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Indian Pediatr 2016;53: 351 |
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Inappropriately Small Sample for Studying
Adverse Events Following Immunization
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*Kanika Goyal and Jacob Puliyel
Department of Pediatrics, St Stephens Hospital, Delhi,
India.
Email: [email protected]
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We refer to the study of common illnesses before and after vaccination
[1]. The authors looked for symptoms like constipation, rash, wheeze,
rhinitis and watering of eyes in 1602 children in the week after
immunization, and found the frequency was same as in the week prior to
immunization (except for fever). It was sought to imply that most
adverse events following immunization (AEFI) are explained by the
background rate. The study was seriously flawed as the sample size
selected was inadequate. The authors noted that no child had hypotonic
hyporesponsive episode (HHE), seizures, pruritus, difficulty in
breathing or breath holding, either during the pre- or post- vaccination
period, and these are the AEFI described by the Brighton Collaboration
(with their specific case definitions) [2].
Incidence of these AEFI is rare. As per WHO
surveillance data, HHE occurs less than 1/1000, seizures in less than
0.5/1000 and anaphylaxis and shock incidence in one per million doses
[3]. Thus, a sample size of 1602 is woefully inadequate to detect real
AEFI. The authors have selected sample size so small that
vaccine-related AEFI (other than fever) was unlikely to be picked up,
and all they would see are routine symptoms unrelated to vaccination.
With these figures, they concluded erroneously that AEFI can be
explained or background rates.
In vaccine trials, randomized controls are used to
look at the background rate. If there is a statistically significant
increase in any adverse effect among the vaccinated, it is likely to be
AEFI caused by the vaccine.
References
1. Kompithra RZ, Sarkar R, Mathew LG, Muliyil J, Kang
G. Study of common illnesses before and after vaccination: A risk
interval approach. Indian Pediatr. 2015;52:933-8.
2. Definition and Application of Terms for Vaccine
Pharacovigilance, Report of CIOMS/WHO Working Group on Vaccine
Pharmacovigilance, Geneva; WHO; 2011; Available from:
http://www.who.int/vaccine_safety/initiative/tools/CIOMS_report_WG_vaccine.pdf
Accessed January 12, 2016.
3. AEFI Guidelines – Ministry of Health and Family Welfare,
Government of India, 2010. Available from:
http://www.pbhealth.gov.in/Immunization/AEFI_Guidelines.pdf Accessed
January 12, 2016.
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