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Original Investigation
May 4, 2020

Patterns of Influenza Vaccination and Vaccine Effectiveness Among Young US Children Who Receive Outpatient Care for Acute Respiratory Tract Illness

Author Affiliations
  • 1Influenza Division, Centers for Disease Control and Prevention, Atlanta, Georgia
  • 2Texas A&M University Health Science Center College of Medicine, Temple
  • 3Baylor Scott & White Health Research Institute, Temple, Texas
  • 4Department of Pediatrics, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania
  • 5Department of Pediatric Emergency Medicine, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania
  • 6Kaiser Permanente Washington Health Research Institute, Seattle, Washington
  • 7Center for Clinical Epidemiology & Population Health, Marshfield Clinic Research Institute, Marshfield, Wisconsin
  • 8Department of Epidemiology, University of Michigan School of Public Health, Ann Arbor
JAMA Pediatr. Published online May 4, 2020. doi:10.1001/jamapediatrics.2020.0372
Key Points

Question  What vaccination sequence is associated with the best protection against influenza for children?

Findings  In this case-control study including 7533 children, vaccine effectiveness against medically attended, laboratory-confirmed influenza was higher among children who received the recommended number of doses compared with children who did not receive the recommended number of doses. Vaccine-naive children aged 2 years or younger who received 2 doses of influenza vaccine in their first vaccination season were less likely to test positive for influenza than those who received 1 dose.

Meaning  Results of this study suggest that the burden of influenza among young children in the US might be reduced by improving adherence to an initial 2-dose series of influenza vaccination in previously unvaccinated children.


Importance  The burden of influenza among young children is high, and influenza vaccination is the primary strategy to prevent the virus and its complications. Less is known about differences in clinical protection following 1 vs 2 doses of initial influenza vaccination.

Objectives  To describe patterns of influenza vaccination among young children who receive outpatient care for acute respiratory tract illness in the US and compare vaccine effectiveness (VE) against medically attended laboratory-confirmed influenza by number of influenza vaccine doses received.

Design  This test-negative case-control study was conducted in outpatient clinics, including emergency departments, at 5 sites of the US Influenza Vaccine Effectiveness Network during the 2014-2015 through 2017-2018 influenza seasons. The present study was performed from November 5, 2014, to April 12, 2018, during periods of local influenza circulation. Children aged 6 months to 8 years 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 using real-time, reverse-transcriptase polymerase chain reaction for influenza for research purposes.

Exposures  Vaccination in the enrollment season with either 1 or 2 doses of inactivated influenza vaccine as documented from electronic medical records, including state immunization information systems.

Main Outcomes and Measures  Medically attended acute respiratory tract infection with real-time, reverse-transcriptase polymerase chain reaction testing for influenza.

Results  Of 7533 children, 3480 children (46%) were girls, 4687 children (62%) were non-Hispanic white, and 4871 children (65%) were younger than 5 years. A total of 3912 children (52%) were unvaccinated in the enrollment season, 2924 children (39%) were fully vaccinated, and 697 children (9%) were partially vaccinated. Adjusted VE against any influenza was 51% (95% CI, 44%-57%) among fully vaccinated children and 41% (95% CI, 25%-54%) among partially vaccinated children. Among 1519 vaccine-naive children aged 6 months to 2 years, the VE of 2 doses in the enrollment season was 53% (95% CI, 28%-70%), and the VE of 1 dose was 23% (95% CI, −11% to 47%); those who received 2 doses were less likely to test positive for influenza compared with children who received only 1 dose (adjusted odds ratio, 0.57; 95% CI, 0.35-0.93).

Conclusions and Relevance  Consistent with US influenza vaccine policy, receipt of the recommended number of doses resulted in higher VE than partial vaccination in 4 influenza seasons. Efforts to improve 2-dose coverage for previously unvaccinated children may reduce the burden of influenza in this population.

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