tell the difference.

• Respiratory infections may become more prevalent again now that Americans are ditching their masks.
• Some cold or flu cases could be easily confused with a mild COVID-19 infection.
• Headaches and runny noses seem to be more common among recent COVID-19 cases than past ones.

Measures to curb the coronavirus' spread, like mask wearing and social distancing, effectively blunted last year's flu season: Just 155 Americans were hospitalized with the flu from October through January. That's compared to around 8,600 during roughly the same time period a year prior.

But as Americans ditch their masks and return to normal activities, experts caution that respiratory infections could become more prevalent again.

"I do anticipate that in the months ahead, if people are not wearing masks - and we've started to see some of this already - that there will likely be an increase of upper respiratory infections in places that are not wearing masks," Rochelle Walensky, director of the Centers for Disease Control and Prevention, said at a White House press briefing last week.

Common colds are particularly likely to spread, since they're a year-round illness (though cold cases typically spike in the spring and winter).

"There's no doubt the colds are coming back," Dr. Monica Gandhi, an infectious disease expert at the University of California, San Francisco, told Insider.

In some instances, colds could be easily confused with a mild COVID-19 case. The following chart shows how COVID-19 symptoms either overlap with or diverge from symptoms commonly associated with colds, flu, and seasonal allergies:

The COVID Symptom Study - a project that tracks self-reported COVID-19 symptoms via an app - suggests that a headache and runny nose are now two leading indicators of a COVID-19 infection in the UK, especially among people who are young or partially vaccinated.

So public-health experts are considering whether the Delta variant - which is now dominant in the UK - is causing a somewhat different set of symptoms than the original strain. It's also possible that average COVID-19 symptoms appear milder lately because more young, healthy people (who are less likely to be vaccinated) are getting infected - or getting official diagnoses - than earlier in the pandemic.

For the most part, though, fully vaccinated people rarely contract COVID-19, let alone develop symptoms. From January to April, just 0.01% of vaccinated Americans - around 10,000 out of 100 million people - got COVID-19 after they were fully immunized, according to a May CDC report. About 27% of those infections were asymptomatic.

That means severe respiratory symptoms among vaccinated people are more likely the result of something other than COVID-19, Gandhi said.

"There are more people hospitalized for other non-COVID respiratory pathogens in the UK right now than there are for COVID-19," Gandhi added. "That's what happens when you mass vaccinate."

Is it COVID-19? Allergies? The common cold?

COVID-19 rarely follows a neat pattern. The CDC estimates that around 30% of cases are asymptomatic, while the remainder can range from mild to severe. The disease can bring a variety of symptoms, the most common of which include fever, cough, loss of smell or taste, headaches, sore throat, and fatigue.

But vaccines could be making symptoms milder overall, so it's difficult to tell what an average case looks like now.

The COVID Symptom Study found that loss of smell was more common among those who were fully vaccinated than those who hadn't been immunized. Meanwhile, fever was more common among unvaccinated than vaccinated people.

"Our hope is it'll get milder," Tim Spector, an epidemiologist at King's College London, recently told Insider. "So it will just become like a cold."

Both colds and COVID-19 tend to develop gradually - whereas the flu and allergies have more abrupt symptoms, as the chart below shows.

On average, people with COVID-19 start to feel sick five days after they were infected, though symptoms can manifest anywhere from two days to two weeks post-infection.

Similarly, people with a common cold may have a sore throat for eight days, a headache for nine to 10 days, and congestion, a runny nose, or cough for more than two weeks. Cold symptoms usually reach their peak within two to three days of infection.

People with the flu, on the other hand, typically feel sick one to four days after exposure.

Allergies tend to last longer - about two to three weeks per allergen - and won't resolve until the allergen leaves the air. Peak allergy season lasts through July this year, so some runny noses may be attributable to pollen, not COVID-19. But getting tested is still the only way to know for sure.