Vaccine effectiveness higher in young children who received full recommended course

The burden of influenza among young U.S. children may be reduced by improving adherence to an initial two-dose series of flu vaccination, researchers reported.

In more than 1,000 previously unvaccinated children (ages 6 months to 2 years), the vaccine effectiveness of two doses of inactivated flu vaccine in the enrollment season was 53% (95% CI 28%-70%), while the vaccine effectiveness of a single dose was 23% (95% CI −11% to 47%), according to Jessie R. Chung, MPH, of the CDC in Atlanta, and co-authors.

Also, those who received two doses were less likely to test positive for influenza versus kids who got only one dose (adjusted odds ratio 0.57, 95% CI 0.35 to 0.93), they wrote in JAMA Pediatrics.

In addition, the adjusted vaccine effectiveness against any influenza was 51% (95% CI 44%-57%) among fully vaccinated children and 41% (95% CI 25%-54%) among partially vaccinated children, the researcher stated.

“The U.S. Advisory Committee on Immunization Practices (ACIP) presently recommends 2 doses of influenza vaccine for children aged 6 months to 8 years who have received fewer than 2 previous doses in their lifetime,” they wrote.

But they pointed out that there is not a lot of data on the “differences in clinical protection following 1 versus 2 doses of initial influenza vaccination.”

In general, the study “provides evidence supporting the need for the administration of 2 doses of the influenza vaccine, particularly in young, previously vaccine-naive children” commented Melissa S. Stockwell, MD, MPH, and Claire Abraham, MD, both of New York-Presbyterian, Columbia University Medical Center in New York City, in an editorial accompanying the study.

They praised the authors for the numerous strengths of the research, particularly the “inclusion of multiple influenza seasons [2014-2015 through 2017-2018] with different vaccines [that] minimized the potential confounding factor of a particularly effective or ineffective vaccine in a specific season.”

However, the exclusion criteria of receiving a dose of live, attenuated flu vaccine in the current season was a study limitation, as was incomplete data on previous flu infections, Stockwell and Abraham noted. Chung and co-authors explained that few children initially received live attenuated influenza vaccine, so they were unable to stratify the results by type of first vaccine.

Other study limitations included the fact that the only U.S. licensed influenza vaccine for children (ages ≤3 years) was a half-dose product until 2018-2019. As a result, “Vaccine effectiveness may differ in children first vaccinated with 1 full-dose vaccine, which we were unable to examine,” the authors wrote.

An important issue that needs more study is why families will get the first dose for their children, but then not see the second dose through, “placing their children at higher risk for contracting a potentially devastating virus,” according to Stockwell and Abraham.

They cited possible barriers such as parents having a change of heart after dose one, insufficient healthcare professional follow-up on the importance of dose two, or simply inconvenience, especially for older children who undergo wellness visits every six months or yearly.

“Whatever the reasons, the difference in vaccine effectiveness between the partially and fully vaccinated children highlights why we as physicians and public health advocates should ensure that all children are being appropriately vaccinated by receiving all of their needed doses,” Stockwell and Abraham wrote.

Chung’s group conducted their test-negative case-control study in outpatient clinics, including emergency departments, at five sites of the U.S. Influenza Vaccine Effectiveness Network from Nov. 2014 to April 2018, during “flu seasons” in those locations.

“The present study extends the previous study using data from the U.S. Influenza Vaccine Effectiveness (Flu VE) Network over 4 more contemporary seasons,” the authors explained.

Children with an acute respiratory tract illness with cough who presented for outpatient care within 7 days of illness onset were included. All children were tested for influenza using real-time, reverse-transcriptase polymerase chain reaction. Vaccination data with either one or two doses of inactivated influenza vaccine came from electronic health records, including state immunization information systems, they wrote.

Out of 7,533 children, 46% were girls, 62% were non-Hispanic white, and 65% were ages <5 years. A little over half (52%) were unvaccinated in the enrollment season, but only 39% were fully vaccinated, and 9% were partially vaccinated.

Chung and co-authors also found that the proportion of children who received two doses within their first vaccination season was higher among those initially vaccinated before age 2 years (65%) versus children first vaccinated at ages 2-4 years (16%) or 5-8 years (8%).

Children who were never vaccinated were younger at enrollment (median age 39 months), less likely to have a high-risk medical condition — such as asthma or reactive airways disease — and more likely to report “excellent general health” versus vaccinated kids.

The authors reported that a single dose of vaccine in the current season did not offer “statistically significant protection against laboratory-confirmed influenza among the small proportion of previously unvaccinated children” (ages ≤2 years).

“The higher risk of infection resulting from underdeveloped immune and respiratory tract systems provides a reason to identify vaccination strategies focusing on this vulnerable population of younger children,” they stressed.

  1. Vaccine effectiveness against medically attended, laboratory-confirmed influenza was higher among children who received the recommended two doses compared with children who did not receive the recommended number of doses.

  2. Vaccine-naive children (ages ≤2 years) who received two doses of inactivated flu vaccine in their first vaccination season were less likely to test positive for influenza than those who received one dose.

Shalmali Pal, Contributing Writer, BreakingMED™

The U.S. Influenza Vaccine Effectiveness Network was supported by the CDC, the University of Michigan, Kaiser Permanente Washington Health Research Institute, Marshfield Clinic Research Institute, University of Pittsburgh, Baylor Scott and White Healthcare, and the NIH.

Chung reported no relationships relevant to the contents of this paper to disclose. Co-authors reported support from, and/or relationships with, the CDC, Abt Associates, MedImmune, AstraZeneca Janssen/Johnson & Johnson, Sanofi Pasteur, Pfizer, and Seqirus.

Stockwell reported grants from the NIH and the CDC, and a relationship with Pfizer. Abraham reported no relationships relevant to the contents of this paper to disclose.

Cat ID: 138

Topic ID: 85,138,730,30,138,139,44,561,653,924