What to Know About Flu Shots

Since the arrival of the novel coronavirus pandemic, the word “vaccine” has been almost exclusively associated with the shots that have been found highly effective in preventing severe illness and death from COVID-19.

But there are still many other kinds of vaccines that should stay on your radar as important health tools, and one of the most important among these shots is the annual flu shot that you take to help reduce your chances of getting influenza.

How Flu Shots Work

“Like all vaccinations, the flu shot aims to provide a susceptible individual a ‘boost’ to their non-existent or waning immunity to a particular infection by presenting a non-infectious version of a pathogen,” says Dr. Charles Bailey, medical director for infection prevention at Providence St. Joseph Hospital and Providence Mission Hospital in Orange County, California.

In the case of the flu shot, this “means that the vaccine contains killed (inactivated) flu virus. Different strains of influenza virus are included, including the ones we know have caused the most death and disability in the past and those predicted to be most prevalent in the forthcoming season,” says Dr. Supriya Narasimhan, an epidemiologist and chief of infectious diseases at Santa Clara Valley Medical Center in San Jose, California.

It’s important to note that the viral particles contained in the vaccine are inactivated, which means the vaccine “cannot cause the flu.” The immune system “develops antibodies when the body is exposed to the flu virus antigens in the inactivated vaccine in about two weeks,” Narasimhan says.

The substance being introduced to the body by the vaccine is incapable of causing the infection, but it helps the immune system identify and remember that this agent is something that it should fight off. Vaccines “train or retrain the immune system to respond to infectious pathogens in a prompt, robust and targeted fashion if and whenever it’s presented with this organism in the future,” Bailey says.

[READ: Is It Safe to Take the COVID-19 Vaccine and the Flu Shot at the Same Time?]

Why You Need the Flu Shot Every Year

Flu shots need to be administered, or boosted, annually because the virus that causes the flu changes or mutates slightly every year. “Flu is a bit different from most vaccine-preventable diseases in that it routinely changes year-to-year, usually in relatively minor ways,” a situation called “drift,” Bailey says. This renders the prior year’s vaccination “less likely to provide the desired level of protection.”

What’s more, “in some years a major change (mutation), called a ‘shift’ occurs.” This happens when the vaccine was formulated to address “observed ‘drift’ changes but could not anticipate the larger ‘shift’ change in the vaccine target,” he explains.

As we’ve seen with the coronavirus, small changes or mutations of the virus’ genetic material can result in big changes in the transmissibility and severity of the virus. The same is true for the virus that causes the flu, and to keep pace with these changes, the flu shot is updated annually to offer better protection.

In addition, “a person’s immune protection from vaccination declines over time and so we need to re-jog this immunological memory periodically, more so for some diseases than others,” Narasimhan says.

The flu vaccine is “reengineered each year to include the most relevant strains (types) of circulating flu viruses so it can provide the best protection,” she adds. “Scientists decide on the composition of the flu vaccine annually and update it prior to each flu season. This is why we need to get the shot every year for the best protection.”

Bailey notes that all current flu vaccines are “quadrivalent,” meaning they cover four different flu strains (two strains each of type). (The CDC reports that Influenza type A tends to be a more severe strain of flu virus and is the type of virus typically responsible for flu pandemics. The H1N1 virus is an influenza type A virus. Influenza type B, on the other hand, is usually less harmful but can cause seasonal epidemics of flu.) These four strains have been “chosen to closely match those flu strains observed and/or predicted to be the most widely circulating during the 2021 to 2022 flu season.”

When to Get Your Flu Shot

Dr. Katrina Miller Parrish, chief quality and information executive with L.A. Care Health Plan, notes that “the worldwide predominant flu virus changes or mutates each year. The flu shot predicts and protects against the most common strains of flu in any given year.” This is why it’s important to get the shot every year.

Bailey says early fall, when the flu shots for the next flu season first become available is the best time to get your shots. “This will provide protection against an unexpectedly early onset of flu season and still provide protection into early spring.”

Getting your shot early in the season is good, but getting a shot even earlier in the year, in July for example, doesn’t help as much, because “the risk of contracting flu before late September is low.” However, the possibility still exists that waning vaccine protection could leave you less well-protected in late spring if a late flu season develops.

The flu shot provides full protection from two weeks after the injection to about six months after injection, Miller Parrish says. “Flu season peaks between November to February, so it’s best to protect yourself from the flu before then.”

While earlier in the season is better, “it’s never too late to get the flu shot so make sure to get it anytime you can,” Miller Parrish adds.

[READ: Should I Get the COVID-19 Booster?]

What to Expect from the Shot

When you go in for your flu shot, your health care provider will ask you to roll up your sleeve to access the meaty part of the upper arm near the shoulder. They’ll swab the area carefully with alcohol or another sterilizing agent. Then, they’ll pierce the skin with a needle that contains the vaccine, which is delivered via a push of the plunger.

The vaccine is administered deep into the muscle. The whole thing is over in a minute or less in most cases. You may feel a pinch when the needle enters the skin, but it’s over so fast, you may barely even realize it’s happened.

Afterwards, you may have some side effects. “Most side effects are minor and very temporary,” Bailey says. These may include:

— Tenderness at the site of injection.



— Muscle aches.

— Weakness or fatigue.

“Although some of these sound like flu symptoms, they’re due to the body’s immune response to the vaccine, not influenza infection itself,” Bailey notes.

Typically, any side effects last a day or two after vaccination, and “are much less severe than actual flu illness,” Miller Parrish says.

In rare cases, you may experience more severe side effects which can indicate a serious allergic reaction such as full body hives or any life threatening symptoms. Seek immediate medical attention if you develop a severe response or your side effects don’t go away in a couple of days.

Still, “the risk of a serious outcome from contracting influenza far outweighs the risk of any potential vaccine-related side effect,” Bailey explains.

Flu Vaccine Effectiveness and Breakthrough Infections

While the flu shot is effective in preventing infections or limiting the risk of complications, such as pneumonia or death, it’s still possible to get the flu, a situation called a breakthrough infection. The Centers for Disease Control and Prevention note that recent studies have pegged the flu vaccine effective rate at between 40% and 60% in most years.

Certain risk factors can increase the chances you’ll develop the flu even after you’ve been vaccinated. These include:

Timing. “After getting the shot, it takes two weeks for the vaccine to take effect,” Miller Parrish explains. “If you get infected with the virus before the two weeks is up, then your body isn’t prepared to fight off the virus yet.” She adds that in some cases, if you get sick with the flu shortly after getting the flu shot, “that means perhaps the flu virus was already in your body when you got vaccinated” or were exposed to it before your immune system had fully responded to the vaccine.

Strain. Narasimhan notes that while there are four flu viruses covered in the current vaccines “there are many more in circulation” and it’s possible to encounter one that’s not included. “Because the flu viruses in particular change constantly, the flu vaccine can have varying efficacy based on how well the strains in the vaccine match the circulating strains of the virus.”

Immune system status. Age, specifically being aged 65 or older, and underlying health conditions, such as autoimmune disorders, asthma and heart disease, can compromise the immune system’s ability to mount a sufficient response.

Late exposure. Waning immunity later in the flu season, especially if that occurs alongside exposure to high levels of virus, can also lead to illness.

While it’s possible to get the flu after being vaccinated against it, the CDC reports that the vaccine really does help prevent severe illness and visits to the doctor. A 2021 study noted that flu vaccination was associated with a 26% lower risk of intensive care unit admission and a 31% lower risk of death from the flu compared to unvaccinated individuals.

Data from the 2019-2020 flu season also indicate that flu vaccination prevented an estimated:

— 7.5 million cases of the flu.

— 3.7 million flu-related medical visits.

— 105,000 flu-associated hospitalizations.

— 6,300 flu-related deaths.

Sometimes when you get sick, it might feel like the flu but actually be something else. Miller Parrish notes that “there are many types of illnesses, including the common cold and COVID-19 that people will confuse with influenza. The flu shot protects against the most common strains of flu in any given year, but doesn’t protect against all viruses.”

[Read: Is It COVID-19, Allergies, Flu or a Cold?]

Getting the Flu Shot Is Even More Important During the Pandemic

Health experts have been saying for years that getting the flu shot annually is an important piece of preventive health care that everyone who’s able to should take advantage of. That advice becomes only more urgent during the coronavirus pandemic, when the risk of co-infection with both the flu and COVID-19 at the same time has increased and could lead to very serious complications or death.

In addition, “the flu shot can protect against flu-related hospitalizations, keeping people out of hospitals that are heavily burdened by COVID-19 patients and by preventing potential COVID-19 exposure,” Miller Parrish says.

Vaccination isn’t just about protecting yourself. “Healthy adults and individuals might be able to fight off the flu quickly, but by getting vaccinated you can protect those around you who might suffer from flu related complications such as the elderly and those who are immunocompromised,” she says.

In 2020, the flu season was mild, which was likely influenced by public health measures — such as masking, social distancing and reduction in opportunities to be in close indoor proximity with lots of other people that were put in place because of the pandemic. In 2021 and beyond, that situation may well change.

“Some experts have predicted a worse than usual flu season (in the 2021 to 2022 season) due to the very low flu cases (in the 2020 to 2021 season), leading to lower immunity on the basis of less recent flu exposure,” Bailey says.

That lower immunity level in combination with fewer mask mandates and the reopening of schools, businesses, restaurants and other places where people may be congregating indoors during flu season, could mean getting your flu shot is even more important for keeping yourself and your community safe and healthy this year.

“Winter is the busiest time for hospitals with high transmission rates of all respiratory infections, including COVID due to travel, indoor activities, the holidays” and other factors, Narasimhan says. “The U.S. health care system is already quite burdened by COVID, especially in states with low COVID vaccine uptake, and health care workers are fatigued after (months) of herculean pandemic effort. A bad flu season overlaid on top of all this can break the health care delivery system.”

And this potential breakdown “has a greater propensity to disproportionately affect rural communities and socioeconomic minorities who have fewer health care options” to begin with, she adds. “COVID has highlighted these disparities like never before. This is why it’s more important than ever to take the flu vaccine this year.”

Get Vaccinated

Bottom line: The flu vaccine is safe and “everyone over 6 months old should get vaccinated for the flu,” Miller Parrish says. “Getting a flu shot is easy” by visiting your doctor’s office, a local pharmacy or a work- or community-based flu vaccination event. “You can go online to find a flu event near you.”

The flu vaccine “protects well against severe influence and its complications including death,” Miller Parrish adds. “It’s our best tool to protect our community against influenza.”

And continuing with the practices taught by the pandemic can also reduce risk, Narasimhan says. “Masking, hand hygiene, avoiding contact with others when sick and prompt diagnosis and treatment all help decrease flu transmission. The antiviral medicines for flu are highly effective and safe.”

More from U.S. News

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Questions to Ask Your Doctor About the COVID-19 Vaccine

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What to Know About Flu Shots originally appeared on usnews.com

Update 01/10/22: This story was previously published at an earlier date and has been updated with new information.

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