Flu activity is rising sharply in the U.S. this season with at least 7.5 million estimated cases so far, largely due to a new variant of influenza type A called subclade K.

Subclade K emerged after this season's flu shot was made. Even though the shot isn't perfectly matched for this variant, experts say the vaccine is still the best line of defense to protect against severe illness, hospitalization and death.

For those who end up contracting the virus, there are treatments that may help reduce the duration of a person's symptoms or severity of illness.

There are four antiviral medications approved by the Food and Drug Administration (FDA) and recommended by the Centers for Disease Control and Prevention (CDC) to treat influenza this season.

Flu activity rises sharply across the US with at least 7.5 million illnesses: CDC

All require a prescription and work best when started as soon as possible after the onset of symptoms, typically under 48 hours, the CDC says.

Testing can help guide treatment, especially to distinguish between COVID-19 and influenza, which have different treatments.

At-home tests for influenza and/or COVID-19 are widely available over the counter, and some pharmacies offer rapid on-site testing if a person doesn't want or need to seek care at a health care facility or emergency room.

"These tests can be really helpful. You can get your result within 15 to 20 minutes as a nasal swab," Dr. Darien Sutton, a board-certified emergency medicine physician and ABC News medical correspondent, told "Good Morning America" on Tuesday.

Some states allow a pharmacist to prescribe and administer appropriate treatment after someone tests positive for the flu. For example, at national pharmacy chain CVS, participating states in its test-to-treat program currently include California, Colorado, Idaho, Illinois, Michigan, Minnesota, Montana, Nebraska, South Dakota, Tennessee, Washington, Wisconsin and Wyoming.

Sutton recommends contacting your pharmacy to get a sense of what your options are.

"Call ahead, and it might save you time and money, and maybe perhaps a trip to the ER if you can get some treatment there," he said.

Types of flu treatment

The CDC says most people who get the flu will not need treatment and can improve with supportive care alone.

For those who are seeking treatment, it is largely guided by a person's underlying health, symptom severity, duration of illness, and age, as well as weighing risks versus benefit.

The CDC recommends antiviral treatment is started as soon as possible for any patient with suspected or confirmed influenza who is hospitalized; has severe, or progressive illness; or is at higher risk for influenza complications.

People who are high risk include young children, people over age 65 and those who are immunocompromised or with underlying health conditions such as diabetes, heart disease or lung problems, the CDC says.

Oseltamivir, zanamivir and peramivir are antiviral medications that work by stopping the virus from spreading in the body.

Flu cases are on the rise in the US. Here's what you need to know

Oseltamivir is available as the brand name drug Tamiflu but also available as a generic drug that may be less expensive.

It's taken for five days, and FDA-approved for people aged two weeks and older, but both the CDC and the American Academy of Pediatrics recommend use for certain high-risk babies who are less than two weeks old as guided by a health care provider.

This drug can also be prescribed to help prevent a flu infection after exposure for some adults and children who are high-risk. The most common side effect is gastrointestinal upset.

Zanamivir, sold under the brand name Relenza, is an inhaled medication and FDA-approved for people aged 7 years and older.

It may also be used as prophylaxis to prevent influenza in people aged 5 years and older, but it is not recommended for use in those with any underlying respiratory disease, including people with asthma because it can cause breathing difficulty.

Peramivir, sold under the brand name Rapivab, can only be used in a health care setting because it is administered through an IV. This drug is FDA-approved for people aged 6 months and older.

A fourth medication called baloxavir marboxil, also known as Xofluza, is a type of antiviral medication that stops influenza from replicating in the body.

It is taken once orally and FDA-approved to treat influenza for all people aged 12 years and older. Kids who are aged 5 to less than 12 years old can also take this drug, but it is only FDA-approved to treat uncomplicated influenza in this age group -- meaning illness without severe symptoms.

Baloxavir can also be used for post-exposure prophylaxis, taken after exposure to prevent infection, in people aged 5 years and older, but it is not recommended for pregnant women, those who are immunocompromised, breastfeeding mothers, outpatients with complicated or progressive illness, or hospitalized patients.

Insurance companies typically cover some type of antiviral treatment for influenza, but options and cost will vary by coverage plan.

For people paying out of pocket, the generic formulation of oseltamivir is likely to be the most affordable but there may be rebates or discounts available for other drugs to consider with a prescribing provider, doctors say.

Jade A. Cobern, MD, MPH, is a practicing physician, board-certified in pediatrics and general preventive medicine, and is a medical fellow of the ABC News Medical Unit.