Evaluation of adverse effects of vaccines: the case-control approach

Use of electronic immunization registry in the surveillance of adverse events following immunization

OBJECTIVE

To describe adverse events following vaccination (AEFV) of children under two years old and analyze trend of this events from 2000 to 2013, in the city of Araraquara (SP), Brazil.

METHODS

This is a descriptive study conducted with data of the passive surveillance system of AEFV that is available in the electronic immunization registry (EIR) of the computerized medical record of the municipal health service (Juarez System). The study variables were: age, gender, vaccine, dose, clinical manifestations and hospitalization. We estimated rates using AEFV as numerator and administered doses of vaccines as denominator. The surveillance sensitivity was estimated by applying the method proposed by the Centers for Disease Control and Prevention. We used Prais-Winsten regression with a significance level of 5.0%.

RESULTS

The average annual rate of AEFV was 11.3/10,000 administered doses, however without a trend in the study period (p=0.491). Most cases occurred after the first dose (41.7%) and among children under one year of age (72.6%). Vaccines with pertussis component, yellow fever and measles-mumps-rubella were the most reactogenic. We highlighted the rates of hypotonic-hyporesponsive episodes and convulsion that were 4.1/10,000 and 1.5/10,000 doses of vaccines with pertussis component, respectively, most frequently in the first dose; 60,0% of cases presented symptoms in the first 24 hours after vaccination, however, 18.6% showed after 96 hours. The sensitivity of surveillance was 71.9% and 78.9% for hypotonic-hyporesponsive episodes and convulsion, respectively.

CONCLUSIONS

The EIR-based AEFV surveillance system proved to be useful and highly sensitive to describe the safety profile of vaccines in a medium-sized city. It was also shown that the significant increase of the vaccines included in the basic vaccination schedule in childhood in the last decade did not alter the high safety standard of the National Immunization Program.

Surveillance of adverse effects following vaccination and safety of immunization programs

The aim of the review was to analyze conceptual and operational aspects of systems for surveillance of adverse events following immunization. Articles available in electronic format were included, published between 1985 and 2009, selected from the PubMed/Medline databases using the key words "adverse events following vaccine surveillance", "post-marketing surveillance", "safety vaccine" and "Phase IV clinical trials". Articles focusing on specific adverse events were excluded. The major aspects underlying the Public Health importance of adverse events following vaccination, the instruments aimed at ensuring vaccine safety, and the purpose, attributes, types, data interpretation issues, limitations, and further challenges in adverse events following immunization were describe, as well as strategies to improve sensitivity. The review was concluded by discussing the challenges to be faced in coming years with respect to ensuring the safety and reliability of vaccination programs.

Uncertainties and anxieties about vaccination, answering parent's concerns

Vaccination is one of the most significant public health interventions responsible for saving millions of lives and preventing significant morbidity. Despite this, there remain concerns about a number of vaccines. Although vaccines are extensively tested, there are genuine questions that many parents have about a number of vaccine-related issues. It is important that practitioners can discuss these concerns because denying or dismissing them may lead parents and patients to seek advice from uninformed sources. This article discusses some of the major concerns about vaccines, including their safety, immunogenicity, and the development of national immunization programs. It concludes that although vaccines are overwhelmingly safe, constant vigilance is required.

Pervasive developmental disorders in Montreal, Quebec, Canada: prevalence and links with immunizations

BACKGROUND

The prevalence of pervasive developmental disorders has increased in recent years. Links with the measles component of the measles-mumps-rubella vaccine and the cumulative exposure to thimerosal through other vaccines have been postulated.

OBJECTIVES

The purpose of this work was to estimate the pervasive developmental disorder prevalence in Montreal, Canada, in cohorts born from 1987 to 1998 and evaluate the relationship of trends in pervasive developmental disorder rates with: (1) changes in cumulative exposure to ethylmercury (thimerosal) occurring through modifications in the immunization schedule of young children and (2) trends in measles-mumps-rubella vaccination use rates and the introduction of a 2-measles-mumps-rubella dosing schedule during the study period.

METHODS

We surveyed 27749 children born from 1987 to 1998 attending 55 schools from the largest Anglophone school board. Children with pervasive developmental disorders were identified by a special needs team. The cumulative exposure by age 2 years to thimerosal was calculated for 1987-1998 birth cohorts. Ethylmercury exposure ranged from medium (100-125 microg) from 1987 to 1991 to high (200-225 microg) from 1992 to 1995 to nil from 1996 onwards when thimerosal was entirely discontinued. Measles-mumps-rubella coverage for each birth cohort was estimated through surveys of vaccination rates. The immunization schedule included a measles-mumps-rubella single dose at 12 months of age up to 1995, and a second measles-mumps-rubella dose at 18 months of age was added on after 1996.

RESULTS

We found 180 children (82.8% males) with a pervasive developmental disorder diagnosis who attended the surveyed schools, yielding a prevalence for pervasive developmental disorder of 64.9 per 10000. The prevalence for specific pervasive developmental disorder subtypes were, for autistic disorder: 21.6 of 10000; for pervasive developmental disorder not otherwise specified: 32.8 of 10000; and for Asperger syndrome: 10.1 of 10000. A statistically significant linear increase in pervasive developmental disorder prevalence was noted during the study period. The prevalence of pervasive developmental disorder in thimerosal-free birth cohorts was significantly higher than that in thimerosal-exposed cohorts (82.7 of 10000 vs 59.5 of 10000). Using logistic regression models of the prevalence data, we found no significant effect of thimerosal exposure used either as a continuous or a categorical variable. Thus, thimerosal exposure was unrelated to the increasing trend in pervasive developmental disorder prevalence. These results were robust when additional analyses were performed to address possible limitations because of the ecological nature of the data and to evaluate potential effects of misclassification on exposure or diagnosis. Measles-mumps-rubella vaccination coverage averaged 93% during the study interval with a statistically significant decreasing trend from 96.1% in the older birth cohorts (1988-89) to approximately 92.4% in younger birth cohorts (1996-1998). Thus, pervasive developmental disorder rates significantly increased when measles-mumps-rubella vaccination uptake rates significantly decreased. In addition, pervasive developmental disorder prevalence increased at the same rate before and after the introduction in 1996 of the second measles-mumps-rubella dose, suggesting no increased risk of pervasive developmental disorder associated with a 2-measles-mumps-rubella dosing schedule before age 2 years. Results held true when additional analyses were performed to test for the potential effects of misclassification on exposure or diagnostic status. Thus, no relationship was found between pervasive developmental disorder rates and 1- or 2-dose measles-mumps-rubella immunization schedule.

CONCLUSIONS

The prevalence of pervasive developmental disorder in Montreal was high, increasing in recent birth cohorts as found in most countries. Factors accounting for the increase include a broadening of diagnostic concepts and criteria, increased awareness and, therefore, better identification of children with pervasive developmental disorders in communities and epidemiologic surveys, and improved access to services. The findings ruled out an association between pervasive developmental disorder and either high levels of ethylmercury exposure comparable with those experienced in the United States in the 1990s or 1- or 2-dose measles-mumps-rubella vaccinations.

Evaluation of vaccine safety after the events of 11 September 2001: role of cohort and case-control studies

As immunization programs world-wide "mature" with high vaccine coverage and near elimination of vaccine-preventable disease, vaccine safety issues have increased in relative prominence. In the wake of events of 11 September 2001, fear of bioterrorism has reemerged. The paradigm of eradicating vaccine-preventable diseases, stopping vaccinations and thereby also eradicating the associated vaccine adverse events (a la smallpox) may unfortunately be obsolete. If all vaccinations have to be continued indefinitely, research is needed more than ever to understand and prevent rare vaccine adverse events. Case-control studies are usually best suited for such purposes, especially when nested within a pre-existing large-linked administrative database cohort to minimize bias. The new clinical immunization safety assessment centers may play an important role in bridging the sometimes conflicting clinical and epidemiologic perspectives in vaccine safety.

Incidence of Adverse Reactions to Vaccines in a Paediatric Population

OBJECTIVE

To detect the appearance and specify the types of adverse reactions to vaccines registered in a paediatric population.

PATIENTS AND METHODS

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.

RESULTS

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.

CONCLUSIONS

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.