Vaccine Safety Studies Research Articles

Vaccine Safety Controversies and the Future of Vaccination Programs

In the years following the hepatitis B vaccination/multiple sclerosis controversy, a number of new issues regarding vaccine safety have been raised, in some cases leading to more debate and confusion. Against this background, an international group of experts was convened to review the current points of view concerning the use of thimerosal as a preservative and its potential risks; the suggested link between thimerosal-containing vaccines and acute lymphoblastic leukemia; the alleged association between aluminum-containing vaccines/macrophagic myofasciitis and general systemic complaints; a possible link between vaccination and autoimmune pathology; and a hypothetical link between measles-mumps-rubella vaccination and autism. At present, there are no data to conclude that childhood vaccines, and in particular hepatitis B vaccine, pose a serious health risk or justify a change in current immunization practice. However, vaccine "scares" continue to have an international impact on immunization coverage. Creating a positive environment for immunization can be achieved by repositioning the value of vaccines and vaccination, supported by evidence-based information. The role of international organizations, the media, and the industry in the implementation of communication strategies was discussed and the impact of litigation issues on vaccination was evaluated. The Viral Hepatitis Prevention Board confirms its commitment to current recommendations for universal and risk group hepatitis B vaccination and further encourages the conduct of vaccine safety studies and the dissemination of their results.

Fever after immunization: current concepts and improved future scientific understanding.

Fever is a common clinical complaint in adults and children with a variety of infectious illnesses, as well as a frequently reported adverse event following immunization. Although the level of measured temperature indicative of a "fever" was first defined in 1868, it remains unclear what role fever has as a physiologic reaction to invading substances, how best to measure body temperature and compare measurements from different body sites, and, consequently, how to interpret fever data derived from vaccine safety trials or immunization safety surveillance. However, even with many aspects of the societal, medical, economic, and epidemiologic meanings of fever as an adverse event following immunization (AEFI) still elusive, it is a generally benign--albeit common--clinical sign. By standardizing the definition and means of assessment of fever in vaccine safety studies, thereby permitting comparability of data, we hope to arrive at an improved understanding of its importance as an AEFI.

Vaccination and ill-health in dogs: a lack of temporal association and evidence of equivalence

Following concerns raised over the safety of canine vaccines, an epidemiological investigation was conducted to evaluate the evidence for a temporal association between vaccination and ill-health in dogs. The owners of a randomly selected population of dogs were sent 9055 postal questionnaires, 4040 of which were returned. No temporal association was found between vaccination and ill-health in dogs after adjusting for potential confounders, such as age. However, reliable inferences from non-significant test results are limited and so equivalence-testing methods were also used to make informative inferences. Results demonstrated that recent vaccination (<3 months) does not increase signs of ill-health by more than 0.5% and may actually decrease it by as much as 5%. This general approach should be used in all field studies of vaccine safety.

Control without separate controls: evaluation of vaccine safety using case-only methods

Epidemiological methods involving only cases are reviewed in the context of vaccine safety studies. These methods include some ecological methods, case-coverage methods, case-crossover and self-controlled case series methods. The properties of the methods are described using examples from the literature. It is argued that such methods, and in particular the self-controlled case series method, are powerful epidemiologic tools meriting the same attention as more traditional cohort and case–control methods.

Incidence of Adverse Reactions to Vaccines in a Paediatric Population

To detect the appearance and specify the types of adverse reactions to vaccines registered in a paediatric population. A 6-month, prospective, observational, multicentre epidemiological vaccine safety study was undertaken in 2002 covering a paediatric population subject to vaccine administration. A two-phase telephone survey of all patients was conducted, comprising an initial call at 1 week and a follow-up call at 30 days after the vaccine administration date. A paediatrician was responsible for diagnosing the specific type of adverse reaction. Of a total sample of 946 children, ranging in age from 0 to 14 years (50.8% girls, 49.1% boys), 191 non-serious suspected adverse reactions were detected, representing 19% of the vaccinated children. Reactions to the diphtheria, tetanus, pertussis acellular and Haemophilus influenzae type b (DTPa + Hib) vaccine appeared in 43.4% of cases, followed by reactions to the measles, mumps, rubella (MMR) [18.4%] and adult tetanus and diphtheria (Td) [17.8%] vaccines. The most frequent types of adverse reactions to vaccines were: injection-site oedema (12.2 per 1000 doses); pain at site of inoculation (10.3 per 1000 doses); temperature not recorded but believed by parents to be very high (4.6 per 1000 doses); and measured temperature indicating fever of 39-40.5 masculineC (4.4 per 1000 doses). Fifty-five percent (n = 21) of cases of injection-site oedema were attributed to DTPa + Hib vaccine (18.8 per 1000 doses), followed by 18.4% (n = 7) attributable to Td vaccine (112 per 1000 doses). Indeed, this latter vaccine was responsible for 43.8% (n = 14; 226 per 1000 doses) of all reported pain at the site of inoculation. MMR vaccine was linked to the occurrence of fever of 39-40.5 masculineC in 52% of cases (n = 10; 29 per 1000 doses). Two children were treated by the emergency services, but there were no deaths or hospitalisations. An active search for subjects with suspected adverse reactions to vaccines led to the detection of reactions that are usually not reported. Primary-care physicians and nurses must be vigilant for information on adverse reactions to vaccines in paediatric populations.

Statistical approaches to establishing vaccine safety.

In a vaccine safety trial, the primary interest is to demonstrate that the vaccine is sufficiently safe, rejecting the null hypothesis that the relative risk of an adverse event attributable to the new vaccine is above a prespecified value, greater than one. We evaluate the exact probability of type I error of the likelihood score test, with sample size determined by normal approximation, by enumeration of the binomial outcomes in the rejection region and show that it exceeds the nominal level. In the case of rare adverse events, we recommend the Poisson approximation as an alternative and develop the corresponding conditional and unconditional tests. We give sample size and power calculations for these tests. We also propose optimal randomization strategies which either (i) minimize the total number of adverse cases or (ii) minimize the expected number of subjects when the vaccine is unsafe. We illustrate the proposed methods using a hypothetical vaccine safety study.

Vaccine trials.

This article reviews some of the issues involved in evaluating vaccines in humans. Vaccine trials are required for licensure and are essential for demonstrating a vaccine's safety and protective efficacy. The formal framework of phase I, II, and III trials is described, with particular emphasis on the choice of hypotheses, trial design, and biases that arise in the context of vaccine trials. However, some aspects of a vaccine's performance cannot be evaluated in clinical trials owing to their relatively small size. Thus, vaccine evaluation must continue after licensure, for example, to evaluate the vaccine with respect to rare reactions, duration of protection, and ecological effects. The article reviews some of the methods commonly used for post-licensure studies of vaccine efficacy and safety.

Relative Incidence Estimation from Case Series for Vaccine Safety Evaluation

A method is described for estimating the relative incidence of clinical events in defined time intervals after vaccination compared to a control period using only data on cases. The method is derived from a Poisson cohort model by conditioning on the occurrence of an event and on vaccination histories. Methods of analysis for event-dependent vaccination histories and survival data are discussed. Asymptotic arguments suggest that the method retains high efficiency relative to the full cohort analysis under conditions which commonly apply to studies of vaccine safety.