ID Week, the annual meeting of the Infectious Disease Society of America, provided valuable insights into past season’s endemic influenza burden and the effectiveness of prevention strategies. Each year, there are from 9million to 49 million influenza cases in the United States, 140,000-960,000 hospitalized cases, and 12,000-70,000 deaths directly attributable to influenza infection. The burden disproportionately falls on infants and adults 65 years of age and older; 11,000-48,000 children are hospitalized, and as many as several hundred children may die from influenza and related complications. School age children (aged 5-19 years) and adults (aged 30-39 years) are a major part of the transmission cycle. Influenza vaccine underlies the prevention strategy for limiting the burden of disease in U.S. populations. ID Week provided new insights into critical questions about influenza vaccines.
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1. What is the effectiveness of influenza vaccine against severe disease (hospitalization) in children? Does it vary by age? By type or subtype?
Angela P. Campbell, MD, MPH, of the Centers for Disease Control and Prevention, and associates presented data on influenza vaccine effectiveness from the New Vaccine Surveillance Network in children for the 2016-2017 and 2017-2018 season (ID Week session 99; Abstract 899). During both 2016-2017 and 2017-2018, H3N2 was the dominant virus and influenza B represented about one-third of cases, and H1N1 was a greater percentage of cases in 2017-2018. Influenza positivity among children younger than 18 years of age admitted to hospital with respiratory disease was 14% among unvaccinated and 8% among vaccinated children; effectiveness again hospitalization was 50%. Vaccine effectiveness (VE) was not statistically different between children younger than 8 years of age and those older that 8 years but did differ by vaccine type. VE was 76% against H1N1 disease, 59% again B disease, and only 33% against H3N2 disease.
Clearly, vaccination with influenza vaccine prevents serious respiratory disease. However, the impact of vaccine will vary by season and by which influenza stains are circulating in the community. The authors concluded that further understanding of the lower VE against H3N2 disease is needed.
2. Does the priming dose of influenza vaccine improve vaccine effectiveness?
Current recommendations call for a two-dose series for influenza vaccine in children aged 6 months through 8 years who have not had prior influenza vaccine. The recommendation is based on evidence demonstrating higher antibody responses in children receiving two doses, compared with a single dose. Using data from the U.S. Influenza Vaccine Effectiveness Network, Jessie R. Chung, MPH, of the CDC, and associates compared VE in children younger than 2 years receiving two doses in the first year of flu immunization (fully immunized), compared with those who received only one dose (partially immunized) (ID Week session 99; Abstract 900). VE was 53% for fully immunized and 23% for partially immunized children. Receipt of a single dose did not provide statistically significant protection against influenza. Surprisingly (to me), of 5,355 children aged 6 months to less than 2 years with no prior influenza vaccine, 1,870 (35%) received only one dose in the season.