Questions & Answers

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When did seasonal influenza vaccine first become available?
The first seasonal 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 virus 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. Seasonal influenza vaccine typically contains both type A and type B viruses. The viruses selected for the vaccine are grown in chicken eggs.

For the inactivated (injectable) vaccine, the viruses are killed with formaldehyde, purified, and packaged in vials or syringes. Live virus vaccine is packaged in a special nasal sprayer. About six months are required to produce seasonal 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 seasonal influenza vaccine?
Many groups of people can benefit from being protected from seasonal influenza.

Annual vaccination against seasonal influenza is recommended for the following groups:

1. All persons age 6 months or older wanting to reduce the likelihood of becoming ill with influenza or of transmitting it to others
2. All adults age 50 years or older
3. Children and teens age 6 months through 18 years
4. Residents of long-term care facilities, nursing homes, and other chronic-care facilities
5. Adults and children who have chronic pulmonary (including asthma), cardiovascular (except hypertension), renal, hepatic, cognitive, neurologic/neuromuscular, hematologic, or metabolic (e.g., diabetes) disorders
6. 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
7. Children and adolescents age 6 months through 18 years on long-term aspirin treatment (who could develop Reye's syndrome if they catch influenza)
8. Women who will be pregnant during the influenza season
9. Healthcare personnel
10. All adults, children, and teens who are household contacts, caregivers, or workplace contacts of persons listed in categories e-g above, of children age 0-59 months, or of adults age 50 years and older

The live virus (nasal spray) influenza vaccine can be given only to healthy, non-pregnant people ages 2 through 49 years. It should not be given to healthy children younger than age 5 years who have recurrent wheezing or have had a wheezing episode within the past 12 months. Children younger than age two years, persons age 50 and older, and anyone with a chronic medical condition (listed in e-g above) should receive inactivated influenza vaccine (injectable), NOT live (nasal spray) influenza vaccine.

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

Who recommends the seasonal 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?
Seasonal influenza vaccine is given each year because immunity decreases after a year and because each year's vaccine is formulated to prevent only that year's anticipated seasonal influenza viruses.

When should persons be vaccinated?
Health experts recommend that patients may be vaccinated as soon as vaccine is available in their clinic, which can be as early as August or September. Vaccination should continue into the winter and spring, even until April or May. Travelers should be aware that the seasonal 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 seasonal 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 seasonal 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 seasonal influenza vaccination. Either inactivated or live virus vaccine may be used; it is preferred that the inactivated vaccine be used for household contacts and caregivers of people with severe immunosuppression that must be in protective isolation.

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 vaccinated against seasonal influenza (i.e., younger than age 6 months) should receive annual seasonal influenza vaccination to protect the infant 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. Less than 1% of vaccine recipients develop symptoms such as fever, chills, and muscle aches for 1 to 2 days following the vaccination. 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.

Healthy children ages 2 through 4 years who received the live attenuated virus (nasal spray) vaccine during clinical trials appeared to have an increased chance of wheezing. Consequently, children with a history of recurrent wheezing or have had a wheezing episode within the past 12 months are not recommended to receive the live nasal spray vaccine; instead, they should be given the inactivated (injectable) vaccine.

Healthy adults receiving the live seasonal influenza vaccine reported symptoms such as cough, runny nose, and sore throat at a rate 3%-10% higher than for placebo recipients. There was no increase in the occurrence of fever.

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. In 1976, the swine flu (injectable) vaccine was associated with a severe illness called Guillain-Barré syndrome (GBS), a nerve condition that can result in temporary paralysis. Injectable seasonal 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, as a precaution, any person without a high risk medical condition who previously experienced GBS within 6 weeks of an influenza vaccination should generally not be vaccinated. Instead, their physician may consider using antiviral drugs during the time of potential exposure to influenza.

What can you tell me about the preservative thimerosal that is in some injectable influenza vaccines 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 seasonal influenza vaccine?
Protection from seasonal 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 seasonal influenza vaccination than are older, frail individuals. It is important to understand that although the vaccine is not as effective in preventing seasonal influenza disease among the elderly, it is effective in preventing complications and death. In general, the immunity following seasonal influenza vaccination rarely lasts longer than a year.

When the "match" between vaccine and circulating strains is close, the injectable (inactivated) vaccine prevents seasonal 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 seasonal influenza shot is 30%-70% effective in preventing hospitalization for pneumonia and seasonal influenza. Among elderly nursing home residents, the shot is most effective in preventing severe illness, secondary complications, and deaths related to seasonal influenza. In this population, the shot can be 50%-60% effective in preventing hospitalization or pneumonia and 80% effective in preventing death from seasonal influenza.

In one large study among children ages 15-85 months, the live, attenuated (nasal-spray) seasonal influenza vaccine reduced the chance of seasonal influenza illness by 92% compared with the placebo. In a study among adults, the participants were not specifically tested for seasonal 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! Neither the injectable (inactivated) vaccine nor the live attenuated (nasal spray) 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 nasal spray influenza vaccine contains live attenuated (weakened) 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. It is always possible that a recently vaccinated person can be exposed to seasonal influenza disease before their antibodies are formed and consequently develop disease. This can result in someone erroneously believing they developed the disease from 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 seasonal influenza vaccination 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 (nasal spray) influenza vaccine is licensed for use only in healthy, nonpregnant individuals ages 2 through 49 years.

The following persons should not be vaccinated with the live, attenuated virus (nasal spray) influenza vaccine; however, most (except infants younger than 6 months) can be vaccinated with the injectable vaccine:

• Persons younger than age two years
• Persons age 50 years or older
• Persons with chronic pulmonary (including asthma) or cardiovascular (excluding hypertension) diseases; persons with renal, hepatic, cognitive, neurologic/neuromuscular, hematologic, or metabolic (e.g., diabetes) disorders; or persons with immunosuppression, including that caused by medications or HIV
• Children ages 2 through 4 years with a history of recurrent wheezing or who have had a wheezing episode in the last 12 months
• Children or adolescents receiving long-term aspirin therapy
• Pregnant adolescents or 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 protective isolation should preferably receive the injectable vaccine over the live (nasal spray) vaccine. Persons having had serious allergic reaction to eggs or to a previous dose of influenza vaccine should not receive either type of influenza vaccine (injectable or nasal spray). 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 vaccination 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 not a valid reason to avoid getting influenza vaccine. Only serious, life-threatening allergies to thimerosal are reasons not to be vaccinated with an influenza vaccine containing thimerosal. 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 2009