Flu (Influenza)

Commonly Asked Questions

How do flu vaccines work? 

• Flu vaccines cause antibodies to develop in the body about two weeks after vaccination. These antibodies provide protection against infection with the viruses that are used to make the vaccine. 
• The seasonal flu vaccine protects against the influenza viruses that research indicates will be most common during the upcoming season. Most flu vaccines in the United States protect against four different flu viruses (“quadrivalent”); an influenza A (H1N1) virus, an influenza A (H3N2) virus, and two influenza B viruses. There are also some flu vaccines that protect against three different flu viruses (“trivalent”); an influenza A (H1N1) virus, an influenza A (H3N2) virus, and one influenza B virus. Two of the trivalent vaccines are designed specifically for people 65 and older to create a stronger immune response. 

Can a flu vaccine give me flu? 

No, a flu vaccine cannot cause flu illness. Flu vaccines that are administered with a needle (flu shots) are currently made in two ways: the vaccine is made either with a) flu vaccine viruses that have been killed (inactivated) and are therefore not infectious, or b) with proteins from a flu vaccine virus instead of flu vaccine viruses (which is the case for recombinant influenza vaccine). The nasal spray vaccine is made with attenuated (weakened) live flu viruses, and also cannot cause flu illness. The weakened viruses are cold-adapted, which means they are designed to only cause infection at the cooler temperatures found within the nose. The viruses cannot infect the lungs or other areas where warmer temperatures exist. 

When should I get vaccinated? 

You should get a flu vaccine before flu viruses begin spreading in your community since it takes about two weeks after vaccination for antibodies to develop in the body and provide protection against flu. Make plans to get vaccinated early in the fall, before flu season begins. CDC recommends that people get a flu vaccine by the end of October. However, getting vaccinated early (for example, in July or August) is likely to be associated with reduced protection against flu infection later in the flu season, particularly among older adults. Vaccination should continue to be offered throughout the flu season, even into January or later. Children who need two doses of vaccine to be protected should start the vaccination process sooner of vaccine to be protected should start the vaccination process sooner because the two doses must be given at least four weeks apart. 

Why do I need a flu vaccine every year? 

A flu vaccine is needed every season for two reasons. First, a person’s immune protection from vaccination declines over time, so an annual vaccine is needed for optimal protection. Second, because flu viruses are constantly changing, flu vaccines may be updated from one season to the next to protect against the viruses that research suggests may be most common during the upcoming flu season. For the best protection, everyone 6 months and older should get vaccinated annually. 

Does the flu vaccine work right away?

No. It takes about two weeks after vaccination for antibodies to develop in the body and provide protection against influenza virus infection. That’s why it’s best to get vaccinated before influenza viruses start to spread in your community. 

Should I get the flu vaccine if I'm pregnant or breastfeeding?

Years of studies and observation show that you can safely get a flu shot at any time, during any trimester, while you are pregnant or breastfeeding. Babies cannot get the vaccine until six months old. Because antibodies from the vaccine pass onto a fetus in the womb and through breast milk, you protect your baby even more by getting vaccinated.  

Pregnant people should not get the nasal spray form of the flu vaccine. Those with a life-threatening egg allergy should not get the flu vaccine, whether pregnant or not.

Can I get seasonal flu even though I got a flu vaccine this year? 

Yes. It’s possible to get sick with flu even if you have been vaccinated (although you won’t know for sure unless you get a flu test). This is possible for the following reasons: 

• You may be exposed to a flu virus shortly before getting vaccinated or during the period that it takes the body to gain protection after getting vaccinated. This exposure may result in you becoming ill with flu before the vaccine begins to protect you. (Antibodies that provide protection develop in the body about 2 weeks after vaccination.) 
• You may be exposed to a flu virus that is not included in the seasonal flu vaccine. There are many different flu viruses that circulate every year. A flu vaccine is made to protect against the three or four flu viruses that research suggests will be most common. 
• Unfortunately, some people can become infected with a flu virus a flu vaccine is designed to protect against, despite getting vaccinated. The protection provided by the flu vaccination can vary widely, based in part on the health and age factors of the person getting vaccinated. In general, a flu vaccine works best among healthy younger adults and older children. Some older people and people with certain chronic illnesses may develop less immunity after vaccination. Flu vaccination is not a perfect tool, but it is the best way to protect against flu infection. 

Vaccine Safety

Flu vaccines are very safe. Like any medicine, vaccines can have side effects. Most people have no side effects from the flu vaccine. While a flu vaccine cannot give you flu illness, there are different side effects that may be associated with getting a flu shot or a nasal spray flu vaccine. These side effects are mild and short-lasting, especially when compared to symptoms of a bad case of flu. 
 

What side effects can occur after getting a flu vaccine?

A flu shot: The viruses in a flu shot are killed (inactivated), so you cannot get flu from a flu shot. Some minor side effects that may occur are: 

• Soreness, redness, and/or swelling where the shot was given 
• Headache (low grade) 
• Fever 
• Muscle aches 
• Nausea 
• Fatigue 

The nasal spray: The viruses in the nasal spray vaccine are weakened and do not cause severe symptoms often associated with influenza illness. In children, side effects from the nasal spray may include: 

• Runny nose 
• Wheezing 
• Headache 
• Vomiting 
• Muscle aches 
• Fever (low grade) 
• In adults, side effects from the nasal spray vaccine may include: 
• Runny nose 
• Headache 
• Sore throat 
• Cough 

Flu and COVID-19

Flu and COVID-19 are both respiratory illnesses that spread among people. But flu and COVID-19 are caused by different viruses. Flu is caused by influenza viruses, but COVID-19 is caused by a new virus (called SARS-CoV-2). Flu and COVID-19 have similar symptoms so it may be hard to tell the difference. Your doctor will test whether you have flu or COVID-19. It is possible to have flu and COVID-19 at the same time. 

The side effects of vaccinations are believed to be generally similar if the Flu and COVID-19 vaccines are administered at the same time or separately. 

Will the flu vaccine prevent COVID? 

The flu vaccine won’t prevent COVID, but it can lower your risk of getting severely ill from COVID by preventing severe flu illness. People can get both COVID and flu at the same time or one after the other, and these individuals are at more risk of severe illness. 

Does a flu vaccine increase your risk of getting COVID-19? 

There is no evidence that getting a flu vaccine increases your risk of getting sick from a coronavirus, like the one that causes COVID-19. 

You may have heard about a study published in January 2020 that reported an association between flu vaccination and the risk of four commonly circulating seasonal coronaviruses, but not the one that causes COVID-19. This report was later found to be incorrect. 

The results from that initial study led researchers in Canada to look at their data to see if they could find similar results in their population. The results from Canada’s study showed that flu vaccination did not increase the risk for these seasonal coronaviruses. The Canadian findings highlighted the protective benefits of flu vaccination. 

The Canadian researchers also identified a flaw in the methods of the first study, noting that it violated the part of the study design that compares vaccination rates among patients with and without flu (test-negative design). This flaw led to the incorrect association between flu vaccination and seasonal coronavirus risk. When these researchers reexamined data from the first study using the correct methods, they found that flu vaccination did not increase the risk for infection with other respiratory viruses, including seasonal coronaviruses. 

If I am at high risk for serious illness from COVID-19 or flu, where is the safest place for me to get a flu vaccine? 

You can safely get a flu vaccine at multiple locations including your doctor’s office, health departments, and pharmacies. You can use VaccineFinder.org to find where flu vaccines are available near you. When going to get a flu vaccine, be sure to practice everyday preventive actions. 

Will there be flu along with COVID-19 in the fall and winter?

While it’s not possible to say with certainty what will happen in the fall and winter, CDC believes it’s likely that flu viruses and the virus that causes COVID-19 will both be spreading. In this context, getting a flu vaccine will be more important than ever. CDC recommends that all people 6 months and older get a yearly flu vaccine. 

If COVID-19 is spreading in my community, should I still go out to get a flu vaccine?

Yes. Getting a flu vaccine is essential to protect your health and your family’s health this season. When getting a flu vaccine, follow CDC guidelines for running essential errands and doctor visits. Continue to wear your mask, keep at least six feet away from others, and wash your hands.