Adverse Events following 12 and 18 Month Vaccinations: a Population-Based, Self-Controlled Case Series Analysis

Kumanan WilsonSteven HawkenJeffrey C. KwongShelley DeeksNatasha S. CrowcroftCarl Van WalravenDoug Manuel



Live vaccines have distinct safety profiles, potentially causing systemic reactions one to 2 weeks after administration. In the province of Ontario, Canada, live MMR vaccine is currently recommended at age 12 months and 18 months.


Using the self-controlled case series design we examined 271,495 12 month vaccinations and 184,312 18 month vaccinations to examine the relative incidence of the composite endpoint of emergency room visits or hospital admissions in consecutive one day intervals following vaccination. These were compared to a control period 20 to 28 days later. In a post-hoc analysis we examined the reasons for emergency room visits and the average acuity score at presentation for children during the at-risk period following the 12 month vaccine.


Four to 12 days post 12 month vaccination, children had a 1.33 (1.29–1.38) increased relative incidence of the combined endpoint compared to the control period, or at least one event during the risk interval for every 168 children vaccinated. Ten to 12 days post 18 month vaccination, the relative incidence was 1.25 (95%, 1.17–1.33) which represented at least one excess event for every 730 children vaccinated. The primary reason for increased events was statistically significant elevations in emergency room visits following all vaccinations. There were non-significant increases in hospital admissions. There were an additional 20 febrile seizures for every 100,000 vaccinated at 12 months.


There are significantly elevated risks of primarily emergency room visits approximately one to two weeks following 12 and 18 month vaccination. Future studies should examine whether these events could be predicted or prevented.

Citation: Wilson K, Hawken S, Kwong JC, Deeks S, Crowcroft NS, Van Walraven C, et al. (2011) Adverse Events following 12 and 18 Month Vaccinations: a Population-Based, Self-Controlled Case Series Analysis. PLoS ONE 6(12): e27897. doi:10.1371/journal.pone.0027897

Editor: Shabir Ahmed Madhi, University of Witwatersrand, South Africa

Received: August 5, 2011; Accepted: October 27, 2011; Published: December 12, 2011

Copyright: © 2011 Wilson et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Funding: This study was supported by the Canadian Foundation for Innovation, the Population Health Improvement Research Network (PHIRN), and by the Institute for Clinical Evaluative Sciences (ICES), which is funded by an annual grant from the Ontario Ministry of Health and Long-Term Care (MOHLTC). The opinions, results and conclusions reported in this paper are those of the authors and are independent from the funding sources. No endorsement by ICES, Ontario MOHLTC or PHIRN is intended or should be inferred. Dr. Wilson holds the Canada Research Chair in Public Health Policy. Dr. Manuel holds the CIHR Chair in Applied Public Health. Dr. Kwong and Professor Keelan are supported by a Career Scientist award from the Ontario Ministry of Health and Long-Term Care. Dr Kwong is also supported by a Clinician Scientist award from the Department of Family and Community Medicine, University of Toronto. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Competing interests: The authors have declared that no competing interests exist.

Full study results can be found here:

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