The Advisory Committee on Immunization Practices (ACIP) of the Centers for Disease Control and Prevention (CDC) has issued recommendations on the prevention and control of influenza for the 2004–2005 influenza season. This report updates the 2003 recommendations by the ACIP for the use of influenza vaccine and antiviral agents.

Epidemics of influenza usually occur during the winter months and are responsible for approximately 36,000 deaths per year in the United States. According to the ACIP, rates of infection are highest among children, but rates of serious illness and death are highest among persons 65 years and older and persons of any age who have medical conditions that place them at increased risk for complications from influenza. Influenza vaccination is the primary method for preventing influenza and its complications. The three primary target groups for annual vaccination are (1) persons who are at increased risk for influenza-related complications; (2) persons 50 to 64 years of age, because they have an elevated prevalence of chronic medical conditions; and (3) persons who live with or care for persons at high risk.

The 2004 recommendations include new or updated information about the influenza vaccine in children six to 23 months of age; vaccination of health care workers with live, attenuated influenza vaccine (LAIV); personnel who may administer LAIV, the 2004–2005 trivalent vaccine virus strains (A/Fujian/411/2002 [H3N2]-like, A/New Caledonia/20/99 [H1N1]-like, and B/Shanghai/361/2002-like); and an assessment of the vaccine supply and timing of influenza vaccination.

The recommendations appear in the May 28, 2004 recommendations and reports series of Morbidity and Mortality Weekly Report, and are available online at http://www.cdc.gov/mmwr/preview/mmwrhtml/rr5306a1.htm.

Recommendations for Using Inactivated and Live, Attenuated Influenza Vaccines

TARGET GROUPS

Groups at high risk for complications from influenza include the following: (1) persons 65 years and older; (2) residents of nursing homes or other chronic-care facilities of any age who have chronic medical conditions; (3) adults and children who have chronic pulmonary or cardiovascular disorders; (4) adults and children who have required regular medical follow-up or hospitalization during the preceding year because of chronic metabolic diseases, renal dysfunction, immunosuppression, or hemoglobinopathies; (5) children and adolescents six months to 18 years of age who are receiving long-term aspirin therapy and, therefore, might be at risk for Reye’s syndrome after influenza infection; (6) women who will be pregnant during the influenza season; and (7) healthy children six to 23 months of age.

SPECIFIC POPULATIONS

According to the ACIP, women who will be pregnant during the influenza season should be vaccinated because of the increased risk for influenza-related complications. The influenza vaccine does not affect the safety of mothers who are breastfeeding or their infants.

The ACIP recommends vaccination of all children six to 23 months of age because of the substantially increased risk for influenza-related hospitalizations in this age group. ACIP continues to recommend influenza vaccination of persons who are at least six months of age who have high-risk medical conditions. The current inactivated influenza vaccine is not approved by the U.S. Food and Drug Administration (FDA) for use in children younger than six months. Vaccinating their household contacts and out-of-home caregivers might decrease the probability of influenza infection among these children.

Persons who have human immunodeficiency virus (HIV) infection, including HIV-infected pregnant women, should be vaccinated because influenza can result in serious illness. Influenza vaccination also can result in the production of protective antibody titers.

Persons at high risk for complications of influenza who were not vaccinated during the preceding fall or winter should consider receiving influenza vaccine before traveling if they plan to travel to the tropics, travel with an organized tour group at any time of year, or travel to the Southern Hemisphere from April through September.

COMPARISON OF LAIV WITH INACTIVATED INFLUENZA VACCINE

Inactivated influenza vaccine and LAIV contain strains of influenza viruses that are antigenically equivalent to the annually recommended strains. Viruses for both vaccines are grown in eggs and administered annually to provide optimal protection against influenza infection. Inactivated influenza vaccine contains killed viruses, whereas LAIV contains attenuated viruses still capable of replication. LAIV is administered intranasally by a sprayer, whereas inactivated influenza vaccine is administered intramuscularly by injection. LAIV is approved for use only among healthy persons five to 49 years of age; inactivated influenza vaccine is approved for use by persons who are at least six months of age, including those who are healthy and those with chronic medical conditions.

Inactivated vaccine is preferred over LAIV for vaccinating health care workers, household members, and others who have close contact with severely immunosuppressed persons during periods when such persons require care in a protected environment. If health care workers receive LAIV, they should refrain from contact with severely immunosuppressed patients for seven days after receiving the vaccine. There is no preference for the use of inactivated influenza vaccine in health care workers or other persons who have close contact with persons with lesser degrees of immunosuppression.

PERSONS WHO SHOULD NOT BE VACCINATED

Inactivated influenza vaccine should not be administered to persons known to have anaphylactic hypersensitivity to eggs or other components of the vaccine without consulting a physician first. Persons with acute febrile illness should not be vaccinated until the symptoms have abated. In persons with minor illnesses, with or without fever, the vaccine is not contraindicated, particularly in children with mild upper respiratory tract infection or allergic rhinitis.

DOSAGE

Dosage recommendations vary by age group (Table 1). In previously unvaccinated children younger than nine years, two doses administered at least one month apart are recom mended. If possible, the second dose should be administered before December. In adults, study results have indicated limited or no improvement in antibody response when a second dose is administered during the same season.

ROUTE

The ACIP recommends the intramuscular route for administering the influenza vaccine. Adults and older children should be vaccinated in the deltoid muscle with a needle length of at least one inch. Infants and young children should be vaccinated in the anterolateral aspect of the thigh using a needle length of 7/8 to one inch for children younger than 12 months.

SIDE EFFECTS AND ADVERSE REACTIONS

Physicians should remind patients that inactivated influenza vaccine cannot cause influenza and that respiratory disease unrelated to the vaccine can occur after vaccination. In adults, the most frequent side effect is soreness at the vaccination site. Fever, malaise, myalgia, and other systemic symptoms can occur after vaccination with inactivated vaccine, and these symptoms most often affect persons who have not been exposed to the influenza virus antigens in the vaccine. Immediate allergic reactions (e.g., hives, anaphylaxis, angioedema, allergic asthma) are rare.

RECOMMENDATIONS FOR USING LIVE, ATTENUATED INFLUENZA VACCINE

LAIV is a vaccination option for healthy persons five to 49 years of age, including persons in close contact with groups at high risk and those wanting to avoid influenza. Possible advantages of LAIV include its ease of administration, its potential to induce a broad mucosal and systemic immune response, and the acceptability of an intranasal rather than intramuscular route of administration.

PERSONS WHO SHOULD NOT BE VACCINATED WITH LAIV

The following populations should not be vaccinated with LAIV: persons younger than five years or older than 50 years; persons with asthma, reactive airways disease, or other chronic disorders of the pulmonary or cardiovascular system; persons with other underlying medical conditions (e.g., renal dysfunction, hemoglobinopathies, metabolic diseases such as diabetes); persons with known or suspected immunodeficiency disease or who are receiving immunosuppressive therapies; children or adolescents receiving aspirin or other salicylates (because of the association with Reye’s syndrome with wild-type influenza infection); persons with a history of Guillain-Barré disease; women who are pregnant; or persons with a history of hypersensitivity, including anaphylaxis, to any of the components of LAIV or to eggs.

PERSONNEL WHO MAY ADMINISTER LAIV

Severely immunosuppressed persons should not administer LAIV. However, other persons at high risk for influenza-related complications may administer LAIV, including persons with underlying medical conditions placing them at high risk or who are likely to be at risk, such as pregnant women, persons with asthma, and person who are at least 50 years of age.

DOSAGE AND ADMINISTRATION

LAIV is intended for intranasal administration only and should not be administered by the intramuscular, intradermal, or intravenous route. LAIV must be stored at 15°C (59°F) or colder, and should not be refrozen after thawing. LAIV is supplied in a prefilled single-use sprayer containing 0.5 mL of vaccine. Approximately 0.25 mL is sprayed into the first nostril while the recipient is in the upright position. An attached dose divider clip is removed from the sprayer to administer the second half of the dose into the other nostril. If the vaccine recipient sneezes after administration, the dose should not be repeated.

LAIV should be administered annually according to the following schedule:

• Children five to eight years of age who are previously unvaccinated at any time with either LAIV or inactivated influenza vaccine should receive two doses (one dose equals 0.5 mL, divided equally between each nostril) of LAIV separated by six to 10 weeks.

• Children five to eight years of age who were previously vaccinated at any time with either LAIV or inactivated influenza vaccine should receive one dose of LAIV. These children do not require a second dose.

• Persons nine to 49 years of age should receive one dose of LAIV.

LAIV may be administered to persons with minor acute illnesses (e.g., diarrhea, mild upper respiratory tract infection with or without fever). However, if clinical judgment indicates that nasal congestion is present that might impede the delivery of the vaccine to the nasopharyngeal mucosa, deferral of administration should be considered until the resolution of the illness.

It is unknown whether the concurrent administration of LAIV with other vaccines affects the safety or efficacy of either LAIV or the simultaneously administered vaccine. The effect on safety and efficacy of LAIV coadministration with influenza antiviral medications has not been studied. Because influenza antivirals reduce replication of influenza viruses, LAIV should not be administered until 48 hours after cessation of influenza antiviral therapy, and influenza antiviral medications should not be administered for two weeks after receipt of LAIV.

SIDE EFFECTS AND ADVERSE REACTIONS

Results of studies show that the incidence of adverse events possibly complicating influenza was not statistically different among LAIV and placebo recipients.

Children. Signs and symptoms reported most often among vaccine recipients include runny nose or nasal congestion, headache, fever, and myalgias. These symptoms were associated more often with the first dose and were self-limited. LAIV is not approved for use among children younger than five years because one study showed an increase in asthma or reactive airways disease in children 12 to 59 months of age who received LAIV.

Adults. Among adults, runny nose or nasal congestion, headache, and sore throat occur more often in those receiving LAIV than in those receiving placebo.

Serious adverse events among healthy children ages five to 17 years or healthy adults ages 18 to 49 years occurred at a rate of less than 1 percent.

RECOMMENDED VACCINES FOR DIFFERENT AGE GROUPS

Health care providers should use inactivated influenza vaccine that has been approved by the FDA for children six months to three years of age. Inactivated influenza vaccine (Fluzone split-virus) is approved for use among persons at least six months of age. Inactivated influenza vaccine is labeled in the United States for use only among persons at least four years of age. LAIV (FluMist) is approved for use by healthy persons five to 49 years of age.

TIMING OF VACCINATION

The annual supply of inactivated influenza vaccine and the timing of its distribution cannot be guaranteed in any year. Information regarding the supply of 2004–2005 vaccine might not be available until late summer or early fall 2004. The CDC and other public health agencies will assess the vaccine supply on a continuing basis through the manufacturing period and make recommendations in the summer preceding the 2004–2005 influenza season regarding the need for tiered timing of vaccination of different risk groups.

Antiviral Agents

The ACIP report also includes recommendations for using antiviral agents for influenza. The section on antiviral agents covers the role of laboratory diagnosis, indications for use, dosage, pharmacokinetics, side effects and adverse reactions, use during pregnancy, and drug interactions. A link to the report and other information regarding influenza can be accessed at http://www.cdc.gov/flu.