Questions & Answers

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When did influenza vaccine first become available?
The first influenza vaccine in the United States became available in 1945.

What kind of vaccine is it?
There are two types of influenza vaccine. The most common influenza vaccine is made from inactivated (killed) viruses. In June 2003, a live influenza vaccine was licensed. It contains live viruses that have been weakened (attenuated).

How are the vaccines made?
Every year, researchers and manufacturers develop a vaccine that contains virus strains they believe will be circulating in the upcoming influenza season. Influenza vaccine contains three viruses—two type A and one type B. The viruses selected for the vaccine are grown in chicken eggs.

For inactivated vaccine, the viruses are killed with formaldehyde, purified, and packaged in vials or syringes. The live vaccine is packaged in a special sprayer. About six months are required to produce influenza vaccine each year.

How is the vaccine given?
The inactivated vaccine is given as an intramuscular injection. The live attenuated vaccine is sprayed into the nose.

Who should get influenza vaccine?
Many groups of people can benefit from being protected from influenza.

Annual vaccination with inactivated vaccine is recommended for the following groups:

• All persons, including school-age children, who want to reduce the risk of becoming ill
  with influenza or of transmitting influenza to others
• Everyone age 50 years or older
• All children age 6-59 months
• Residents of long-term care facilities, nursing homes, and other chronic-care facilities
• Adults and children who have chronic pulmonary (including asthma), cardiovascular (except hypertension), renal, hepatic, hematological or metabolic disorders (including diabetes mellitus)
• Anyone who has a condition (e.g., spinal cord injury or seizure disorder) that can affect their ability to cough out their respiratory secretions or that can increase the risk for aspiration
• Anyone whose immune system is weakened because of the following: HIV/AIDS or other diseases that affect the immune system, long-term treatment with drugs such as steroids, or cancer treatment with x-rays or drugs
• Children and adolescents age 6 months-18 years on long-term aspirin treatment (who could develop Reye's syndrome if they catch influenza)
• Women who will be pregnant during the influenza season
• Healthcare personnel
• Healthy household contacts (including children) and caregivers of children younger than age 5 years and/or adults age 50 years and older
• Healthy household contacts (including children) and caregivers of persons with medical conditions that put them at higher risk for severe complications from influenza

The live nasal spray vaccine may only be used in healthy, nonpregnant persons age 2 through 49 years. Children younger than age two years, persons age 50 and older, and anyone with a chronic medical condition (listed above) should receive inactivated influenza vaccine (injectable), NOT live influenza vaccine.

What are the unique features of giving influenza vaccine to children compared with adults?
Children age 6 months through 8 years should receive two doses of influenza vaccine the first time they receive this vaccine, separated by at least 4 weeks. If a child age 6 months through 8 years only received one dose in their first year of vaccination, he/she should receive two doses the subsequent vaccination season.

Who recommends the influenza vaccine?
The Centers for Disease Control and Prevention (CDC), the American Academy of Pediatrics (AAP), the American Academy of Family Physicians (AAFP), the American College of Physicians (ACP), and the American College of Obstetricians and Gynecologists (ACOG) all recommend this vaccine.

How often should this vaccine be given?
Influenza vaccine is given once a year because immunity decreases after a year and because each year's vaccine is formulated to prevent only that year's anticipated influenza viruses.

When should persons be vaccinated?
The time to get influenza vaccine is whenever influenza vaccine becomes available for use. Vaccination should continue into the winter and spring, even until April or May. Travelers should be aware that the influenza season typically occurs from April to September in the Southern Hemisphere and throughout the year in the tropics. If they missed vaccination in the previous season, they should still be vaccinated before they travel, even if it’s in the following spring or summer.

Are there recommendations for the prevention of influenza outbreaks in institutions?
The most important factor in preventing outbreaks is annual vaccination of all occupants of the facility and all persons working or volunteering in the facility who share the same air as the high-risk occupants. Groups that should be targeted include physicians, nurses, and all other personnel in hospitals, long-term care facilities, other care facilities, and outpatient settings who have contact with high-risk patients in all age groups.

Should siblings of a person with a chronic illness receive influenza vaccine even though the chronically ill person has been vaccinated?
Yes. All household contacts (who are age six months or older) of persons with "high-risk" conditions, of people age 50 years and older, or of children from birth through age 59 months, should receive annual influenza vaccination. Either inactivated or live vaccine may be used, except for household contacts and caregivers of people with severe immunosuppression in the care of a protective environment, who should receive only inactivated vaccine.

Should siblings of a healthy child who is younger than age 6 months be vaccinated?
Yes, all household contacts of children too young to be protected against influenza with vaccination should receive annual influenza vaccination to protect the younger child from serious infection. This is very important because these infants are too young to be vaccinated and are most vulnerable to complications from influenza.

Is it safe for pregnant women to get influenza vaccine?
Yes. In fact, vaccination with the inactivated vaccine is recommended for women who will be pregnant during the influenza season. Pregnant women are at increased risk for serious medical complications from influenza. One recent study found that the risk of influenza-related hospitalization was four times higher in healthy pregnant women in the fourteenth week of pregnancy or later than in nonpregnant women. In addition, vaccination of the mother will provide some protection for her newborn infant.

The live intranasal vaccine is not licensed for use in pregnant women. However, pregnant women do not need to avoid contact with persons recently vaccinated with this vaccine.

Vaccination is recommended for all persons, including breastfeeding mothers, who are contacts of infants or children from birth through age 59 months because infants and young children are at higher risk for influenza complications and are more likely to require medical care or hospitalization if infected. Women who are breastfeeding may receive either type of influenza vaccine unless the vaccine is not appropriate because of other medical conditions.

How safe is this vaccine?
Influenza vaccine is very safe. The most common side effects of the injectable (inactivated) influenza vaccine include soreness, redness, or swelling at the site of the injection. These reactions are temporary and occur in 15%–20% of recipients. Fewer than 1% of vaccine recipients develop such symptoms as fever, chills, and muscle aches. These symptoms are more likely to occur in a person who has never been exposed to the influenza virus or vaccine. Experiencing these non-specific side effects does not mean that you are getting influenza. These symptoms can persist for 1 to 2 days.

In clinical trials, the most common side effects of the intranasal influenza vaccine among adults were runny nose or nasal congestion (28%–78%), headache (16%–44%), and sore throat (15%–25%). Among children, side effects included runny nose or nasal congestion (20%–75%), headache (2%–46%), and fever (0%–26%).

Serious adverse reactions to either vaccine are very rare. Such reactions are most likely the result of an allergy to a vaccine component, such as the egg protein left in the vaccine after growing the virus. A vaccine, like any medicine, is capable of causing serious allergic reactions.

The risk of an influenza vaccine causing serious harm, or even death, is very rare. In 1976, the swine flu (injectable) vaccine was associated with an illness called Guillain-Barré  syndrome (GBS), a nerve condition that can result in temporary paralysis. Injectable influenza vaccines since then have not been clearly linked with GBS, because the disease is so rare it is difficult to obtain a precise estimate of any increase in risk. However, if there is a risk of GBS from current influenza vaccines, it is estimated at one or two cases per million persons vaccinated—much less than the risk of severe influenza, which can be prevented by vaccination.

What can you tell me about the preservative thimerosal that is in injectable influenza vaccine and the claim that it might be associated with the development of autism?
Thimerosal is a very effective preservative that has been used to prevent bacterial contamination in vaccines for more than 50 years. It is comprised of a type of mercury known as ethylmercury. It is different from methylmercury, which is the form that is in fish and seafood. At very high levels, methylmercury can be toxic to people, especially to the neurological development of infants.

In recent years, several very large scientific studies have determined that thimerosal in vaccines does not lead to serious neurologic problems, including autism. Nonetheless, because we generally try to reduce people’s exposure to mercury if at all possible, the vaccine manufacturers have voluntarily changed their production methods to produce vaccines that are now free of thimerosal or have only trace amounts. They have done this because it is possible to do, not because there was any evidence that the thimerosal was harmful.

How effective is influenza vaccine?
Protection from influenza vaccine varies by the similarity of the vaccine strain(s) to the circulating strains, and the age and health of the recipient. Healthy persons younger than age 65 years are more likely to have protection from their influenza vaccination than are older, frail individuals. It is important to understand that although the vaccine is not as effective in preventing influenza disease among the elderly, it is effective in preventing complications and death. In general, the immunity following influenza vaccination rarely lasts longer than a year.

When the "match" between vaccine and circulating strains is close, the injectable (inactivated) vaccine prevents influenza in about 70%-90% of healthy persons younger than age 65 years. Among elderly persons living outside chronic-care facilities (such as nursing homes) and those persons with long-term (chronic) medical conditions, the influenza shot is 30%-70% effective in preventing hospitalization for pneumonia and influenza. Among elderly nursing home residents, the shot is most effective in preventing severe illness, secondary complications, and deaths related to influenza. In this population, the shot can be 50%-60% effective in preventing hospitalization or pneumonia and 80% effective in preventing death from influenza.

In one large study among children aged 15-85 months, the nasal-spray influenza vaccine (FluMist®) reduced the chance of influenza illness by 92% compared with placebo. In a study among adults, the participants were not specifically tested for influenza. However, the study found 19% fewer severe febrile respiratory tract illnesses, 24% fewer respiratory tract illnesses with fever, 23-27% fewer days of illness, 13-28% fewer lost work days, 15-41% fewer health care provider visits, and 43-47% less use of antibiotics compared with placebo.

Can the vaccine cause influenza?
No! This common misconception keeps many people from getting the influenza vaccine.

Neither the injectable vaccine nor the intranasal vaccine can cause influenza. The injectable influenza vaccine contains only killed viruses and cannot cause influenza disease. Fewer than 1% of people who are vaccinated develop influenza-like symptoms, such as mild fever and muscle aches, after vaccination. These side effects are not the same as having the actual disease.

The intranasal influenza vaccine does not cause influenza either. The intranasal influenza vaccine contains live attenuated viruses that can produce mild symptoms similar to a cold. While the viruses are able to replicate in the nose and throat tissue and produce protective immunity, they are attenuated and do not replicate effectively in the lung. Consequently, they cannot produce influenza disease.

Protective immunity develops 1 to 2 weeks after vaccination. Some people who get vaccinated later in the season (December or later) may get influenza shortly afterward, but the disease they develop is the result of being exposed to someone with the virus before the vaccine produced immunity, not the result of the vaccination.

Also, to many people "the flu" is any illness with fever and cold symptoms. If they get any viral illness, they may blame it on the influenza shot or think they got "the flu" despite being vaccinated. Influenza vaccine only protects against certain influenza viruses, not all viruses.

Who should NOT receive influenza vaccine?
In general, the inactivated (injectable) influenza vaccine can be given to most everyone except children younger than age 6 months, persons with a history of a serious allergic reaction to eggs or to a previous dose of influenza vaccine (see additional contraindications below). The live, attenuated (intranasal) influenza vaccine is licensed for use only in healthy, nonpregnant individuals age 2 through 49 years.

The following persons should not be vaccinated with live virus intranasal influenza vaccine:

• Persons younger than age two years
• Persons age 50 years or older
• Persons with asthma, reactive airway disease or other chronic disorders of the pulmonary or cardiovascular systems; persons with other underlying medical conditions, including metabolic diseases such as diabetes, renal dysfunction, and hemoglobinopathy; or persons with known or suspected immune deficiency diseases or who are receiving immunosuppressive therapies
• Children ages 2 through 4 years with a history of recurrent wheezing
• Children or adolescents receiving long-term aspirin therapy
• Pregnant women
• Healthcare workers, household members, and others who have close contact with severely immunocompromised individuals during the periods in which the immunosuppressed person requires care in a protective environment

Persons having had serious allergic reaction to eggs or to a previous dose of influenza vaccine should not receive either type of influenza vaccine (inactivated or live). Persons with a history of serious egg allergies who are at increased risk for influenza or its complications should consult with their healthcare provider regarding referral to an allergist to determine if the vaccine can be given following treatment for desensitization.

Persons with a history of Guillain-Barré syndrome should also consult with their physician before receiving this vaccine, so that the potential risks and benefits of influenza immunization can be weighed.

Persons who are moderately or severely ill at the time of their influenza vaccine appointment should usually wait until their symptoms are improved before getting the vaccine.

Some people believe they are allergic to thimerosal, the preservative used in some brands of influenza vaccine, because in the past they developed eye irritation after using eye drops containing thimerosal. Past eye irritation is no reason to avoid getting influenza vaccine. Only serious, life-threatening allergies to thimerosal are a reason not to be vaccinated. Most brands of influenza vaccine are packaged in vials or syringes that contain natural rubber or latex. Persons with a severe allergy to latex generally should not receive vaccine packaged in these vials or syringes.

Questions and answers about influenza disease

Reviewed by the Centers for Disease Control and Prevention, October 2007