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.

spukkato/Getty Images

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.

The data strongly support the current recommendations for a priming dose, especially in young children, in the first season of influenza vaccine and warrants increased efforts to increase the update of second doses during the season. Hopefully we can do better in 2019!

3. Should we wait to vaccinate with influenza vaccine?

Some evidence suggests that waning immunity to influenza vaccine, primarily in those aged 65 years and older, may explain increased disease activity toward the end of influenza season. Other explanations include increasing viral diversity throughout the season, resulting in reduced effectiveness. Do such concerns warrant delaying immunization? The onset and peak of influenza season varies by year; in October 2019, 3% of tests performed on patients with respiratory illness were influenza positive. The trade-offs for delaying immunization until October are the unpredictability of onset of influenza season, the requirement for two doses in infants, the need for 2 weeks to achieve peak antibody concentrations, and the potential that fewer individuals will be vaccinated. Kathy Neuzil, MD, MPH, from the Center for Vaccine Development and Global Health, University of Maryland School of Medicine, reviewed recent modeling (for adults aged 65 years and older) and reported that delaying vaccine programs until October is associated with greater burden of hospitalization if 14% fewer individuals (who would be vaccinated in August/September) are vaccinated (ID Week; Session 940).

In response to these concerns, the CDC recommendations for 2019 are that, in children aged 6 months through 8 years who need two doses, start early so that you can achieve both doses before influenza season (MMWR 2019 Aug 23;68[3]:1-21).In older children and adults, who need only a single dose, early vaccination (August and early September) may lead to reduced protection late in the influenza season?

4. How can we optimize vaccine impact?

Vaccine impact refers to the affect on a population level and not at an individual level. Meagan C. Fitzpatrick, PhD, from the Center for Vaccine Development and Global Health, University of Maryland School of Medicine, evaluated the benefits of our moderately effective influenza vaccines (VE 40%-60%) to the population beyond those who are vaccinated. Her conclusions were that even a modestly effective vaccine prevents 21 million cases of influenza, 129,000 hospitalizations, and 62,000 deaths. And that two-thirds of the deaths prevented are from herd benefit (or indirect effects). Although both coverage and vaccine effectiveness are important, she reported that population impact was most sensitive to coverage, compared with vaccine effectiveness. Dr. Fitzpatrick found that targeting school-age children 6-19 years of age and adults 30-39 years of age maximizes the public health benefits (herd effects) of influenza vaccine. In 2018 season, influenza coverage was 63% for at least one dose in children aged 6 months through 17 years and 45% in adults aged 18 years and older; in the two target age groups 5-17 and 30-39 years, coverage was 59% and approximately 35%, respectively (ID Week; Session 939).

Dr. Stephen I. Pelton, professor of pediatrics and epidemiology, Boston University schools of medicine and public health. — Dr. Stephen I. Pelton

Clearly, even our modestly effective influenza vaccines have significant public health benefit in protecting the U.S. populations from serious disease and death. Efforts to increase vaccine uptake in school-age children, both those with and without comorbidity, and the 30- to 39-year-old adult cohort would likely further reduce the burden of serious disease from influenza.

In summary, despite a vaccine that is only moderately effective, there is clear evidence to support current recommendations of universal immunization beginning at 6 months of age. Optimizing the impact of the flu vaccine requires increasing coverage, including two doses for those less than 8 years of age being immunized for the first time. Delaying until October 1 is a good idea only if the same number of individuals will receive influenza vaccine, otherwise the hypothetical benefit is lost.

Dr. Pelton is professor of pediatrics and epidemiology at Boston University schools of medicine and public health and is senior attending physician, Boston Medical Center. Dr. Pelton has investigator-initiated research awards to Boston Medical Center from Pfizer and Merck Vaccines. He also received honorarium as an advisory board member, participation in symposium and consultation from Seqirus and Merck Vaccine, Pfizer, and Sanofi Pasteur. Email him at pdnews@mdedge.com.

Flu vaccine: Larger impact on influenza burden than you thought?