Birth order matters: Eldest more likely to visit hospital after vaccinations

December 4, 2013

Ottawa and Toronto, Canada — First-born children have a higher incidence of post-vaccination emergency room (ER) visits and admissions than later-born children for vaccinations up to 12 months of age, says a new Ontario study from researchers at the Institute for Clinical Evaluative Sciences (ICES) uOttawa, the Ottawa Hospital Research Institute and Public Health Ontario.

“There are a number of possible explanations for this including a combination of evolving parental decision-making and diminishing anxiety with the experience gained from each additional child born,” says Dr. Kumanan Wilson, senior author on the study, a scientist at the Ottawa Hospital Research Institute and adjunct scientist at ICES uOttawa. "However, the possibility of a biological or environmental explanation also needs to be considered.”

Published in PLOS ONE, the researchers examined records for all children born in Ontario between April 1, 2006, and March 31, 2009, correlating ER visits and hospital admissions immediately following each of the two, four, six and 12-month vaccinations with each child’s birth order.

“We undertook this study because previous research has suggested that larger numbers of siblings or later birth order may reduce a child’s chances of developing allergies and asthma. We wanted to see if this phenomenon carried through to vaccine response, and this appears to be the case,” says Steven Hawken, lead analyst at ICES uOttawa who was the first author of the study.

The strongest effects were observed following the two-month and four-month vaccinations. For the four-month vaccination, first-born children were 70 per cent more likely to go to a hospital after vaccination compared to later-born children. This represents 157 additional events per 100,000 vaccinated.

“To help alleviate parental anxiety, I would encourage physicians to take the time to inform parents of expected adverse reactions following vaccination,” adds Dr. Wilson. “This is particularly important for first-time parents.”

Parents should also be encouraged to talk to their physicians about what to expect following immunization.

The study “Association between Birth Order and Emergency Room Visits and Acute Hospital Admissions Following Pediatric Vaccination: A Self-controlled Study,” was published today in PLOS ONE. (

Authors: Steven Hawken, Jeffrey C. Kwong, Shelley L. Deeks, Natasha S. Crowcroft, Robin Ducharme, Douglas G. Manuel and Kumanan Wilson.

More detailed study findings are available on the ICES website.

For more information, please contact:
Paddy Moore,
Ottawa Hospital Research Institute
(o) 613-737-8899 x73687 or (c) 613-323-5680

About the Ottawa Hospital Research Institute
The Ottawa Hospital Research Institute (OHRI) is the research arm of The Ottawa Hospital and is an affiliated institute of the University of Ottawa, closely associated with the university’s Faculties of Medicine and Health Sciences. OHRI includes more than 1,700 scientists, clinical investigators, graduate students, postdoctoral fellows and staff conducting research to improve the understanding, prevention, diagnosis and treatment of human disease. Research at OHRI is supported by The Ottawa Hospital Foundation.

About ICES
ICES is an independent, non-profit organization that uses population-based health information to produce knowledge on a broad range of health care issues. Our unbiased evidence provides measures of health system performance, a clearer understanding of the shifting health care needs of Ontarians, and a stimulus for discussion of practical solutions to optimize scarce resources. ICES knowledge is highly regarded in Canada and abroad, and is widely used by government, hospitals, planners, and practitioners to make decisions about care delivery and to develop policy.
Follow ICES on Twitter: @ICESOntario

About Public Health Ontario
Public Health Ontario is a Crown corporation dedicated to protecting and promoting the health of all Ontarians and reducing inequities in health. Public Health Ontario links public health practitioners, front-line health workers and researchers to the best scientific intelligence and knowledge from around the world.
Follow Public Health Ontario on Twitter: @publichealthON