What's the latest to know about COVID, flu vaccines as respiratory virus season begins?

As the United States heads into the cold-weather months, respiratory virus season has also arrived, with cases of influenza and COVID-19 likely to increase.

The Centers for Disease Control and Prevention (CDC) currently recommends that everyone 6 months and older stay up-to-date and get a flu vaccine and a COVID vaccine, and that it's safe to get both at the same time.

"For flu and COVID, not only does the vaccine reduce…the chance of getting sick, it certainly – even for those people who get sick – reduces their chance of getting severely sick," Dr. Cameron Wolfe, a professor of medicine in the division of infectious diseases at Duke University School of Medicine, told ABC News.

Here's what you need to know about what vaccines are available, and who is eligible to receive them:

What to know about COVID-19 vaccines

For COVID-19, there is an updated vaccine that's formulated to target variants that are currently circulating that are related to XBB, an offshoot of the Omicron variant.

There are formulations made by Pfizer-BioNTech and Moderna for those aged 6 months and older, and a formulation made by Novavax for those aged 12 and older.

"There's a different number of doses that you have to get depending on how many vaccines you've already received in the past," Dr. Shivanjali Shankaran, an associate professor in the department of internal medicine in the division of infectious diseases at Rush University in Chicago, told ABC News.

For those who are between 6 months and 4 years old, the CDC recommends getting two doses of Moderna or three doses of Pfizer if they are unvaccinated. If they've been previously vaccinated, the CDC recommends one or two doses of Moderna or Pfizer, depending on the previous number of doses.

For those aged 5 to 11, the CDC recommends one dose of either Moderna or Pfizer, regardless of previous vaccination status.

When it comes to Americans aged 12 and older, it's recommended that those who are unvaccinated receive one dose of Pfizer or Moderna or two doses of Novavax. Those who have been previously vaccinated are recommended to receive one dose of the Pfizer, Moderna or Novavax updated vaccine.

Those who are moderately or severely immunocompromised may require more doses.

"There's the gain for any individual by taking the vaccine and there's the most gain for people who are immunosuppressed, have heart or lung conditions, or older adults," Wolfe told ABC News. "You know, if you're a 25-year-old who lives with a patient who's had a lung transplant, this is not a bad thing for you to get. In fact, it would be highly encouraged."

What to know about flu vaccines

For the majority of those aged 6 months and older, the CDC recommends receiving the standard quadrivalent flu vaccine, which protects against four different strains of the virus. If this is an infant or child’s first time getting a flu vaccination they should get two doses this season, each at least four weeks apart.

However, for those who are aged 65 and older, the CDC recommends getting one of three vaccines: the high-dose flu shot, the adjuvanted flu shot, or the recombinant flu vaccine, all of which are quadrivalent.

The high-dose shot contains four times the amount of antigen – that is, the protein molecule identified with the virus – to trigger a stronger immune response, while the recombinant shot contains three times the amount of antigen. The adjuvanted shot is made with an ingredient – an adjuvant – that helps create an even stronger immune response.

"There is a broadly available shot, of course, [but] there is a high-dose vaccine that's a stronger dose and provides that additional protection, because those who are older in age have more of a challenge to mount an effective response. So getting that high-dose shot or adjuvant is important, because it can provide that critical protection," said Dr. John Brownstein, an epidemiologist and chief innovation officer at Boston Children's Hospital and ABC News contributor.

Traditionally, flu vaccines are made using an egg-based manufacturing process, so if someone has an egg allergy they can instead receive the cell-culture-based flu vaccine, which uses influenza viruses grown in cell cultures rather than in eggs. However, the CDC says the standard vaccine should be safe to receive, even for those allergic to eggs.

"The flu vaccines that are available this year are safe to be taken if you do have an underlying egg allergy," Shankaran said. "As long as someone can monitor you, which I think most places do, anyway."

There is also a nasal spray flu vaccine, made with a live modified virus, which can be given to those between ages 2 and 49. It's specifically not recommended for those who are immunosuppressed or pregnant.

Health officials typically suggest getting the flu shot by Halloween but stress that it's never too late, because the flu season can last into the spring months.

Shots that could be around the corner

In addition to the vaccines that are now available, there are several clinical trials for both flu and COVID vaccines that are currently undergoing clinical trials.

This includes a flu vaccine from Pfizer using mRNA technology, which was used to develop the COVID-19 vaccine. There's also an mRNA universal flu vaccine, developed by researchers at the National Institute of Allergy and Infectious Diseases Vaccine Research Center.

Pfizer, Moderna and Novavax are also all working to develop a combination COVID-19 and flu vaccine that would offer protection from both viruses in a single shot.

"I do think that having a combo shot – if the clinical data suggests that it's safe and effective, clearly – will be more appealing to people to have things available in a single shot as opposed to multiple shots, especially if it's challenging for people now that they're trying to chase both COVID and flu vaccines, and maybe not always available at the same time," said Brownstein. "It creates convenience, and potentially just more ease of administration overall, and hopefully reduced costs."