Article : Flu shot facts & side effects

By Rachael Rettner - Senior Writer Live Science

Flu shots are recommended for everyone ages 6 months and older.

The seasonal flu shot is a yearly vaccine administered to protect against the flu, or influenza. In the United States, flu shots are recommended for everyone ages 6 months and older, according to the Centers for Disease Control and Prevention.

The flu can be a very serious illness, especially in young children, adults ages 65 and over, those with underlying health conditions, and pregnant women. The flu shot is the best way to protect yourself and family from the flu, the CDC says.

Strains of the flu virus are constantly changing, so a new flu vaccine is made each year. Scientists make the vaccine before the flu season starts by predicting which flu strains are likely to be the most common during the upcoming season.

"Since the flu virus frequently drifts in its genetic composition, you have to reformulate the vaccine, and this is one of the reasons that people have to [get a flu shot] on an annual basis," said Dr. William Schaffner, a preventive medicine and infectious disease expert at Vanderbilt University School of Medicine.

Exactly when the flu season starts and ends is unpredictable, so health officials recommend that people get their flu shot in early fall, preferably by the end of October, the CDC says. The same recommendation applies this year during the COVID-19 pandemic. Flu activity typically peaks in January or February.

"We'd like to get as many people protected against influenza before influenza becomes active in communities across the country," Schaffner said.

Most flu vaccines are given before Thanksgiving, Schaffner said, but people can still get their shot throughout the winter months. Each season's flu shot expires in June of that year, but Schaffner said that he would consider it "too late" to get a flu vaccine after March, unless a person is traveling to the Southern Hemisphere (where the flu season will be starting).

After vaccination, it takes a person about two weeks to build up immunity against the flu.

People can visit the CDC's to find flu shot locations.


Children younger than 6 months cannot get a flu shot. Those who've had a severe allergic reaction to a flu vaccine in the past should generally not be vaccinated, the CDC says.

You should not get the flu vaccine if you have a high fever. (You should wait until the fever is gone.)

However, if you have minor illness, like a mild cold or a headache, you can still get a flu shot, Schaffner said. "The vaccine does perfectly well in those folks."

But people who have COVID-19 should not get a flu shot until they have met the criteria to discontinue isolation, according to the CDC. Although people generally can get a flu shot if they have a mild illness, they should not go get a flu shot while they could be contagious with COVID-19 to avoid exposing health care workers and other patients to the virus, the CDC says.


According to the CDC, mild side effects from the flu shot include soreness, redness or swelling at the injection site, low-grade fever and aches. Only about 1% to 2% of people who get a flu shot will have fever as a side effect, Schaffner said. These mild effects should go away within a few days.

Rare but serious side effects can occur, including allergic reactions. Symptoms of serious side effects include difficulty breathing, swelling around the eyes or lips, hives, racing heart, dizziness and high fever. If you experience serious side effects, you should seek medical care immediately, the CDC says. In addition, it's important to ask your health-care provider to file what a Vaccine Adverse Event Reporting System report either online or by calling VAERS at 1-800-822-7967, the CDC says.

For children, side effects from the flu nasal spray can include runny nose, wheezing, headache, vomiting, muscle aches and fever. For adults, side effects from the spray version of the vaccine include runny nose, headache, sore throat and cough. These side effects last a short time compared with the actual flu illness, the CDC says.


The effectiveness of the seasonal flu vaccine depends upon several factors, including how well the flu strains in the vaccine match the strains in circulation. Some studies show that when strains in the vaccine are a good match with the ones that are circulating, vaccinated individuals are 60 percent less likely to catch the flu than people who aren't vaccinated, according to the CDC.

Flu vaccine effectiveness can also vary depending on the person being vaccinated — the vaccine tends to work best in healthy adults and older children, and less well in older adults.

For instance, a 2013 study from the CDC found that the year's flu vaccine was not very effective in adults ages 65 and over: Older people who got the vaccine were just as likely to visit the doctor for flu symptoms as those who did not get the vaccine.

But other studies suggest that individuals who do get sick develop less serve symptoms if they are vaccinated. A 2013 study published in the journal Clinical Infectious Diseases found that people who got the flu shot were less likely to be hospitalized with the flu.

There are some studies that suggest the high-dose flu vaccine given to individuals 65 and older may provide better protection for older adults. The high-dose flu vaccine contains four times the dose of the standard vaccine, Schaffner said. A 2014 study in the New England Journal of Medicine found that the high-dose vaccine provides 24 percent more protection against the flu than the standard dose, Schaffner said.


Flu shots protect against three or four strains of flu virus. Trivalent flu vaccines protect against two influenza A strains — H1N1 and H3N2 — and one influenza B strain. Quadrivalent flu vaccines protect against the same strains as the trivalent vaccine, as well as an extra influenza B strain.

In addition to the standard-dose flu vaccine given through a needle, flu shots are available in several different forms. These include a high-dose version for those ages 65 and older; a "cell-based" version that's grown in animal cells rather than hen's eggs and is approved for people ages 4 and older; a "recombinant" vaccine that does not use the full influenza virus or chicken eggs in the production process and is approved for people ages 18 and older; and a nasal spray, which is approved for healthy people ages 2 to 49, but not for pregnant women.

There is also a needle-free flu shot, delivered by a so-called jet injector, which uses a high-pressure stream of fluid to inject the vaccine, the CDC says. It is approved for adults ages 18 to 64. 

Two new flu vaccines have been licensed for the 2020-2021 season, both for use in adults ages 65 and older. One of the new vaccines is a quadrivalent high-dose vaccine, which was previously available only as a trivalent vaccine, the CDC says. The second new vaccine is a quadrivalent adjuvanted vaccine that has an additional influenza B component.


The composition of the 2020-2021 flu shot will be different from last season's flu shot. Specifically, all three components of the trivalent flu vaccine (H1N1, H3N2 and influenza B) have been updated compared with last year's shot. According to the CDC, the 2020-2021 trivalent egg-based flu shot will contain the following strains of the flu virus:

  •  A/Guangdong-Maonan/SWL1536/2019 (H1N1)pdm09-like virus — This is the H1N1 component that is different from last year's flu shot.  
  •  A/Hong Kong/2671/2019 (H3N2)-like virus — This is the H3N2 component that is different from last year's flu shot. 
  •  B/Washington/02/2019 (B/Victoria lineage)-like virus  — This is the influenza B strain component that is different from last year's shot. 

The 2020-2021 quadrivalent vaccine will also contain a second influenza B strain called "B/Phuket/3073/2013-like (Yamagata lineage) virus," which was also included in last season's quadrivalent vaccine.  


Flu activity has been unusually low in the U.S. during the 2020-2021 flu season, according to the CDC. As of Feb. 13, all states were reporting minimal flu activity, which is not typically for this time of year  — the U.S. usually sees its highest levels of flu activity in January and February.

The rate of hospitalizations so far this flu this season is 0.6 hospitalizations per 100,000 people. That's extremely low — for comparison, during the mild flu season of 2011-2012, the hospitalization rate was 1.6 times higher at this time of year compared with the 2020-2021 season, the CDC said.

Even amid the COVID-19 pandemic, the U.S. is still testing for flu. But out of nearly 25,000 samples from sick people that were tested in the second week of February, just 14 samples, or 0.1%, were positive for flu.

The remarkably low levels of flu this year may be due to precautions taken to prevent the spread of COVID-19, such as mask wearing and social distancing as well as school and work closures. But experts are worried that the flu could make a comeback at an unexpected time of year as immunity to circulating flu viruses wanes in the population, Live Science previously reported.


Should I get a flu shot during the COVID-19 pandemic?

Yes. The CDC says that getting a flu shot during the 2020-2021 season is "more important than ever" in light of the COVID-19 pandemic. Getting a flu shot is important not only to reduce your risk of flu, but also to reduce the burden on the healthcare system responding to COVID-19.

Will a flu vaccine protect against COVID-19?

No, flu shots do not protect against SARS-CoV-2, the virus that causes COVID-19. (Separate vaccines against SARS-CoV-2 have been authorized in the U.S. and are currently being rolled out to the population.) But flu shots do protect against strains of seasonal flu viruses and can reduce your risk of getting sick from the flu, as well as the risk of hospitalization and death from flu, the CDC says.

Can you get the flu and COVID-19 at the same time?

Yes, you can have the flu and COVID-19 — or another respiratory illness — at the same time. And there have been reports of this happening —  for example, the first death of COVID-19 outside of China occurred in a man in the Philippines who was also infected with influenza and Streptococcus pneumoniae, according to the University of MinnesotaScientists are still studying how common dual infections with flu and COVID-19 are, the CDC says. 

Because flu and COVID-19 have similar symptoms, symptoms on their own can't differentiate the two illnesses. Testing can determine if a person has flu or COVID-19 (or both). The U.S. Food and Drug Administration has already issued emergency use authorizations to several tests that can detect flu and COVID-19 at the same time, with a single sample.

How can I safely get a flu vaccine when COVID-19 is spreading in my area?

The CDC has issued guidance for giving vaccinations during a pandemic, and you can ask your doctor, pharmacist or health department if they are following this guidance before you get a shot. The guidance, which is geared towards those giving flu shots, includes recommendations such as screening people for symptoms of COVID-19 before they get their shot; providing appointment times to avoid crowding; ensuring staff wear medical face masks, and in some cases, eye protection and gloves; requiring face masks for those getting flu shots and using markers to help with physical distancing for those waiting for shots.


"It's a myth that you can get flu from the flu vaccine," Schaffner said.

The viruses in the flu shot are killed, so people cannot get the flu from a flu vaccine. However, because it takes about two weeks for people to build up immunity after they get the flu vaccine, some people may catch the flu shortly after they're vaccinated, if they are exposed to the flu during this time period.

Some people may also mistakenly attribute symptoms of a cold to the vaccine, Schaffner said.

The nasal spray vaccine contains a "live attenuated" flu virus, but the virus is weakened so that it cannot cause the flu. The viruses in the nasal spray can't replicate in the warm temperatures of the lungs and other parts in the body. However, because temperatures in the nose are colder, the virus causes a small infection in the nose. This infection does not cause symptoms in most people, but in some people, it causes symptoms such as runny nose and sore throat, Schaffner said.

This local infection will prompt the body to make antibodies against the flu virus, Schaffner said. "That provides better protection against the real flu, which is of course, is a virus that can make you seriously ill," Schaffner said.


Yes. Studies show flu vaccines are safe for women in any stage of pregnancy, the CDC says. There are several reasons why it's important for pregnant women to get a flu shot, Schaffner said.

"Pregnant women, when they get influenza, have a tendency to get a more severe disease," and are at increased risk for complications and hospitalization from the disease, Schaffner said.

In addition, flu vaccination in pregnancy helps to protect the baby against flu during the first six months of life, when the baby is too young to receive a flu shot, Schaffner said. The mother "passes that protection on to her newborn baby," Schaffner said.


Antibiotics only kill bacteria, but the flu is caused by a virus.

There are antiviral drugs that can fight flu infections, Cunningham said, but they've only been shown to work when they're given within 48 hours of the start of symptoms. "Most people, by the time they go to the doctor, they're past the 48-hour mark," he said.

For patients hospitalized with severe flu, the drugs may help, he said. But they aren't a cure, and for most people who aren't hospitalized, these drugs may only cut down on the duration of the flu by a day or two.


There are two reasons why doctors recommend that people get the flu vaccine every year, Cunningham said.

For one, the strains of the flu virus that are circulating change from year to year. "It's like the common cold — there's more than one type of virus that causes the flu," and, in fact, there are hundreds of flu viruses, he said.

Each year, health officials identify the virus strains that are the most likely to cause illness during the upcoming flu season, according to the CDC.

Second, the immunity you develop after getting the shot wanes by the following year. "If you get your shot in August, you'll be safe through March, but those antibodies won't be for the next flu season," Cunningham said.


Thimerosal — a preservative that contains mercury — has never been shown to be harmful, Cunningham said. The type of mercury linked with nervous system damage is methyl mercury, he said. Concerns over levels of methyl have led to recommendations that pregnant women avoid eating large amounts of certain types of fish, such as swordfish.

In contrast, thimerosal is an ethyl mercury compound.

Still, because the preservative raised controversy, especially over a now-disproven link to autism, it was taken out of almost all U.S. vaccines starting in 2001, Cunningham said.

The injectable form of the flu vaccine is available to health care providers as large, multidose bottles and small vials carrying individual doses. A tiny amount of thimerosal is added to the multidose bottles to ensure that no bacteria will grow in the vaccine, Cunningham said. The individual-dose bottles contain no thimerosal. 

The nasal spray form of the flu vaccine also contains no thimerosal, he noted.


"The flu is certainly a very serious disease," Cunningham said.

Every year, between 15 million and 60 million cases of the flu are reported in the U.S., Cunningham said. More than 200,000 people with the flu are admitted to hospitals yearly. And between 3,000 and 50,000 people in the U.S. die of the flu yearly. During the 2019-2020 flu season, early estimates by the CDC suggest 38 million Americans were infected with the flu and 22,000 people died from it.

One reason people may not perceive the flu as being serious is that cases of the "stomach flu" are mistaken for influenza virus infections. "True influenza is an infection of the lungs and respiratory tract," Cunningham said. Infected people may develop a high fever, body aches and nasal congestion, he said.

People with the stomach flu — which is commonly caused by a virus called norovirus — have diarrhea, cramping and other gastrointestinal symptoms. Influenza does not cause such symptoms.

This article is for informational purposes only, and is not meant to offer medical advice.