About a week before Pennsylvania shut down last March, at the start of the pandemic, Amanda Bernstein felt short of breath. Then she had a dry cough, followed by a fever.

Bernstein, a 29-year-old insurance analyst, asked for a coronavirus test, but her doctor said he didn’t have one. Instead, he tested her for influenza. When that came back negative, he said she probably had a “bug,” and indeed, she was fine after a few days of Tylenol and cough syrup.

“I’ll never know if I had covid,” Bernstein said. “If I’d have known more back then about how serious it was, I’d have been much more worried.”

Six months later, on the verge of another flu season, Americans know a lot more about covid-19, the disease caused by the novel coronavirus. Unfortunately, doctors may be just as much in the dark when it comes to distinguishing it without a lab test for influenza or several other common seasonal viruses, including adenovirus or RSV. All of these ailments often share initial symptoms that may include a cough, fever, fatigue and muscle aches.

“We’re going to be totally confused,” predicted Arnold Monto, an epidemiologist at the University of Michigan School of Public Health.

To date, covid-19 has just one common telltale symptom, Monto said, which is a distinctive loss of taste and smell. More than 85 percent of patients reported this symptom in a recent European study of 417 people with mild to moderate cases of the disease.

People with influenza or a bad cold and nasal congestion may also lose their sense of smell. The difference with covid-19 is that the loss is sudden and more profound — affecting even the ability to distinguish between sweet and bitter — and may happen without a stuffy nose, said Carl Philpott, a University of East Anglia ear, nose and throat expert.

Philpott, one of the authors of a small study recently described in a letter to the editor in the journal Rhinology, gave smell and taste tests to 30 people matched for age and sex. Ten had been diagnosed with covid-19; 10 had bad colds and 10 served as a healthy control group.

In an email, Philpott said the finding offers a way to discriminate between covid-19 and influenza or a cold when conventional tests aren’t available. A University of East Anglia statement said the finding supported the theory that covid-19 infects the brain and central nervous system. Philpott said people who suspect they have covid-19 might screen themselves at home, by sniffing products such as coffee and oranges.

A rarer covid-19 symptom that may offer another warning sign is dyspnea, or difficulty breathing. The Centers for Disease Control and Prevention lists this as one of only two symptoms (along with loss of taste and smell) not commonly shared with influenza. Researchers have reported it in about 50 percent of patients with severe cases of the disease, saying it tends to appear about a week after onset, and usually signals a serious respiratory problem.

Doctors will be on the alert for these two symptoms in the coming weeks, yet several experts said there’s still enough overlap with symptoms of other respiratory illnesses to raise concerns that they’ll be flummoxed.

“Let’s just imagine you have a person in their 60s with diabetes and high blood pressure who has a fever and cough,” said Paul Sax, clinical director of the Division of Infectious Diseases at Brigham and Women’s Hospital in Boston. In trying to distinguish between influenza and covid-19, he said, “There’s no way a physician can confidently tell one from the other.”

The stakes of uncertainty are high, given that swift differentiation between covid-19 and the flu could help save lives. Time-tested anti-viral medications, approved by the Food and Drug Administration, can fight influenza and similar illnesses even as they’re powerless against covid-19. But patients with covid-19 should be quarantined as soon as possible, given that on average it is nearly twice as contagious as influenza. What’s more, people infected with covid-19 can be contagious for about twice as long as for those with the flu.

Covid-19 is also about ten times more lethal than influenza, as Anthony S. Fauci, the director of the National Institute of Allergy and Infectious Diseases, has noted. In barely six months, more than 182,000 Americans have died of covid-19, compared with between 12,000 and 61,000 annual deaths from influenza, according to the CDC. This is true even as 80 percent of covid-19 cases are classified as mild or asymptomatic — not requiring hospitalization — according to the World Health Organization. Influenza has a similar share of mild cases.

If coronavirus test results were as rapidly delivered in the United States as they are in many other countries, doctors would have less cause for concern about the elusive task of trying to tell the difference from flu on the phone or in the office, experts said.

Covid-19 outpatient tests are now much more widely available throughout the United States than they were last March, but in some parts of the country where labs are backlogged, delays in reporting results are still common. A doctor in Georgia recently tweeted to complain about delays of two to three weeks, which would make contact tracing impossible.

Quest Diagnostics reports that it has performed 12.5 million tests for the coronavirus so far with results now available in one to two days. In contrast, doctors can test for influenza in their offices and get results within an hour, Monto said. As the flu season approaches, manufacturers have been working on a combined test for the coronavirus and influenza with plans to have it available in public hospitals in coming weeks, said Erica Shenoy, associate chief of the Infection Control Unit at Massachusetts General Hospital. This, she said, might eventually help reduce demand for test supplies.

Researchers studying the novel coronavirus suspect that the main reason it’s so much more lethal than influenza is because of the fierce way our immune systems may overreact to a relatively unfamiliar infection.

The CDC has been studying a rare but deadly immune-system reaction to covid-19 in young people, known as MIS-C for multisystem inflammatory syndrome in children, in which various body parts become inflamed, including the heart, lungs, kidneys, brain, eyes and gastrointestinal organs.

A gentler immune system response has been a factor in “covid toes,” the painful, itchy rash that has appeared on the toes and fingers of many children and young adults with otherwise mild or asymptomatic cases.

A major unknown is what may happen when flu and covid-19 strike simultaneously. Early on in the pandemic but at the tail end of last year’s flu season, Stanford University researchers reported that 20 percent of a sample of patients had both illnesses at once. While the phenomenon has yet to be studied in depth, some experts worry that having both at the same time could increase the toll on the immune system, worsening symptoms and impairing recovery.

The Stanford team drew no conclusions about that concern. “That’s an experiment we’re going to conduct in October, whether we want to or not,” said one of the authors, clinical virologist Benjamin Pinsky. But their study emphasized that having one illness offers no protection against also having the other.

Bernstein, who has been sheltering in place with her boyfriend and their cat, Sgt. Pepper, said the couple has so faithfully isolated themselves since March that she hopes they’ll stay healthy through the coming flu season. She nonetheless plans to get vaccinated this month — and has stocked up on toilet paper and tuna, in case the double-whammy of flu and covid-19 brings back the chaos of March.

What to watch out for

Influenza and covid-19 share so many symptoms that doctors expect it will be challenging to tell them apart in the coming flu season. Below is a list of major symptoms of each disease, according to the Centers for Disease Control and Prevention. Not everyone who has either disease will have any or all of these symptoms.

Influenza

Fever/chills

Cough

Sore throat

Runny or stuffy nose

Muscle or body aches

Headaches

Fatigue

Vomiting and diarrhea

Covid-19

Fever/chills

Cough

Shortness of breath or difficulty breathing

Fatigue

Muscle or body aches

Headache

New loss of taste or smell

Sore throat

Congestion or runny nose

Nausea or vomiting

Diarrhea

CDC advice

If you are ill with covid-19 or think you might have been exposed, the CDC recommends that you stay home except to get medical care. Stay in touch with your doctor, calling before you go to the office. Get rest and stay hydrated. Avoid public transportation, ride-sharing or taxis. But call a doctor immediately if you or someone you know is showing any of these symptoms:

Trouble breathing

Persistent pain or pressure in the chest

New confusion

Inability to wake or stay awake

Bluish lips or face