A combination vaccine for flu and COVID-19 is in the works. Here's what to know.

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As the holiday season draws near, more Americans may be sporting two Band-Aids after receiving both their flu shot and the new COVID-19 bivalent booster.

Vaccine developers are looking to relieve people from the unpleasantness of getting two shots by creating one that offers strong protection against both viruses.

Moderna and Pfizer-BioNTech, companies that have led the COVID-19 vaccination strategy, say they're beginning trials to assess the safety, efficacy and dosage of their candidate vaccine that combines four flu strains and two coronavirus strains.

Health experts say these combination vaccines could be available as early as next flu season. Here's what to know.

What's the difference between flu and COVID-19?

Influenza: An infection of the respiratory system including the nose, throat and lungs.

Flu symptoms: Fever, chills, muscle aches, cough, congestion, runny nose, headache and fatigue.

Flu strains: There are four types of influenza viruses – A, B, C and D – but the strains that typically cause seasonal flu illness are influenza A and B. The Centers for Disease Control and Prevention says influenza C primarily causes mild disease and influenza D is normally seen in cattle, not people.

Depending on the flu season in the U.S, the CDC estimates the virus causes:

  • 9 million to 40 million cases.

  • 140,000 to 710,000 hospitalizations.

  • 12,000 to 52,00 deaths.

COVID-19: First discovered in December 2019 and caused by SARS-CoV-2.

COVID-19 symptoms: Fever or chills, cough, shortness of breath, fatigue, muscle or body aches, and new loss of taste or smell.

The coronavirus may also cause respiratory symptoms similar to the flu, but research has shown other parts of the body may also be affected by the disease. In some people, it can also cause "long COVID," or life-altering symptoms that endure for months or even years after an initial infection.

COVID-19 strains: Dozens of coronavirus strains have popped up all over the world. But a few came out on top over the course of the pandemic, most prominently the delta and omicron variants.

The BA.5 subvariant of omicron made up more than 99% of new cases in August but now makes up only about 40%. Coronavirus variants that are gaining momentum include BQ.1 and BQ.1.1, which are also subvariants of omicron.

The U.S. has reported more than 97.6 million cases and 1.07 million deaths from COVID-19, according to Johns Hopkins University data.

What flu and COVID vaccines do we currently use?

Influenza: The type of flu shot a person gets is based on their age, but each kind is designed to protect against four flu strains circulating that season. Most standard dose flu vaccines are approved for people 6 months and older, and they are made using virus grown in eggs.

Other shots include:

  • Cell-based: Contains virus grown in cells, approved for people 6 months and older.

  • Recombinant: Made using recombinant technology without flu viruses, approved for people 18 years and older.

  • Attenuated: Egg-based shot that contains a weakened live virus, approved for ages 2 to 49.

  • High-dose or adjuvanted: Egg-based shot with a high-dose or a special ingredient that helps create a stronger immune response, approved for people 65 and older.

COVID-19: The most widely used shots in the United States are messenger RNA vaccines created by Pfizer-BioNTech and Moderna.

In a COVID-19 vaccine, mRNA spurs cells to make a spike protein normally found on the surface of the coronavirus. That way, when the immune system sees the actual virus, it will recognize the protein and attack the virus before it can do serious damage.

Both companies offer a primary series vaccine that contains only the original coronavirus and a bivalent booster that is also meant to generate spike proteins from the BA.4 and BA.5 omicron subvariants.

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What would the new vaccines do?

Ideally: They would protect against COVID-19 and the flu with a single shot. There are no approved flu vaccines with mRNA technology, but companies are looking to change that.

An mRNA-based flu vaccine would be less costly to manufacture and may produce a better immune response in the elderly, said Dr. Daniel Kuritzkes, division chief of infectious diseases at Brigham and Women's Hospital.

Vaccine developers are using their candidate flu vaccines that use mRNA technology and combining it with their bivalent booster. The combination vaccines induce an immune response specific to the four strains of flu circulating that season, the original coronavirus, and the BA.4/BA.5 subvariants.

"What it should do it generate high-titer neutralizing antibody against influenza (and COVID), and it should also be safe," Kuritzkes said. "We don’t want people getting much worse reactions in terms of fever, chills and muscle aches as a consequence of getting a combination vaccine."

Moderna, Pfizer: What's in the works

Pfizer-BioNTech: The companies have announced the start of Phase 1 trials to evaluate the safety, efficacy and optimal dose level of its combination candidate vaccine. The trial includes 180 participants ages 18 to 64, with the first person receiving a dose the beginning of November. Researchers will follow up with participants after six months, the companies said in a statement.

The combination vaccine includes authorized bivalent vaccine and a new mRNA-based flu vaccine candidate, which is now in Phase 3 trials.

Moderna: The Cambridge-based company is slightly ahead of the curve. It already has its Phase 1/2 trial fully enrolled with participants ages 18 to 75.

The flu-COVID candidate vaccine combines Moderna’s original COVID-19 mRNA vaccine with its candidate mRNA flu vaccine, which is also in Phase 3 trials.

Can you get flu and COVID-19 at the same time? Would a new vaccine help?

Experts: Co-infections with respiratory viruses happen "all the time," especially in early childhood, said Dr. Pedro Piedra, professor of molecular virology and microbiology at Baylor College of Medicine.

It’s possible to co-detect the flu and COVID-19 in the same person – what was dubbed “flurona” last year – but health experts say it’s difficult to determine which virus is causing the principle infection and related symptoms.

There has yet to be data in human participants, but health experts say in theory a combination vaccine should protect against both a flu and coronavirus infection.

"Having combination vaccines for respiratory pathogens that circulate during the fall and winter months is an ideal type of vaccine ... as long as (they're) safe, well tolerated and effective," Piedra said.

Combining vaccines is nothing new, he said. Although none exist for respiratory viruses, many common childhood vaccines protect against multiple diseases and contain multiple vaccines.

Where can I get a flu and COVID-19 vaccine now?

Health officials say Americans can get their flu and COVID-19 shots in the same visit.

Doctor's office: Experts advise to call ahead before making an appointment to make sure the vaccine you want or need is available.

Pharmacy: Major pharmacy retailers, such as Walgreens and CVS, offer both vaccines and accept walk-ins or appointments online.

Vaccines.gov: Americans unsure of where to find the closest available vaccine can visit this website and choose a vaccine based on age and manufacturer.

Follow Adrianna Rodriguez on Twitter: @AdriannaUSAT.

Health and patient safety coverage at USA TODAY is made possible in part by a grant from the Masimo Foundation for Ethics, Innovation and Competition in Healthcare. The Masimo Foundation does not provide editorial input.

This article originally appeared on USA TODAY: Flu, COVID vaccine: Pfizer, Moderna working on combination shot