Exploring indirect protection associated with influenza immunization - A systematic review of the literature

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Vaccine. 2019 Oct 21. pii: S0264-410X(19)31328-3. doi: 10.1016/j.vaccine.2019.09.086. [Epub ahead of print] Exploring indirect protection associated with influenza immunization - A systematic review of the literature.

Friedman L¹, Renaud A², Hines D², Winter A², Bolotin S³, Johnstone J⁴, Kwong JC⁵, McGeer AJ⁶, Crowcroft NS⁷, Warshawsky BF⁸.
Author information

1 Public Health Ontario, 480 University Avenue, Suite 300, Toronto, Ontario M5G 1V2, Canada. Electronic address: lindsay.friedman@oahpp.ca. 2 Public Health Ontario, 480 University Avenue, Suite 300, Toronto, Ontario M5G 1V2, Canada. 3 Public Health Ontario, 480 University Avenue, Suite 300, Toronto, Ontario M5G 1V2, Canada; Dalla Lana School of Public Health, University of Toronto, 155 College Street, Toronto, Ontario M5T 3M7, Canada. Electronic address: shelly.bolotin@oahpp.ca. 4 Public Health Ontario, 480 University Avenue, Suite 300, Toronto, Ontario M5G 1V2, Canada; Dalla Lana School of Public Health, University of Toronto, 155 College Street, Toronto, Ontario M5T 3M7, Canada; Department of Laboratory Medicine and Pathobiology, University of Toronto, 1 King's College Circle, Toronto, Ontario M5S 1A8, Canada; Department of Microbiology, Mount Sinai Hospital, 600 University Avenue, Toronto, Ontario M5G 1X5, Canada. Electronic address: jennie.johnstone@oahpp.ca. 5 Public Health Ontario, 480 University Avenue, Suite 300, Toronto, Ontario M5G 1V2, Canada; Dalla Lana School of Public Health, University of Toronto, 155 College Street, Toronto, Ontario M5T 3M7, Canada; ICES, 2075 Bayview Avenue, Toronto, Ontario M4N 3M5, Canada; Department of Family and Community Medicine, University of Toronto, 500 University Avenue, 5th floor, Toronto, Ontario M5G 1V7, Canada; Toronto Western Family Health Team, Toronto Western Hospital, 399 Bathurst Street, Toronto, Ontario M5T 2S8, Canada. Electronic address: jeff.kwong@utoronto.ca. 6 Dalla Lana School of Public Health, University of Toronto, 155 College Street, Toronto, Ontario M5T 3M7, Canada; Department of Laboratory Medicine and Pathobiology, University of Toronto, 1 King's College Circle, Toronto, Ontario M5S 1A8, Canada; Department of Microbiology, Mount Sinai Hospital, 600 University Avenue, Toronto, Ontario M5G 1X5, Canada. Electronic address: allison.mcgeer@sinaihealthsystem.ca. 7 Public Health Ontario, 480 University Avenue, Suite 300, Toronto, Ontario M5G 1V2, Canada; Dalla Lana School of Public Health, University of Toronto, 155 College Street, Toronto, Ontario M5T 3M7, Canada; Department of Laboratory Medicine and Pathobiology, University of Toronto, 1 King's College Circle, Toronto, Ontario M5S 1A8, Canada; ICES, 2075 Bayview Avenue, Toronto, Ontario M4N 3M5, Canada. Electronic address: natasha.crowcroft@utoronto.ca. 8 Public Health Ontario, 480 University Avenue, Suite 300, Toronto, Ontario M5G 1V2, Canada; Department of Epidemiology and Biostatistics, Schulich School of Medicine and Dentistry, Western University, 1151 Richmond Street, London, Ontario N6A 5C1, Canada. Electronic address: bryna.warshawsky@oahpp.ca.

Abstract

BACKGROUND:

Influenza causes significant annual morbidity and mortality, particularly in older adults, for whom influenza vaccine effectiveness (VE) is also lower. Immunizing one group (e.g., children) against influenza may indirectly protect another group (e.g., older adults) against influenza and its complications.
METHODS:

We updated previous systematic reviews on indirect protection against influenza by searching MEDLINE and EMBASE for relevant human studies published until January 4, 2017. We abstracted and critically appraised English language publications that reported or provided information to calculate indirect VE against influenza, as a percentage, in non-institutional settings. We developed a term called 'estimated actual protection' to explore the relationship between indirect protection and the product of direct VE and relative vaccine coverage. We calculated estimated actual protection for a subset of studies that reported coverage and indirect VE for: laboratory-confirmed influenza; outpatient care for respiratory illness; influenza-associated emergency visits; or influenza-associated hospitalizations. We ran linear mixed models to compare estimated actual protection against indirect VE for the four outcomes, and graphed the data.
RESULTS:

Of 2320 unique records identified, we abstracted and appraised 26 articles describing 24 studies. The majority of included studies reported at least one outcome suggesting that immunizing one group reduced influenza-related outcomes in another group. Critical appraisal of the abstracted studies identified recurring methodological weaknesses, such as lack of laboratory-confirmed influenza. Our exploratory analyses of 18 studies indicated a positive but not statistically significant relationship between estimated actual protection and indirect protection for each of the four outcomes.
CONCLUSIONS:

Our systematic review and exploratory analyses suggest influenza immunization provides some level of indirect protection. However, our critical appraisal highlights the need for a standardized and consistently applied approach to measuring indirect protection against influenza to fill existing knowledge gaps. Additionally, the concept of estimated actual protection requires validation.
Crown Copyright ? 2019. Published by Elsevier Ltd. All rights reserved.


KEYWORDS:

Indirect protection; Influenza vaccine; Seasonal influenza; Vaccine effectiveness

PMID: 31648907 DOI: 10.1016/j.vaccine.2019.09.086
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