ID Consult

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


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 [email protected].


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