Is it time for your flu shot? What to know about effectiveness and how long it lasts
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Based on wastewater samples, the current flu season hasn't taken off yet in the U.S., although once it does, it spreads rapidly across the country, Alexandria Boehm, program director of WastewaterSCAN, a nonprofit monitoring network and a professor of civil and environmental engineering at Stanford University, said. "Once it starts in one state it starts everywhere at the same time," she said. "It's not a slow wave."
With flu, wastewater levels match what's happening in the community, she said. Last year, influenza took off in mid-December and lasted through mid-April.
Flu season typically peaks around February, according to the Centers for Disease Control and Prevention, although the influenza virus can continue circulating through May.
In adults, immunity provided by flu vaccines can wane over time. So adults who get their flu shots very early — such as in August or early September — may lose some of that protection toward the end of flu season, said Dr. Amesh Adalja, an infectious disease physician and senior scholar at the Johns Hopkins Center for Health Security.
Adalja recommends adults get their flu shots by late October in order to be protected by the beginning of flu season, which can vary depending on where you live.
Flu vaccines are safe
Flu vaccines and their ingredients are rigorously tested and found to be very safe. Over the past 50 years, hundreds of millions of Americans have safely received flu shots.
“Really, these are extremely safe vaccines,” said Dr. Robert Kim-Farley, a professor at the Fielding School of Public Health at the University of California, Los Angeles. “There’s nothing people need to be alarmed about.”
The most common side effect of flu shots is a sore arm, Adalja said.
Some people worry that flu shots increase the risk of Guillain-Barre syndrome, a serious but rare disorder in which the immune system attacks nerve cells, causing muscle weakness and sometimes paralysis. The actual risk from current flu shots is very low, with one to two additional cases of the syndrome for every million doses of vaccine, according to the CDC.
It’s important to note that viruses also can cause the syndrome, Adalja said. In fact, people have a higher risk of developing the syndrome from flu than from the vaccine.
Fears of Guillain-Barre syndrome stem from a 1976 outbreak of swine flu, in which 45 million Americans received a new vaccine. That flu shot caused one additional case of Guillain-Barre for every 100,000 people vaccinated.
Flu vaccines save lives
Flu vaccination prevented 9.8 million illnesses; 4.8 million medical visits; 120,000 hospitalizations; and 7,900 deaths during the 2023-2024 season, according to the Centers for Disease Control and Prevention.
Yet, only 32% of people hospitalized for flu last year had been vaccinated, according to the CDC.
Last season’s flu shot reduced the risk of testing positive for flu during an outpatient visit by 56%. Although people who are vaccinated sometimes still get infected with flu, their illness tends to be much milder compared to people who aren’t vaccinated, Adalja said.
While they’re not perfect, flu vaccines can reduce the risk of infection and make people less contagious, at least for part of the flu season. Flu shots “do provide some level of protection to close contacts for a period of time,” Adalja said.
Flu vaccines tend to be less protective than other vaccines for multiple reasons, Adalja said. First, flu viruses themselves change every year, which means the vaccine needs to change, as well. Because flu shots take several months to produce, vaccine makers need to begin production in the spring, long before flu season begins.
Manufacturers in the United States select which viral strain to include in fall vaccines based on the influenza strains circulating in the Southern Hemisphere, Kim-Farley said. This year’s flu shots protect against three strains of influenza, including two types of influenza A and one type of influenza B.
Flu shots don't cause flu
Injectable flu shots are made with pieces of the flu virus, Adalja said. These pieces stimulate the immune system to make protective antibodies in order to combat any flu viruses that enter the body.
Nasal flu vaccines, which are approved for people 2 through 49, contain live viruses that have been weakened to prevent them from causing the flu, Bryant said.
Because the virus is live, it can cause mild symptoms, including a fever over 100 degrees in children ages 2 through 6; a runny nose and nasal congestion in people ages 2 through 49; and a sore throat in adults ages 18 through 49.
People can now order the intranasal flu vaccine online and use it at home.
Flu vaccines take 2 weeks for full protection
Some people who develop symptoms such as a fever or cough after receiving their flu vaccines assume their flu shot was ineffective. But because it takes the immune system about two weeks to make antibodies against the influenza virus, it can take two weeks for people to be fully protected, Banks said.
Meanwhile, there are dozens of viruses and bacteria that can cause flu-like symptoms — such as fever, chills, cough, body aches and headaches — including enterovirus, parainfluenza and more.
Flu shots can be safely given with other vaccines
“Extensive testing is required for every vaccine, and we have a lot of research showing that getting multiple vaccines at the same time is safe,” Bryant said. “What happens when we delay or we space out vaccines is that we’re really just increasing the amount of time that children remain vulnerable to infection.”
Some parents worry that multiple vaccines given in one visit overwhelm a baby’s immune system. But babies and young children are exposed to as many as 6,000 viruses, bacteria and other substances that stimulate the immune system -- known as antigens -- in a single day. In comparison, the entire pediatric immunization schedule exposes them to 165 antigens, Bryant said.
Pediatricians and health officials recommend grouping vaccinations together because it’s more convenient for parents to bring a baby to a clinic once, rather than multiple times, Kim-Farley said. Children are more likely to receive all recommended vaccines if they are given multiple shots in one visit.
Some parents fear that giving the flu shot in combination with other vaccines causes their children excess stress. In fact, a study examining stress hormone levels in infants found no difference in stress levels between babies who received one vaccine compared to those who received several. Spacing out vaccines increases a child’s total stress by causing them to receive injections on multiple days.
Flu vaccines don’t contain harmful ingredients
Contrary to what is shared on social media, flu vaccines do not contain fetal cells, blood products, human DNA, soy, gluten, latex or microchips.
Vaccines have five major ingredients: antigens, which are viral proteins or sugars that stimulate the immune system to make protective antibodies; adjuvants, which boost the immune response; stabilizers, which keep the vaccine stable over time; preservatives, which prevent the growth of fungi and bacteria; and residual byproducts.
Some flu shots contain dead or weakened viruses as antigens. Others have only viral proteins. Although some flu vaccines grown in eggs have traces of residual egg protein, the amount is so minuscule that most people with egg allergies can still safely be vaccinated.
Flu shots do not contain aluminum, used in trace amounts in some shots, such as hepatitis and pneumococcal vaccines, to help generate a strong immune response.
Although President Donald Trump has called for removing aluminum from vaccines, studies show that the tiny amounts of aluminum in vaccines do not cause harm. In fact, babies consume more aluminum from breastmilk or formula than vaccines.
Some social media influencers have expressed concerns about preservatives in vaccines. A CDC advisory committee in June recommended removing a preservative called thimerosal from vaccines. But that wasn’t a major change; flu vaccines in prefilled syringes are already thimerosal-free. Thimerosal was only used in multidose vials of vaccines -- which account for only about 4% of flu shots -- as a way to prevent dangerous bacteria and fungi, Adalja said.
Although studies show that thimerosal isn’t harmful — and doesn’t cause autism — manufacturers took the preservative out of routine pediatric vaccinations in 1999.
Most flu shots don’t contain adjuvants; the one exception is a flu shot designed for people older than 65, whose immune systems need an extra boost, which uses a non-aluminum adjuvant.
Dr. Virginia Banks, an infectious disease specialist in Youngstown, Ohio, said she recently opted for a vaccine recommended for people older than 65.
“I’m way over 65, but I look in the mirror and I think to myself, ‘You don’t need that,’” she said. “But then I realize, ‘Yeah, yeah you do.’”
The flu is not just a bad cold
Even during relatively mild flu seasons, thousands of Americans die from the virus.
Flu season was particularly brutal last year, sickening at least 47 million Americans, hospitalizing 610,000 and killing 27,000, according to the CDC.
Those deaths included 281 children, the largest number of pediatric deaths in 15 years, when the world was hit by the H1N1 pandemic, according to the CDC.
Of the children who died from flu last year, 89% were unvaccinated or weren’t full vaccinated, compared to 82% during the 2023-2024 flu season.
Last season’s flu also led 109 children to develop a rare, severe neurologic complication called influenza-associated encephalopathy, which can cause altered mental status and seizures. Nineteen percent of affected children died, according to the CDC.
Research shows that Black people have the highest flu-related hospitalization rates, followed by Native American people and Hispanic people.
Racial and ethnic disparities are greatest among children; rates of severe outcomes, such as hospitalization and death, are up to four times higher among racial and ethnic minority children under age 4 compared to white children. People from racial and ethnic minority communities are also less likely to be completely vaccinated against the flu.
How much do flu shots cost without insurance?
Children can be vaccinated for free through the federal Vaccines for Children program.
The Affordable Care Act requires that commercial insurance plans provide flu shots for free. People can also get free flu shots through Medicare, Medicaid or the Veterans Health Administration. Without insurance, a flu shot can cost $20 to $130 out of pocket.
Many adults without insurance can receive free or reduced-cost flu shots at federally qualified health centers or public health departments.
Young, healthy adults and children die of the flu every year
Although infants, older adults, pregnant women and people with chronic illnesses have the highest risk of hospitalization or death from the flu, the virus can also kill healthy people, often by causing pneumonia. Forty-four percent of children who died from influenza were previously healthy.
The CDC recommends flu shots for everyone older than 6 months old. Vaccinating pregnant women against the flu helps protect both prospective mothers, as well as their infants, at least for the first few months of life.
Fewer Americans are getting flu shots
Flu vaccination rates have fallen since 2020, caught up in a post-pandemic backlash against vaccines, as well as the mistaken belief that influenza isn’t a serious threat, research suggests.
The percentage of American adults who report having received or planning to receive a flu shot dropped from 60% in 2020 to 54% in 2024.
Vaccination rates have fallen among children, as well, dropping from 64% in 2019-2020 to 49% in 2024-2025.
“People have been told to get Covid shots and RSV shots and flu shots, and people are just a little bit vaccine fatigued,” Banks said.
Low vaccination rates may have contributed to the severity of last year’s flu season, in which influenza-related hospitalization rates were the highest since 2010-2011.
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25 Oct 2025, 8:01 pm
