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Similarities and Differences between Flu and COVID-19​

From the CDC:

What is the difference between Influenza (Flu) and COVID-19?

Influenza (flu) and COVID-19 are both contagious respiratory illnesses, but they are caused by different viruses. COVID-19 is caused by infection with a coronavirus first identified in 2019, and flu is caused by infection with influenza viruses.

COVID-19 seems to spread more easily than flu. However, as more people become fully vaccinated against COVID-19, the spread of the virus that causes COVID-19 should slow down. More information is available about COVID-19 vaccines and how well they work.

Compared to flu, COVID-19 can cause more serious illnesses in some people. COVID-19 can also take longer before people show symptoms and people can be contagious for longer. More information about differences between flu and COVID-19 is available in the different sections below.

Because some of the symptoms of flu, COVID-19, and other respiratory illnesses are similar, the difference between them cannot be made based on symptoms alone. Testing is needed to tell what the illness is and to confirm a diagnosis. People can be infected with both flu and the virus that causes COVID-19 at the same time and have symptoms of both influenza and COVID-19.

While more is learned every day about COVID-19 and the virus that causes it, there are still things, such as post-COVID conditions, that are unknown. This page compares COVID-19 and flu, given the best available information to date.

Signs and Symptoms

Similarities:

Both COVID-19 and flu can have varying degrees of signs and symptoms, ranging from no symptoms (asymptomatic) to severe symptoms. Common symptoms that COVID-19 and flu share include:

• Fever or feeling feverish/having chills
• Cough
• Shortness of breath or difficulty breathing
• Fatigue (tiredness)
• Sore throat
• Runny or stuffy nose
• Muscle pain or body aches
• Headache
• Vomiting and diarrhea
• Change in or loss of taste or smell, although this is more frequent with COVID-19.

Flu Symptoms

COVID-19 Symptoms

How Long Symptoms Appear After Exposure and Infection

Similarities:

For both COVID-19 and flu, 1 or more days can pass between when a person becomes infected and when he or she starts to experience illness symptoms.

Differences:

If a person has COVID-19, it could take them longer to experience symptoms than if they had flu.

Flu

Typically, a person experiences symptoms anywhere from 1 to 4 days after infection.

Flu Symptoms

COVID-19

Typically, a person experiences symptoms about 5 days after being infected, but symptoms can appear 2 to 14 days after infection.

COVID-19 Symptoms

How Long Someone Can Spread the Virus

Similarities:

For both COVID-19 and flu, it’s possible to spread the virus for at least 1 day before experiencing any symptoms.

Differences:

If a person has COVID-19, they could be contagious for a longer time than if they had flu.

Flu

Most people with flu are contagious for about 1 day before they show symptoms.

Older children and adults with flu appear to be most contagious during the initial 3-4 days of their illness but many people remain contagious for about 7 days.

Infants and people with weakened immune systems can be contagious for even longer.

How Flu Spreads

COVID-19

How long someone can spread the virus that causes COVID-19 is still under investigation.

It’s possible for people to spread the virus for about 2 days before experiencing signs or symptoms (or possibly earlier) and remain contagious for at least 10 days after signs or symptoms first appeared. If someone is asymptomatic or their symptoms go away, it’s possible to remain contagious for at least 10 days after testing positive for COVID-19. People who are hospitalized with severe disease and people with weakened immune systems can be contagious for 20 days or longer.

How COVID-19 Spreads

How it Spreads

Similarities:

Both COVID-19 and flu can spread from person-to-person between people who are in close contact with one another (within about 6 feet). Both are spread mainly by large and small particles containing virus that are expelled when people with the illness (COVID-19 or flu) cough, sneeze, or talk. These particles can land in the mouths or noses of people who are nearby and possibly be inhaled into the lungs. In some circumstances, such as indoor settings with poor ventilation, small particles might be spread further than 6 feet and cause infections.

Although most spread is by inhalation, it may be possible that a person can get infected by touching (for example, shaking hands with someone who has the virus on their hands) or by touching a surface or object that has virus on it, and then touching their own mouth, nose, or eyes.

Both flu viruses and the virus that causes COVID-19 can be spread to others by people before they begin showing symptoms; by people with very mild symptoms; and by people who never experience symptoms (asymptomatic people).

Differences:

While the virus that causes COVID-19 and flu viruses are thought to spread in similar ways, the virus that causes COVID-19 is generally more contagious than flu viruses. Also, COVID-19 has been observed to have more superspreading events than flu. This means the virus that causes COVID-19 can quickly and easily spread to a lot of people and result in continual spreading among people as time progresses.

How Flu Spreads         How COVID-19 Spreads

People at Higher-Risk for Severe Illness

Similarities:

Both COVID-19 and flu illness can result in severe illness and complications. Those at highest risk include:

• Older adults
• People with certain underlying medical conditions (including infants and children)
• Pregnant people

Differences:

Overall, COVID-19 seems to cause more serious illnesses in some people.

Serious COVID-19 illness resulting in hospitalization and death can occur even in healthy people.

Some people that had COVID-19 can go on to develop post-COVD conditions or multisystem inflammatory syndrome (MIS)

People at Increased Risk of COVID-19 Severe Illness

Complications

Similarities:

Both COVID-19 and flu can result in complications, including:

• Pneumonia
• Respiratory failure
• Acute respiratory distress syndrome (fluid in the lungs)
• Sepsis (a life-threatening illness caused by the body’s extreme response to an infection)
• Cardiac injury (for example, heart attacks and stroke)
• Multiple-organ failure (respiratory failure, kidney failure, shock)
• Worsening of chronic medical conditions (involving the lungs, heart, or nervous system or diabetes)
• Inflammation of the heart, brain, or muscle tissues
• Secondary infections (bacterial or fungal infections that can occur in people who have already been infected with flu or COVID-19)

Differences:

Flu

Most people who get flu will recover on their own in a few days to two weeks, but some people will experience severe complications, requiring hospitalization. Some of these complications are listed above. Secondary bacterial infections are more common with influenza than with COVID-19.

Diarrhea is more common in young children with flu than in adults with flu.

Flu complications

COVID-19

Additional complications associated with COVID-19 can include:

• Blood clots in the veins and arteries of the lungs, heart, legs or brain
• Multisystem Inflammatory Syndrome in Children (MIS-C) and in Adults (MIS-A)

Long COVID is a range of symptoms that can last weeks or months after first being infected with the virus that causes COVID-19 or can appear weeks after infection. Long COVID can happen to anyone who has had COVID-19, even if their illness was mild, or if they had no symptoms.

Approved Treatments

Similarities:

People at higher risk of complications or who have been hospitalized for COVID-19 or flu should receive supportive medical care to help relieve symptoms and complications.

Differences:

Flu

Prescription influenza antiviral drugs are FDA-approved to treat flu.

People who are hospitalized with flu or who are at increased risk of complications and have flu symptoms are recommended to be treated with antiviral drugs as soon as possible after illness onset.

Flu Treatment

COVID-19

The National Institutes of Health (NIH) has developed guidance on treatment of COVID-19external icon, which is regularly updated as new evidence on treatment options emerges.

The Food and Drug Administration (FDA) has approved one drug, remdesivir (Veklury®), to treat COVID-19. FDA has issued emergency use authorizationsexternal icon (EUAs) to allow healthcare providers to use investigational products that are not yet approved, or that are approved for other uses, to treat patients with COVID-19 if certain legal requirements are metexternal icon.

What to Do If You Are Sick with COVID-19

Vaccine

Similarities:

Vaccines for COVID-19 and flu are approved and/or authorized for emergency use (EUA) by FDA.

Differences:

Flu

There are multiple FDA-licensed influenza vaccines produced annually to protect against the 4 flu viruses that scientists expect will circulate each year.

Flu Vaccines

COVID-19

Three COVID-19 vaccines have been authorized for use by FDA under an EUA. Other vaccines to prevent COVID-19 are under development.

COVID-19 Vaccines

Key Facts About Seasonal Flu Vaccine

From the CDC:

Flu Vaccination

Why should people get vaccinated against flu?

Influenza is a potentially serious disease that can lead to hospitalization and sometimes even death. Every flu season is different, and influenza infection can affect people differently, but millions of people get flu every year, hundreds of thousands of people are hospitalized and thousands to tens of thousands of people die from flu-related causes every year. An annual seasonal flu vaccine is the best way to help protect against flu. Vaccination has been shown to have many benefits including reducing the risk of flu illnesses, hospitalizations and even the risk of flu-related death in children.

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.

Who Should Not Be Vaccinated?

Different influenza (flu) vaccines are approved for use in different age groups. In addition, some vaccines are not recommended for certain groups of people. Factors that can determine a person’s suitability for vaccination, or vaccination with a particular vaccine, include a person’s age, health (current and past) and any allergies to flu vaccine or its components. For more information, visit Who Should and Who Should NOT get a Flu Vaccine.

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 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, because the two doses must be given at least four weeks apart.

Where can I get a flu vaccine?

Flu vaccines are offered in many doctor’s offices, clinics, health departments, pharmacies and college health centers, as well as by many employers, and even in some schools.

Even if you don’t have a regular doctor or nurse, you can get a flu vaccine somewhere else, like a health department, pharmacy, urgent care clinic, and often your school, college health center, or workplace.

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 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.

Flu and COVID-19

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 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 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 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 correct methods, they found that flu vaccination did not increase risk for infection with other respiratory viruses, including seasonal coronaviruses.

Vaccine Effectiveness

Influenza (flu) vaccine effectiveness (VE) can vary. The protection provided by a flu vaccine varies from season to season and depends in part on the age and health status of the person getting the vaccine and the similarity or “match” between the viruses in the vaccine and those in circulation. During years when the flu vaccine match is good, the benefits of flu vaccination will vary, depending on factors like the characteristics of the person being vaccinated (for example, their health and age), what influenza viruses are circulating that season and, potentially, which type of flu vaccine was used. For more information, see Vaccine Effectiveness – How well does the Flu Vaccine Work. For information specific to this season, visit About the Current Flu Season.

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. Protection provided by flu vaccination can vary widely, based in part on 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.

What protection does a flu vaccine provide if I do get sick with flu?

Some people who get vaccinated may still get sick. However, flu vaccination has been shown in some studies to reduce severity of illness in people who get vaccinated but still get sick. A 2017 study showed that flu vaccination reduced deaths, intensive care unit (ICU) admissions, ICU length of stay, and overall duration of hospitalization among hospitalized adults with flu. Another study in 2018 showed that a vaccinated adult who was hospitalized with flu was 59 percent less likely to be admitted to the ICU than someone who had not been vaccinated. Among adults in the ICU with flu, vaccinated patients on average spent 4 fewer days in the hospital than those who were not vaccinated.

Vaccine Benefits

What are the benefits of flu vaccination?

There are many reasons to get an influenza (flu) vaccine each year. Because of the on-going COVID-19 pandemic, getting a flu vaccine during 2020-2021 will be more important than ever. Flu vaccines will not prevent COVID-19, but they will reduce the burden of flu illnesses, hospitalizations and deaths on the health care system and conserve scarce medical resources for the care of people with COVID-19.

Below is a summary of the benefits of flu vaccination and selected scientific studies that support these benefits.

• Flu vaccination can keep you from getting sick with flu.

• Flu vaccine prevents millions of illnesses and flu-related doctor’s visits each year. For example, during 2019-2020, flu vaccination prevented an estimated 7.5 million influenza illnesses, 3.7 million influenza-associated medical visits, 105,000 influenza-associated hospitalizations, and 6,300 influenza-associated deaths.
• During seasons when the flu vaccine viruses are similar to circulating flu viruses, flu vaccine has been shown to reduce the risk of having to go to the doctor with flu by 40 percent to 60 percent.
• Flu vaccination can reduce the risk of flu-associated hospitalization for children, working age adults, and older adults.

• Flu vaccine prevents tens of thousands of hospitalizations each year. For example, during 2019-2020 flu vaccination prevented an estimated 105,000 flu-related hospitalizations.
• A 2014 study showed that flu vaccine reduced children’s risk of flu-related pediatric intensive care unit (PICU) admission by 74% during flu seasons from 2010-2012.
• In recent years, flu vaccines have reduced the risk of flu-associated hospitalizations among older adults on average by about 40%.

• A 2018 study showed that from 2012 to 2015, flu vaccination among adults reduced the risk of being admitted to an intensive care unit (ICU) with flu by 82 percent.
• Flu vaccination is an important preventive tool for people with chronic health conditions.

• Flu vaccination has been associated with lower rates of some cardiac events among people with heart disease, especially among those who had had a cardiac event in the past year.
• Flu vaccination can reduce worsening and hospitalization for flu-related chronic lung disease, such as in persons with chronic obstructive pulmonary disease (COPD).

• Flu vaccination also has been shown in separate studies to be associated with reduced hospitalizations among people with diabetes and chronic lung disease.
• Many people at higher risk from flu also seem to be at higher risk from COVID-19.
• Flu vaccination helps protect women during and after pregnancy.

• Vaccination reduces the risk of flu-associated acute respiratory infection in pregnant women by about one-half.
• A 2018 study that included influenza seasons from 2010-2016 showed that getting a flu shot reduced a pregnant woman’s risk of being hospitalized with flu by an average of 40 percent.
• A number of studies have shown that in addition to helping to protect pregnant women, a flu vaccine given during pregnancy helps protect the baby from flu for several months after birth, when he or she is not old enough to be vaccinated.
• Flu vaccine can be lifesaving in children.

• A 2017 study was the first of its kind to show that flu vaccination can significantly reduce a child’s risk of dying from flu.
• Flu vaccination has been shown in several studies to reduce severity of illness in people who get vaccinated but still get sick.

• A 2017 study showed that flu vaccination reduced deaths, intensive care unit (ICU) admissions, ICU length of stay, and overall duration of hospitalization among hospitalized flu patients.
• A 2018 study showed that among adults hospitalized with flu, vaccinated patients were 59 percent less likely to be admitted to the ICU than those who had not been vaccinated. Among adults in the ICU with flu, vaccinated patients on average spent 4 fewer days in the hospital than those who were not vaccinated.
• *References for the studies listed above can be found at Publications on Influenza Vaccine Benefits. Also, see the A Strong Defense Against Flu: Get Vaccinated! fact sheet.

Vaccine Side Effects (What to Expect)

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). 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.

What side effects can occur after getting a 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 bad case of flu.

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

If these problems occur, they begin soon after vaccination and usually are mild and short-lived. A flu shot, like other injections, can occasionally cause fainting. Tell your provider if you feel dizzy or have vision changes or ringing in the ears. As with any medicine, there is a very remote chance of a vaccine causing a severe allergic reaction, other serious injury, or death. People who think that they have been injured by a flu vaccine can file a claim for compensation from the National Vaccine Injury Compensation Program (VICP).

Where can I find information about vaccine supply?

Information about vaccine supply is available on CDC’s Vaccine Supply & Distribution.Edit”Key Facts About Seasonal Flu Vaccine”

Influenza Vaccine Safety Questions & Answers

Are flu vaccines safe?

Flu vaccines have a good safety record. Hundreds of millions of Americans have safely received flu vaccines over the past 50 years, and there has been extensive research supporting the safety of flu vaccines.

A flu vaccine is the first and best way to reduce your chances of getting the flu and spreading it to others. CDC recommends that everyone 6 months of age and older receive a flu vaccine every year.

Can I get the flu from the flu vaccine?

No, the flu vaccine cannot cause flu. The vaccines either contain inactivated virus, meaning the viruses are no longer infectious, or a particle designed to look like a flu virus to your immune system. While the nasal spray flu vaccine does contain a live virus, the viruses are changed so that they cannot give you the flu.

Do flu vaccines cause any side effects?

Like any medical product, vaccines can cause side effects. Side effects of the flu vaccine are generally mild and go away on their own within a few days.

Common side effects from the flu shot include:

• Soreness, redness, and/or swelling from the shot
• Headache
• Fever
• Nausea
• Muscle aches

The flu shot, like other injections, can occasionally cause fainting.

Some studies have found a possible small association of injectable flu vaccine with Guillain-Barré syndrome (GBS). Overall, these studies estimated the risk for GBS after vaccination as fewer than 1 or 2 cases of GBS per one million people vaccinated. Other studies have not found any association. GBS also, rarely, occurs after flu illness. Even though GBS following flu illness is rare, GBS is more common following flu illness than following flu vaccination. GBS has not been associated with the nasal spray vaccine.

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Are there signs or symptoms that should cause concern after getting a flu vaccine?

With any vaccine, look for any unusual conditions, such as a high fever, behavior changes, or signs of a severe allergic reaction after vaccination.

Signs of a severe allergic reaction can include:

• Difficulty breathing
• Hoarseness or wheezing
• Swelling around the eyes or lips
• Hives
• Paleness
• Weakness
• A fast heart beat or dizziness

Life threatening allergic reactions to the flu shot are rare. These signs would most likely happen within a few minutes to a few hours after the vaccine is given.

What should I do if I think I am having a severe reaction to a flu vaccine?

If you think it is a severe allergic reaction or other emergency that can’t wait, call 9-1-1 and get to the nearest hospital. Otherwise, call your doctor.

Afterward, the reaction should be reported to the Vaccine Adverse Event Reporting System (VAERS). Your doctor might file this report, or you can do it yourself through the VAERS websiteexternal icon, or by calling 1-800-822-7967.

What should I do if I think I have been injured by the flu vaccine?

If you believe you have been injured by a flu vaccine you may be eligible to receive compensation from the federal government for your injuries if certain criteria are met. To learn more visit the National Vaccine Injury Compensation Program websiteexternal icon or call 1-800-338-2382.

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Are there some people who should not receive a flu vaccine?

CDC recommends everyone 6 months of age and older should receive an annual flu vaccination with rare exceptions. Individuals who can’t get the flu shot include:

• Children younger than 6 months, since they are too young to get a flu shot.
• Individuals with severe, life-threatening allergies to flu vaccine or any ingredient(s) in the vaccine.

Individuals should talk with their doctor before getting the flu shot if they:

• Have had a severe allergy to eggs or any of the ingredients in the vaccine.
  • See Special Considerations Regarding Egg Allergy for more information about egg allergies and flu vaccine.
• Have had Guillain-Barré syndrome (GBS).
• Are not feeling well.

There are multiple flu vaccines available, and not all flu vaccines can be given to people of all ages. Talk to your doctor if you have any questions regarding which flu vaccine options are best for you and your family. See Vaccination: Who Should Do It, Who Should Not and Who Should Take Precautions) for more information.

Should pregnant women receive a flu vaccine?

Yes, pregnant women should get a flu shot to protect themselves and their developing babies. To learn more about flu vaccine safety during pregnancy, visit Flu Vaccine Safety and Pregnancy.

How is the safety of flu vaccines monitored?

CDC and the Food and Drug Administration (FDA) closely monitor the safety of vaccines approved for use in the United States. CDC uses two primary systems to monitor the safety of flu vaccines:

1. Vaccine Adverse Event Reporting System (VAERS): an early warning system that helps CDC and FDA monitor problems following vaccination. Anyone can report possible vaccine side effects to VAERS. Generally, VAERS reports cannot determine if an adverse event was caused by a vaccine, but these reports can help determine if further investigations are needed.
2. Vaccine Safety Datalink (VSD): A collaboration between CDC and nine health care organizations which allows ongoing monitoring and proactive searches of vaccine-related data.

People with egg allergies can receive any licensed, recommended age-appropriate influenza vaccine (IIV4, RIV4, or LAIV4) that is otherwise appropriate. People who have a history of severe egg allergy (those who have had any symptom other than hives after exposure to egg) should be vaccinated in a medical setting, supervised by a health care provider who is able to recognize and manage severe allergic reactions. Two completely egg-free (ovalbumin-free) flu vaccine options are available: quadrivalent recombinant vaccine and quadrivalent cell-based vaccine.