A possible link between immunization and asthma is of great concern to many parents. As the authors of the paper I discuss today have stated: “It is important that researchers clarify this issue, because… the perception that immunization causes asthma may become a significant determinant of parents’ attitudes toward routine vaccination of their children.”
Fortunately, Dr. Ran Balicer and his colleagues published a systematic literature review of this very subject in the journal Pediatrics in November 2007. Dr. Balicer and his co-authors are with the Department of Epidemiology at Ben-Gurion University of the Negev in Israel. I hope it makes everybody happy that neither Dr. Balicer nor any of his co-authors work for a pharmaceutical company or the CDC, nor did they accept any money from drug companies during their research. The title of their review is, “Is Childhood Vaccination Associated With Asthma? A Meta-Analysis Of Observational Studies.”
The aim of the study was to study the available evidence on the association of whole cell pertussis (whooping cough) and BCG vaccination with the risk of asthma in childhood and adolescence. The major electronic medical databases (Medline, National Library of Medicine Gateway, and Cochrane Library) were searched, and reference lists of publications were reviewed for relevant birth-cohort studies and randomized controlled trials from 1966 to March 2006. 71 original scientific studies were found and read in full.
Studies had to meet the following criteria to be included in the meta-analyses:
(1) Directly compared vaccinated and unvaccinated children.
(2) Validated vaccination status by medical charts.
(3) Used preset criteria to define asthma.
Seven studies of pertussis vaccination (with a total of 186,663 patients) and five studies of BCG vaccination (with a total of 41,479 patients) met the authors’ inclusion criteria. Here are the results in two tables. An odds ratio of 1.00 means that there is no relationship between immunization and asthma.
No statistically significant association was detected between either whole cell pertussis or BCG vaccination and incidence rates of asthma during childhood and adolescence. The authors conclude: “Currently available data…do not support an association, [causal] or protective, between receipt of the BCG or whole cell pertussis vccine and risk of asthma in childhood and adolescence.” These are the two vaccines that have been studied the most in this context. The authors hope, as do I, that “these findings could be used to rellieve parental concerns that could otherwise lead to vaccination refusal.”
As a scientist, I need to add two caveats:
1. This is still not a 100 percent settled issue for two reasons, which Balicer et al. openly acknowledge:
(A) “The multitude of biases in studies that have used a birth-cohort design stress the need for additional adequately controlled, large-scale studies.”
(B) Balicer et al. ranked studies on quality. The pertussis vaccination study that they ranked as the highest quality was the UK Retrospective Study of Farooqi and Hopkin. (Farooqi IS, Hopkin JM. Early childhood infection and atopic disorder. Thorax 1998; 53: 927-32.) This investigation did report a positive association between pertussis immunization and asthma, with an odds ratio of 1.44 (95% CI: 1.17 - 1.85), even though this was the only study that did find such an association.
2. A recent study of children born in Manitoba, Canada in 1995 found that Delay in diphtheria, pertussis, tetanus vaccination is associated with a reduced risk of childhood asthma (McDonald KL, Huq SI, Lix LM, Becker AB, Kozyrskyj AL. J Allergy Clin Immunol. 2008;121: 626-31). However, this was a study of the timing of vaccination among vaccinated children. As the authors of the study admit: “To our knowledge we are the first to report that delay in adminstration of the first dose of DPT immunization is significantly associated with reduced risk of developing asthma in childhood.”
In sum: A recent systematic literature review of high-quality studies that directly compared vaccinated and unvaccinated children, validated vaccination status by medical charts, and used preset criteria to define asthma found no association between childhood vaccination and asthma.