Thanksgiving leftovers are a distant memory, and December's extra travel, shopping and family commitments are already straining nerves, budgets and immune systems. It's officially "the holidays" — which also means we're well into a new flu season.

It's never too late to benefit from a flu shot, even into December and January, says Dr. William Schaffner, an infectious disease specialist at Vanderbilt School of Medicine in Nashville.

"Giving influenza vaccine to people with heart disease, for example, is as important as giving them a statin," Schaffner says, yet many people living with chronic illness — and even many of their health care providers — don't realize how vital flu vaccination is to managing their condition.

The vaccine's ability to prevent flu cases last year hovered around 44 percent overall; it was about 59 percent effective for young children and just 16 percent in adults over 65. But even that low number for older adults elides how much death and disability the vaccine prevented.

"Effectiveness" only refers to the ability of the vaccine to prevent illness, and the vaccine does so much more than that," says Dr. LJ Tan, chief strategy officer for the nonprofit Immunization Action Coalition, based in St. Paul, Minn.

Extra benefits from the flu shot for certain groups

In older adults, the flu shot prevents the loss of quality of life that can result from influenza complications, Tan says. That protection is also important for people with chronic conditions beyond heart disease, such as diabetes, asthma and lung or liver disease. Doctors say flu shots are also indicated for patients with suppressed immune systems, and most people who have an autoimmune condition.

Pregnant women can be especially vulnerable to influenza complications. Just under 1 in 10 women aged 15-44 are pregnant each flu season, but, according to a CDC report in October, pregnant women accounted for 1 in every 3 or 4 people hospitalized with influenza over the past seven flu seasons. Under-vaccination is part of the problem: The same report found only 54% of pregnant women reported getting the flu vaccine during the 2018-2019 season.

What to expect this year

Flu cases don't typically peak until well after the start of the new year, but infections often begin picking up in December. Some states, including Texas and North Carolina, are already reporting the first influenza deaths of the season, including those of at least 10 children. Most children each year who die from the flu had not been vaccinated, and many were healthy until their infection.

Serese Marotta, chief operating officer of the nonprofit advocacy organization Families Fighting Flu, says her 5-year-old son Joseph was healthy until he caught the flu and died in 2009.

"Sadly, there are many families like mine that have been adversely affected by flu," she says. Misconceptions about the flu — such as the erroneous belief that it only kills people with underlying conditions — persist because the virus "is so dynamic and difficult to understand," Marotta says. "Flu kills more Americans than all other vaccine-preventable diseases, and as such, it deserves our respect and attention."

This year hospitals are already admitting children with flu complications. Dr. Bernhard Wiedermann, an infectious disease physician at Children's National Hospital in Washington, D.C., says his hospital had 23 cases during the week before Thanksgiving, alone. To his surprise, 20 of them were influenza B, which is "usually more prominent at the tail end of the season in the spring," Wiedermann says. He doesn't know what, if anything, that portends for the upcoming season, but it highlights just how little public health officials know until the flu season is in full swing.

"Projections about flu are tricky," Schaffner says. "If you've seen one flu season, you've seen one season. Each one is distinctive."

The past two years have been the worst, in terms of flu cases and complications, in the past decade. An estimated 37 to 43 million Americans had flu last season, resulting in as many as 647,000 hospitalizations and as many as 61,200 deaths. Preliminary numbers from 2017-2018 are similar: 45 million infections, 810,000 hospitalizations and 61,000 deaths.

This season, flu cases seem to be taking off quickly, Schaffner says, though whether that's "a harbinger of things to come" is impossible to know. He points to last year's "unprecedented double-barreled season," in which H1N1 infections peaked early, followed by an unexpected second peak of H3N2, which is typically the most severe strain.

What's new in this year's shot?

This year's flu vaccines contain four inactivated or live, attenuated strains: two influenza A strains (an H1N1 strain and an H3N2) and two B strains. The two B strains are the same as in last year's formulation, but this year's H1N1 and H3N2 vaccine strains are different from last year, based on recommendations from the World Health Organization. WHO infectious disease experts consider which strains have been recently circulating in different parts of the globe when deciding what to recommend for the next season, but their predictions can be as much art as science. Curveballs like last year's two-peak season are always possible. The flu virus is a master of adaptation — it has evolved to be extremely good at infecting humans — so it's frequently a few steps ahead of scientists.

That's why any protection is better than none, Tan says. Even during years with low vaccine effectiveness, "if it's not in your arm, it's 0% effective," he notes.

Flu shot options

As in past years, people have multiple options for flu vaccines this year. All the standard flu shots for 2019-2020 are quadrivalent — containing all four strains —according to the CDC. The only seasonal flu shots with trivalent (three-strain) options this year are the two recommended for people age 65 and older — a high-dose version and the one with an "adjuvant," an extra ingredient that boosts the body's immune response to the vaccine.

Hate needles? The live nasal flu vaccine – a nasal spray — is available this year for people ages 2-49 who are not pregnant and don't have any other restrictions. A lack of effectiveness kept earlier versions of a nasal flu vaccine off pharmacy shelves for a few years; but it's been reformulated and was reintroduced last year.

Adults who want to avoid needles but can't get the nasal vaccine might opt for the jet injector vaccine, which shoots a high-pressure stream of fluid into the arm.

Patients who prefer to avoid vaccines made with eggs have two options: the recombinant flu vaccine and cell-based vaccines. The recombinant vaccine combines one gene from a naturally occurring flu virus with another virus, and is grown inside cells from insects, instead of chicken eggs. A cell-based vaccine relies on virus grown inside mammal cells.

Why myths about flu and flu shots persist

Too often, doctors say, people shy away from the shot because of some falsehoods or misconception they've heard about the flu vaccine.

"When you repeat a myth often enough, it takes on its own life," Tan says. Inaccurate information seems more prevalent for the flu vaccine than for any other vaccine, he says, likely because it's the only one recommended for nearly everyone, every year.

"There's a lot more chatter," Tan says. "The adults are chattering, nurses are chattering, parents are chattering, adolescents are chattering." That chatter and heavy media coverage can amplify misinformation, making it harder for people to know what to believe.

Some myths are simply misconstrued — from a kernel of truth.

For example, Schaffner says, one of the leading reasons people opt out of the vaccine is that they don't think it works well enough. The flu shot's effectiveness does, in fact, fluctuate quite a bit from year to year.

But here's the rest of the story: Even if you do catch the flu despite having been vaccinated, "your illness is very likely to be less severe" than if you'd skipped the shot, Schaffner says. "It's more likely to be shorter, and you're much less likely to get the complications of pneumonia, being hospitalized and dying."

Need more reasons to get vaccinated? Here are five of the most common myths about flu shots — and a strong dose of science-based facts to dispel the fiction.

1. Can getting the flu vaccine give you the flu or make you sick?

Fact: The flu shot can't give you the flu.

It's not biologically possible to catch an illness from the inactivated vaccine, and even the weakened live virus in the nasal vaccine cannot cause the flu. Anyone who gets sick after the flu shot caught the bug either just before or after getting vaccinated. It only takes 2-5 days to incubate a flu virus but two weeks for the vaccine to start working, so if you catch the flu in the waiting room, you still could fall ill even though you got the vaccine that day. That said, some people may feel under the weather from side effects of the flu shot, such as a headache, nausea, low fever or similar symptoms. These are normal responses to the vaccine in some people because they are normal responses from an immune system fighting a pathogen.

2. Aren't deaths from the flu exaggerated?

Fact: Deaths from influenza range from a few thousand to tens of thousands every U.S. flu season.

The number of flu deaths varies wildly from one year to the next depending on the dominant strains (H3N2 is usually the deadliest). But the total U.S. deaths are always in the thousands, ranging from 3,349 deaths in 1986-87 to a high of 48,614 deaths in 2003-04, according to a 2010 study by the CDC. People usually die from complications of the flu — especially pneumonia — rather than the flu itself, but wouldn't have died if they hadn't caught the flu.

3. Don't flu vaccines contain dangerous ingredients, such as mercury, formaldehyde and antifreeze?

Fact: Flu shot ingredients do not pose a risk to most people.

Vaccines do not contain antifreeze. Formaldehyde is used during vaccine manufacturing to inactivate the virus so it cannot cause disease. Any trace amounts remaining in the final vaccine fall well below what naturally occurs in fruits and vegetables or the levels that the human body produces itself.

Single-dose vials of flu vaccine do not contain any mercury compounds. The larger, multi-dose vials contain a preservative called thimerosal, which breaks down into 49% ethylmercury and is used to prevent bacterial contamination of the vaccine container. Unlike the methylmercury found in fish that can build up in the body, ethylmercury is made of larger molecules that cannot enter the brain; they exit the body in about a week. Vaccines made with thimerosal have been extensively studied and are safe, but if you don't want it for whatever reason, request a single-dose vaccine.

Some people have allergies to some of the ingredients in flu vaccines, such as gelatin, thimerosal or antibiotics; if you think that might be the case for you, ask your physician which type of flu vaccine is right for you.

4. Should pregnant women avoid the flu shot?

Fact: Pregnant women are particularly advised to get the flu shot.

The flu vaccine is not only safe for pregnant mothers but especially recommended since pregnancy increases the risk of flu complications for the mom and the baby. From 2010-2018, pregnant women accounted for 24% to 34% of influenza-associated hospitalizations, though only 9% of women aged 15-44 are pregnant each year. Plus, the protection from maternal antibodies extends to the baby after birth. Hospitalizations are 40% lower among vaccinated pregnant women and 72% lower among their infants, from birth through age 6 months, compared to those who don't get vaccinated. Flu shots are also linked to a lower risk of miscarriage and stillbirth, likely in part because infections during pregnancy, including influenza, are linked to greater miscarriage risk. Flu vaccination is even associated with reduced risk of other birth complications, such as a preterm or underweight baby.

5. Don't pharmaceutical companies make a massive profit from flu vaccines?

Fact: Revenue from vaccines comprise a tiny proportion of pharma profits­, and make it possible for companies to continue making the vaccines in the event of a pandemic.

Total revenue from influenza vaccines is estimated by the WHO to have been about $2.2 billion in 2018. But total pharmaceutical industry revenue was nearly $1.2 trillion – making flu vaccines only about 0.18% of total industry revenue. By comparison, just one drug, Humira, which is used to treat autoimmune conditions, accounted for $20 billion in 2018. Each of the top 15 most profitable drugs in 2018 bring in more than double the amount from the entire flu vaccine market each year.

If pharmaceutical companies didn't make a profit off vaccines, they likely wouldn't manufacture them, resulting in thousands more deaths, health care analysts note. Keeping production going also ensures vaccines are available if a pandemic occurs. Meanwhile, influenza itself is quite expensive, costing the US about $4.6 billion annually in direct medical costs and as much as $87 billion annually in all costs.

Lastly, Wiederman points out, remember this: Getting vaccinated also helps protects the family, friends and coworkers swirling around you each day — particularly the people whose age or health conditions put them at higher risk for flu complications.

"I would think there's not a single person out there that doesn't have contact regularly with someone with risk factors," Wiederman says. The long list of people especially vulnerable to a bad case of flu includes toddlers and seniors, pregnant women and anyone with asthma or other chronic illness. "When we get vaccinated," he says, "we lessen the chances that they will get infected, too."

Tara Haelle is a freelance health and science writer based in Peoria, Ill. She's on Twitter: @tarahaelle