The effectiveness of influenza vaccine is recognized to vary widely from season to season. At least two factors are critical for determining the likelihood that flu vaccine will be successful in preventing illness.
First, the demographics of who is being immunized (primarily age and presence of comorbidity) and second, the "match" between the circulating flu viruses and that year’s flu vaccine. When the flu vaccine is a poor match with circulating viruses, less benefit from flu vaccination will be observed; in years when the "match" between vaccine and circulating virus is good, substantial reduction in influenza respiratory illness in children and adults is observed. Recently, a second influenza B antigen has been added (creating quadrivalent vaccines) to improve the match with influenza B strains that may circulate in the community.
Dr. Stephen I Pelton
In February 2014, the Centers for Disease Control and Prevention reported midseason vaccine effectiveness estimates (MMWR 2014 Feb 21;63:137-42).
The major circulating virus was influenza A "2009 H1N1" virus and the "match" between vaccine strains and circulating strains was considered good. The CDC’s midseason vaccine effectiveness estimate was 61% for all age groups (95% confidence interval, 52%-68%), reinforcing the value of influenza vaccine for disease prevention in both children and adults. Flu vaccine reduced the risk of seeking medical attention for flulike illness by 60% for both children and adults.
Another factor that may determine the effectiveness of influenza vaccine in children is whether the individual receives live attenuated influenza vaccine (LAIV) or trivalent or quadrivalent inactivated influenza vaccine (IIV). The CDC has been considering the question "should LAIV be recommended preferentially over IIV in healthy children 2-8 years of age?" based on data from a limited number of studies. Canada, United Kingdom, Israel, and Germany have each expressed a preference for LAIV in their recent recommendations. The CDC working group evaluated published studies primarily restricted to those focused on healthy children, those with both LAIV and IIV cohorts, those studying the U.S. licensed and similar vaccines, and those in English. Their literature review identified five randomized trials and five additional observational studies. Lab-confirmed influenza in symptomatic children was the primary outcome; influenza related mortality and hospitalization also were considered.
The efficacy of LAIV was originally established in four randomized, placebo-controlled clinical trials. Each study was completed over two influenza seasons.
In the Belshe study (N. Engl. J. Med. 1998;338:1405-12), the efficacy compared with placebo was 93% in the first season and 100% in the second (after revaccination).
In a second study (Pediatrics 2006;118:2298-312), efficacy compared to placebo was 85% in the first season and 89% in the second (after revaccination).
Subsequently, randomized studies comparing LAIV with IIV in children younger than 8 years of age demonstrating the relative benefits of LAIV were reported (N. Engl. J. Med. 2007;356:685-96; Pediatr. Infect. Dis. J. 2006 ;25:870-9). A reduction greater than or equal to 50% in laboratory-confirmed influenza cases in the LAIV cohorts compared with the trivalent inactivated vaccine groups was observed. Greater efficacy was reported both in groups that were influenza vaccine naive as well as those with prior immunization. No reductions in hospitalization and medically-attended acute respiratory illness were reported for the LAIV cohorts; however, the quality of the data was judged to be less robust than for laboratory-confirmed disease. For children aged 9-18 years, no differences in laboratory-confirmed influenza were reported.
The mechanism for improved efficacy of LAIV in young children (2-8 years) is largely unknown. LAIV may elicit long-lasting and broader humoral and cellular responses that more closely resembles natural immunity. It also has been hypothesized that LAIV is more immunogenic than IIV as a priming vaccine, and IIV is more effective in boosting preexisting immunity. It is possible that is one explanation for why LAIV is more effective in young children, and that no differences are observed in older children and adults. It also has been suggested that LAIV may elicit an antibody that is more broadly protective against mismatched influenza strains.
In June, the Advisory Committee on Immunization Practices (ACIP) proposed new recommendations regarding the use of LAIV and IIV for young healthy children. ACIP affirmed that both LAIV and IIV are effective in prevention of influenza in children, but recommended that LAIV be used for healthy children aged 2-8 years when both vaccines are available and there are no contraindications or precautions to its use. When LAIV is not immediately available, IIV should be used. Vaccination should not be delayed to procure LAIV.
ACIP restated previous contraindications and precautions to administration of LAIV. Those with contraindications to LAIV should receive inactivated vaccine. These include: