by Staff Writer, MedPage Today
A double dose of the influenza vaccine was more effective than a single dose, supporting the U.S. flu vaccine policy, researchers reported.
Among 7,533 children between age 6 months and 8 years, a primer dose and subsequent booster dose had greater vaccine efficacy than a single dose (58% vs 46%), and was associated with a lower odds of influenza after data adjustment (adjusted odds ratio 0.61, 95% CI 0.40-0.94), reported Jessie R. Chung, MPH, of the CDC in Atlanta, and colleagues.
In a subset of vaccine-naive children 2 years old and younger, two doses of the vaccine were far more effective than one (53% vs 23%), and were associated with a lower chance of influenza (aOR 0.57, 95% CI 0.35-0.93), the researchers wrote in JAMA Pediatrics.
Altogether, just 39% of children in the cohort were considered fully vaccinated, defined as receiving one or zero doses in prior flu seasons and two doses in the current season, or two or more doses in a prior season and one or more in the current season.
“Promoting efforts to improve influenza vaccine coverage particularly with two doses in the first vaccination season may reduce the burden of influenza illness among young children, who are particularly vulnerable to complications and death from influenza infection,” Chung and co-authors wrote.
The CDC’s Advisory Committee on Immunization Practices recommends that children between age 6 months and 8 years who have never been vaccinated or who received only a single dose in previous seasons receive two doses, administered at least 4 weeks apart.
Notably, this study did not evaluate vaccine effectiveness in children first vaccinated with one full-dose vaccine, as the only licensed vaccine was a half-dose product until the 2018-2019 flu season, the authors reported.
These findings reinforce prior studies that have consistently shown children who were completely vaccinated were “more likely to achieve a protective level of antibody titers and less likely to have an office visit for influenza-like illness,” noted Melissa Stockwell, MD, MPH, and Claire Abraham, MD, both of Columbia University in New York City, in an accompanying editorial.
They added that the study also re-emphasizes the need to understand what barriers — like parental hesitancy, inconvenience, or a lack of counseling — exist for families not receiving the recommended dosing, as less than half of children in this study were fully vaccinated.
Altogether the study highlights for physicians that it “may be helpful to communicate to parents that, especially for vaccine-naive children, receipt of one dose does not offer significant protection against influenza,” Stockwell and Abraham wrote.
The study enrolled children with acute respiratory tract illness and cough who received outpatient care in the Flu Vaccine Effectiveness (VE) Network, which includes five sites across the country, over the 2014-2015 and 2017-2018 flu seasons. Illness was confirmed through nasal and oropharyngeal swab specimens that were tested through reverse-transcriptase polymerase chain reaction.
If children were vaccinated within 2 weeks of illness onset, had already received two doses of the vaccine in the enrollment season, or had received a live attenuated influenza vaccine that season, they were excluded. Data were adjusted for the study site, influenza season, age, calendar time, and comorbidities like asthma.
Of the 7,533 children included, 46% were female, 62% were white, and 65% were younger than 5 years old. One in five children were never vaccinated, while two-thirds (68%) were vaccinated before the current season, and 12% were vaccinated for the first time in the current season.
Kids who were administered the double dose within their first vaccination season (60%) tended to be younger at the time of their first vaccination than kids given a single dose (median 8 vs 11 months) and were more often non-Hispanic white (68%) and privately insured (40%), the authors reported.
Residual confounding may play a role in the findings, particularly because families were financially compensated and children receiving the recommended dosing may have different care-seeking behavior than those who did not, the authors noted. Additional limitations involve the relatively small sample sizes across the four flu seasons and the lack of stratification by type of initial vaccine, the researchers added. Finally, they said, the study focused primarily on influenza A(H3N2) circulation with some influenza B and little influenza A(H1N1)pdm09.
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