Concerns about the flu spreading in the U.S. are growing as the U.K. continues to see a spike in cases among children and young adults.

The increased number of cases in the U.K., could be a predictor for the flu season in the U.S., according to ABC News chief medical correspondent Dr. Tara Narula.

"We know that England or other places can be a marker for what is going to happen here, because their flu season happens a few weeks earlier than ours," Narula said on "Good Morning America" Monday, adding, "We have low numbers of cases so far but they are increasing."

Some hospitals are starting to implement flu season visitor restrictions, including the Detroit Medical Center and Children's Hospital of Michigan, which are allowing, as of Monday, up to two visitors per patient and only those 13 years of age and older are permitted on inpatient hospital floors or in observation units.

According to data from the Centers for Disease Control and Prevention, flu activity in the U.S. is up at least 7% in the last week, and so far, there have been nearly 2 million illnesses, 19,000 hospitalizations, and 730 deaths from the flu.

Last year, the U.S. saw an extremely severe flu season, with 560,000 hospitalizations and approximately 38,000 deaths from the flu, the CDC reported in September.

Meanwhile, in the U.K., where flu season started earlier than usual this year and has yet to peak, doctors are seeing increasing flu activity that's currently at "medium" level with hospitalizations for the flu around 7.79 per 100,000, according to UK Health Security Agency data.

Some hospitals, such as the Sherwood Forest Hospitals in Nottinghamshire, have also reintroduced face mask policies in some hospital areas in light of the uptick in flu cases.

Hospitals issue new guidelines amid rising flu cases

What flu variant is circulating?

The predominant flu strain that is currently circulating the most in the U.S. is the subclade K variant.

"The strain that is circulating most … subclade K, which is a variant of H3N2, seems to have a little bit more severe symptoms and maybe more severely affects the elderly and children," Narula said Monday.

How to protect kids amid new subclade K flu strain

What are the symptoms of the flu?

Flu symptoms can vary from mild to severe intensity, and according to the CDC, they can come on suddenly.

Symptoms can include as fever, chills, cough, sore throat, runny or stuffy nose, body or muscle aches, headaches, fatigue, and vomiting or diarrhea, two symptoms which tend to be more common in children rather than adults.

People who are at more at risk of developing complications from the flu, according to the CDC, include older adults over 65, children younger than 2, and people with underlying conditions such as asthma and chronic lung disease or heart disease. Complications from the flu may vary, but they may include pneumonia, sinusitis, ear infections, and kidney and respiratory failure.

How to protect against the flu

Although the subclade K flu variant is not a perfect match for this year's flu vaccine, doctors and the CDC still recommend everyone over the age of 6 months to receive one as a protective measure.

"When they look at studies abroad in England, they have found that the vaccine lowers the risk of hospitalizations for children by about 70% to 75%, which is really good, and adults by about 30% to 40%," Narula said.

"Everyone thinks [the flu is] not that serious, it's just a cold, but no, it can really cause complications and death," Narula added.

Emergency physician Dr. Stephanie Widmer also recommended practicing regular hygienic habits to stay healthy.

"On top of getting the vaccine, washing your hands often, covering your coughs and sneezes, avoid[ing] touching your face and staying home if you're feeling sick are all things [to do]," Widmer said on "ABC News Live."