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Influenza immunization: Live or inactivated?
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Immunization with influenza vaccine remains the single most effective action to reduce risk of infection and disease from influenza virus.
Since 2010, recommendations from the CDC have included influenza immunization for all infants and children aged at least 6 months. Influenza virus strains contained in this season’s vaccines are unchanged from the 2010-2011 season. The vaccine includes two influenza A subtypes (H1N1 and H3N2) and one influenza B subtype. Vaccines containing these strains are available as several different products from several manufacturers. These products also differ by mode of administration — intramuscular injection and intranasal spray. A new intradermal product (Fluzone Intradermal, Sanofi-Pasteur) is available this year, but this product is FDA-labeled for patients aged 18 to 64 years.
Reports from the CDC and the AAP for the use of influenza vaccine for the 2011-2012 season do not state a preference for trivalent inactivated vaccine or live-attenuated influenza vaccine (FluMist, MedImmune) for efficacy, although indications for TIV and LAIV differ by age, presence of underlying medical conditions and other factors. Some comparative efficacy data are available from clinical studies.
In a recently published meta-analysis, Osterholm and colleagues reviewed the published literature from 1967 to early 2011 for controlled trials and observational studies that assessed influenza vaccine efficacy for children and adults, in which influenza disease was documented by culture or reverse transcription polymerase chain reaction (RT-PCR). These researchers used more stringent criteria for documentation of infection and vaccine efficacy (culture or RT-PCR) as compared with other studies that have used serology, which has been criticized for reduced sensitivity for diagnosis of influenza infection. Thirty-one studies were selected for analysis (17 controlled trials and 14 observational studies). Six studies of eight influenza seasons evaluated LAIV in children aged 6 months to 7 years, and the median vaccine efficacy was 78%.
No trials meeting inclusion criteria for this meta-analysis were found for children aged 2 to 17 years or adults aged at least 65 years for TIV, nor for LAIV for children aged 8 to 17 years. Vaccine efficacy of all studies evaluated was found to be highest for LAIV for children aged 6 months to 7 years.
Relatively few controlled trials have directly compared TIV with LAIV in children. The AAP policy statement on influenza prevention and control for the 2011-2012 season said influenza vaccine efficacy varies according to the age of the patient. The CDC report on influenza said direct comparative data for TIV and LAIV are limited and are insufficient to determine whether TIV or LAIV is more effective in certain settings or populations.
Belshe and colleagues compared TIV with LAIV in infants and children aged 6 to 59 months (n=7,852) without a recent episode of wheezing (within 42 days of enrollment) or severe asthma in a randomized, double blind manner during the 2004-2005 season. The primary endpoint was vaccine efficacy in preventing culture-confirmed influenza-like illness. In the children receiving LAIV, there were 54.9% fewer cases of influenza as compared with children receiving TIV (153 cases vs. 338 cases, respectively; P<.001). The highest efficacy of LAIV was demonstrated for antigenically well-matched and drifted viruses. Overall, there was no significant difference in medically significant wheezing between the two groups.
However, an increased risk of medically significant wheezing (within 42 days of vaccination) was shown for infants aged younger than 12 months who received LAIV. Hospitalization rates for any cause also were higher among infants aged younger than 12 months who received LAIV. Several other comparative studies have shown LAIV to have higher efficacy than TIV in infants and children, although these studies have evaluated children with other underlying medical conditions, such as asthma or recurrent respiratory tract infections.
Vaccine differences
Most influenza vaccine products used in the pediatric population are available as TIV. These vaccine products contain no live virus. In contrast, one product contains live-attenuated virus, LAIV. TIV vaccine products differ by FDA-labeled age indications, with one product labeled for use in children aged at least 6 months (Fluzone). LAIV is FDA-labeled for use in those aged 2 to 49 years.
Other than viral composition (ie, inactivated virus vs. live virus), an important distinction between TIV and LAIV vaccine products is labeled indication. TIV products are labeled for use in infants and children aged at least 6 months (varies with specific products), including children with underlying medical conditions, such as pulmonary and cardiac conditions. LAIV is labeled for use only in otherwise healthy children aged at least 2 years. More specifically, LAIV should not be given to the following:
- Children with chronic pulmonary (including asthma), cardiac, metabolic (including diabetes mellitus), neuromuscular/neurodevelopmental, or renal conditions, and hemoglobinopathies;
- Children with immunodeficiencies or children receiving immunosuppressive therapies;
- Pregnant females;
- Children receiving salicylates (including aspirin);
- Children with copious nasal congestion;
- Children who have received other live-virus vaccines within the previous 4 weeks (although live-virus vaccines can be given on the same day as LAIV); and
- Children aged 24 to 59 months with a history of wheezing episodes (ie, if the answer to the following question is “Yes:” In the previous 12 months, has a health care professional ever told you that your child had wheezing?).
An additional difference in influenza vaccine products that may be important to some parents or caregivers is thimerosal content. No single-use influenza vaccine product (TIV and LAIV) contains thimerosal. Only the multidose TIV vaccine products, including Fluzone, Fluvirin (Novartis Vaccines and Diagnostics Limited) and Afluria (CSL Limited), contain thimerosal.
Edward A. Bell, PharmD, BCPS, is professor of clinical sciences at Drake University College of Pharmacy, Blank Children’s Hospital, in Des Moines, Iowa. Bell is also a member of the Infectious Diseases in Children Editorial Board. Disclosure: Dr. Bell reports no relevant financial disclosures.
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