Is it the 'worst cold ever,' the flu or COVID-19?

Doctors in the United States are bracing for a "twindemic" of flu and coronavirus spikes. Germany bought extra flu vaccines. Tens of thousands of people in Britain are looking up "worst cold ever" on search engines.

In countries with relatively high vaccination rates such as the United States and in Europe, it could get tricky this winter for the immunized to tell a nasty cold from a breakthrough case of COVID. It's also hard to predict how bad this flu season will be after last year's historically low flu rates during lockdowns.

With children back in school in many parts of the world and travel picking up in tourist spots, health-care professionals worry that the flu season could come roaring back and are urging people to get their shots.

Q: Why are health officials worried about the flu this year?

A: Health experts say Americans have built up less natural immunity against influenza because so few were infected in 2020. The comeback of common viruses including RSV brought toddlers, who were not exposed as babies, to U.S. hospitals with severe cases this year.

While a feared collision of infections remained at bay last winter, viruses will have more potential to spread this fall in venues opening up again. This has medical staff worried that flu and COVID-19 admissions could surge together in the next few months.

Many countries have warned of this: Britain expanded its free annual flu shot drive to cover more people, while France ordered 30 percent more of the seasonal flu vaccine than it did last year.

A: The illnesses come from distinct viruses but often have similar symptoms. Some of the signs that COVID-19 and the flu share include fever, cough and chills. The common cold is usually milder than the flu, with a runny or stuffy nose more likely.

The loss of taste or smell - a possible COVID-19 symptom - can help set it apart, although people with allergies or stuffy noses might find it harder to smell, too. Those with coronavirus can also take more time to show symptoms and be contagious for longer than people with flu infections.

Q: How can you tell if you have a breakthrough coronavirus infection?

A: Most people will struggle to know for sure based on symptoms alone, without a nose swab to rule out the coronavirus. And if vaccinated people test less often, COVID infections could get dismissed as mild colds or go unnoticed.

British scientist Tim Spector, a founder of the ZOE app that tracks coronavirus symptoms, has said that people thinking they have colds when they really have COVID-19 could help fuel the pandemic. People who have had their coronavirus shots and are knocked out by heavy colds in Britain could be breakthrough cases without knowing it, he added, which could pose a risk if they don't get tested.

A: It's "doubly important this year" to get a flu shot, according to Rochelle Walensky, director of the U.S. Centers for Disease Control and Prevention - not only to protect yourself but also to ease the strain on hospitals. The CDC is also urging everyone eligible to get a coronavirus vaccine.

The CDC says people can take the COVID shot along with others "without regard to timing." Already, U.S. manufacturer Moderna has plans to make a single shot that combines a booster with a flu vaccine.

It is possible to get sick with both COVID-19 and the flu, infectious-disease doctors have found, although studies are still looking at how common that is.

Q: Can I still get COVID-19 if I have the coronavirus vaccine?

A: People who had their coronavirus shots can still get sick. But experts stress that vaccines remain the best weapon against the disease.