ID Consult

Influenza: A vaccine we love to hate


 

References

The Centers for Disease Control and Prevention, American Academy of Pediatrics, and American Academy of Family Physicians recommend that everyone 6 months of age and older get a seasonal flu vaccine. Emphasizing influenza vaccination in children recognizes the high burden of morbidity and significant mortality associated with influenza in young children as well as their role in transmission in the community.

In 2015-2016, the CDC reported 83 influenza deaths in children, and estimated the rate of hospitalization for children younger than 4 years of age to be 42/100,000 (at press time). In 2015-2016, the H1N1 strain was dominant in the community overall, with influenza B being most prevalent late in the season. The CDC estimates that nearly 75% of children less than 24 months and 68% between 2 and 4 years of age were immunized this year. Overall vaccine efficacy in children 6 months through 8 years was reported at 47% last season from a CDC study using a study design that compares vaccination odds among influenza reverse transcription polymerase chain reaction (RT-PCR)–positive cases and RT-PCR–negative controls.

Influenza virus vaccines are unique in that they are updated, often annually, to include the most current hemagglutinin (HA) antigens based on estimates from circulating strains. In the United States, influenza vaccine manufacturers submit a supplement to their license and obtain Food and Drug Administration approval. These applications require only a limited study of safety in approximately 300 adults, essentially to verify attenuation (Influenza Other Respir Viruses. 2016. doi: 10.111/irv.1283). They do not require clinical proof of efficacy or even a threshold of immunogenicity.

Dr. Stephen Pelton

At the June 2016 CDC’s Advisory Committee on Immunization Practices (ACIP) meeting, data were presented comparing the efficacy of this season’s live attenuated influenza vaccine (LAIV) with inactivated influenza vaccine (IIV) by age and specific influenza type and subtype. Data from the U.S. Flu Vaccine Effectiveness (VE) Network, a consortium of five CDC-funded sites that conducts annual studies of influenza vaccine effectiveness, failed to demonstrate efficacy for LAIV in children aged 2-8 years. There was an absence of efficacy against the primary circulating strain, A(H1N1). This contrasted with the 62% efficacy report for IIV against A(H1N1).

The concern for efficacy for LAIV was not limited to 2015-2016; efficacy was poor in 2013-2014 during a year in which A(H1N1) was the dominant virus as well, and in 2014-2015 when the prevalent strain was a drifted A(H3N2). The lack of efficacy in 2015-2016 and 2013-2014 when A(H1N1) was the prevalent strain was especially enigmatic given its high efficacy against A(H1N1) between 2009 and 2011. Studies of LAIV from Astra Zeneca and the U.S. Department of Defense were consistent with those from the U.S. Flu VE Network; however, there were discordant data from Finland where vaccine efficacy was present. As a result of these studies, the ACIP voted that LAIV should not be used during the 2016-2017 flu season. This vote reinforces the importance of monitoring the effectiveness of annual flu vaccination and other public health interventions.

ACIP recommendations for 2016-2017

• Children younger than 2 years of age and those with chronic health problems such as asthma, diabetes, and disorders of the brain or nervous system are at especially high risk of developing serious flu complications.

• Annual influenza immunization, with either the IIV or recombinant influenza vaccine (RIV), for everyone 6 months and older, remains the only effective strategy for decreasing influenza disease in the community.

• LAIV should not be used during the 2016-2017 flu season.
ACIP recommendations must be reviewed and approved by the CDC’s director before becoming CDC policy. The final annual recommendations on the prevention and control of influenza with vaccines will be published in CDC Morbidity and Mortality Weekly Report (MMWR) Recommendations and Reports in late summer or early fall.

Flu vaccines available for children for 2016-2017

• The trivalent flu vaccine protects against three flu viruses; two influenza A viruses and an influenza B virus. Standard dose trivalent shots are manufactured with viruses grown in eggs. These are approved for children aged 6 months and older. There are different brands of this type of vaccine; each specific formulation has different age-based approvals.

• The quadrivalent flu vaccine protects against four flu viruses; two influenza A viruses and two influenza B viruses. A standard dose quadrivalent formulation is available for children; one brand is approved for children 6 months and older while others are approved for those 3 years and older.

• A cell-based vaccine, developed through a manufacturing process different from the traditional egg-based manufacturing process, was approved as a quadrivalent formulation for use in children 4 years of age and older.

Unanswered questions for the 2016-2017 influenza season

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