Quality Assessments of HMO Diagnosis Databases Used to Monitor Childhood Vaccine Safety

Objective: To assess the quality of automated diagnoses extracted from medical care databases by the Vaccine Safety Datalink (VSD) study.

Methods: Two methods are used to assess quality of VSD diagnosis data. The first method compares common automated and abstracted diagnostic categories (“outcomes”) in 1-2% simple random samples of study populations. The second method estimates positive predictive values of automated diagnosis codes used to identify potential cases of rare conditions (e.g., acute ataxia) for inclusion in nested case-control medical record abstraction studies.

Results: There was good agreement (64-68%) between automated and abstracted outcomes in the 1-2% simple random samples at 3 of the 4 VSD sites and poor agreement (44%) at 1 site. Overall at 3 sites, 56% of children with automated cerebella ataxia codes (ICD-9 = 334) and 22% with “lack of coordination” codes (ICD-9 = 781.3) met objective clinical criteria for acute ataxia.

Conclusions: The misclassification error rates for automated screening outcomes substantially reduce the power of screening analyses and limit usefulness of screening analyses to moderate to strong vaccine-outcome associations. Medical record verification of outcomes is needed for definitive assessments.

Predictive Value of ICD-9-CM Codes Used in Vaccine Safety Research

Objectives: To assess how well selected ICD-9-CM diagnosis codes predict adverse events; to model bias and power loss when vaccine safety analyses rely on unverified codes.

Methods: We extracted chart verification data for ICD-9-CM diagnosis codes from six Vaccine Safety Datalink (VSD) publications and modeled biases and power losses using positive predictive value (PPV) estimates and ranges of code sensitivity.

Results: Positive predictive values were high for type 1 diabetes (80%) in children, relative to WHO criteria, and intussusception (81%) in young children, relative to a standard published case definition. PPVs were moderate (65%) for inpatient and emergency department childhood seizures and low (21%) for outpatient childhood seizures, both relative to physician investigator judgment. Codes for incident central nervous system demyelinating disease in adults had high PPV for inpatient codes (80%) and low PPV for outpatient codes (42%) relative to physicians’ diagnoses. Modeled biases were modest, but large increases in frequencies of adverse events are required to achieve adequate power if unverified ICD-9-CM codes are used, especially when vaccine associations are weak.

Conclusions: ICD-9-CM codes for type 1 diabetes in children, intussusception in young children, childhood seizures in inpatient and emergency care settings, and inpatient demyelinating disease in adults were sufficiently predictive for vaccine safety analyses to rely on unverified diagnosis codes. Adverse event misclassification should be accounted for in statistical power calculations.

Predictive value of ICD-9-CM codes used in vaccine safety research

OBJECTIVES

To assess how well selected ICD-9-CM diagnosis codes predict adverse events; to model bias and power loss when vaccine safety analyses rely on unverified codes.

METHODS

We extracted chart verification data for ICD-9-CM diagnosis codes from six Vaccine Safety Datalink (VSD) publications and modeled biases and power losses using positive predictive value (PPV) estimates and ranges of code sensitivity.

RESULTS

Positive predictive values were high for type 1 diabetes (80%) in children, relative to WHO criteria, and intussusception (81%) in young children, relative to a standard published case definition. PPVs were moderate (65%) for inpatient and emergency department childhood seizures and low (21%) for outpatient childhood seizures, both relative to physician investigator judgment. Codes for incident central nervous system demyelinating disease in adults had high PPV for inpatient codes (80%) and low PPV for outpatient codes (42%) relative to physicians' diagnoses. Modeled biases were modest, but large increases in frequencies of adverse events are required to achieve adequate power if unverified ICD-9-CM codes are used, especially when vaccine associations are weak.

CONCLUSIONS

ICD-9-CM codes for type 1 diabetes in children, intussusception in young children, childhood seizures in inpatient and emergency care settings, and inpatient demyelinating disease in adults were sufficiently predictive for vaccine safety analyses to rely on unverified diagnosis codes. Adverse event misclassification should be accounted for in statistical power calculations.

Vaccines and autism: evidence does not support a causal association

A suggested association between certain childhood vaccines and autism has been one of the most contentious vaccine safety controversies in recent years. Despite compelling scientific evidence against a causal association, many parents and parent advocacy groups continue to suspect that vaccines, particularly measles-mumps-rubella (MMR) vaccine and thimerosal-containing vaccines (TCVs), can cause autism.

Immunity to tetanus is protective against the development of multiple sclerosis

Following allegations that Hepatitis B vaccination causes or triggers multiple sclerosis (MS), several epidemiological studies have been conducted to evaluate the association between MS and vaccination. In one study conducted in the US, a significant protective effect on the development of MS was observed for tetanus immunization. We reviewed the medical literature and found two additional recent studies, as well as several older studies, which also observed a significant protective effect of tetanus immunization on the development or progression of MS. Furthermore, decreased humoral and cellular immunity to tetanus toxoid has been observed among MS patients. We postulate that naturally acquired or vaccine-induced immunity to tetanus has a protective effect against the development and progression of MS. We also postulate that this link to tetanus is in part responsible for the gender, age, geographic and socio-economic distribution of MS, as well as its pattern among migrants. The biological basis for this protective effect could be an unspecific boost of bystander suppression of auto-immunity as shown for other infections. Our hypothesis can be tested in several ways. The simplest approach would be to compare tetanus exposure and MS occurrence on a population level. Stronger support would come from the re-analysis of previous studies that have information at the individual level on both tetanus exposure, whether induced or natural, and on the development of MS. Laboratory evidence could be sought by testing the effect of tetanus toxoid on experimental allergic encephalomyelitis, the experimental animal model of MS.

Medically recorded allergies and the risk of childhood acute lymphoblastic leukaemia

Data on five allergic conditions were abstracted from the medical records of 180 cases of childhood acute lymphoblastic leukaemia (ALL) and 718 matched controls. Odds Ratios (OR) and 95% Confidence Intervals (CI) were estimated for composite variables and for individual allergies using conditional logistic regression modelling. Allergies were divided into late and early diagnoses (those made within the year before the matched case's ALL diagnosis and those made earlier, respectively). Among the early diagnoses, atopy or hives was significantly associated with ALL (OR=2.20; 95% CI: 1.16-4.16). Significant associations were found for late diagnoses of atopy or hives (OR=3.78; 95% CI: 1.00-14.29) and of asthma (OR=3.10; 95% CI: 1.39-6.95). None of the other allergic conditions were associated with ALL. These results are contrary to those of prior studies of childhood ALL and allergy.

Quality assessments of HMO diagnosis databases used to monitor childhood vaccine safety

OBJECTIVE

To assess the quality of automated diagnoses extracted from medical care databases by the Vaccine Safety Datalink (VSD) study.

METHODS

Two methods are used to assess quality of VSD diagnosis data. The first method compares common automated and abstracted diagnostic categories ("outcomes") in 1-2% simple random samples of study populations. The second method estimates positive predictive values of automated diagnosis codes used to identify potential cases of rare conditions (e.g., acute ataxia) for inclusion in nested case-control medical record abstraction studies.

RESULTS

There was good agreement (64-68%) between automated and abstracted outcomes in the 1-2% simple random samples at 3 of the 4 VSD sites and poor agreement (44%) at 1 site. Overall at 3 sites, 56% of children with automated cerebella ataxia codes (ICD-9 = 334) and 22% with "lack of coordination" codes (ICD-9 = 781.3) met objective clinical criteria for acute ataxia.

CONCLUSIONS

The misclassification error rates for automated screening outcomes substantially reduce the power of screening analyses and limit usefulness of screening analyses to moderate to strong vaccine-outcome associations. Medical record verification of outcomes is needed for definitive assessments.

Allogeneic bone marrow transplantation (BMT) for adults with acute lymphoblastic leukemia (ALL): predictive role of minimal residual disease monitoring on relapse

We developed a PCR-based method to monitor clonogenic IgH VDJ rearrangement as a possible predictor of relapse in patients with acute B-ALL after allogeneic bone marrow transplantation (BMT). We studied 23 patients at diagnosis, before and after BMT. At the time of BMT, 13 patients were in first complete remission, eight in second complete remission and two in relapse. Four patients were PCR negative before BMT and remained PCR negative also after BMT (-/- pattern). They are still in remission after a median follow-up of 41 months. Nineteen patients were MRD-positive before BMT: three were PCR negative at first determination after BMT (+/- pattern) and maintain remission. Sixteen patients were PCR-positive at first determination after BMT (+/+ pattern): five became PCR negative (+/+/- pattern) (four with chronic graft-versus-host disease (GVHD) and two after donor lymphocyte infusions (DLI)). Nine patients remained PCR-positive (+/+/+ pattern) (four remain in remission, and six relapsed); two patients died before transplant. In conclusion, PCR negative patients before BMT remained negative post-BMT; many pre-BMT positive patients had initial MRD positivity after BMT: 37% of them achieved a molecular remission with cGVHD or DLI.

Autism and measles-mumps-rubella vaccination: controversy laid to rest?

It has been suggested that vaccination, particularly with measles-mumps-rubella (MMR) vaccine, may be related to the development of autism. The main evidence for a possible association is that the prevalence of autism has been increasing at the same time that infant vaccination coverage has increased, and that in some cases there is an apparent temporal association in which autistic characteristics are first noted shortly after vaccination. Although the prevalence of autism and similar disorders appears to have increased recently, it is not clear if this is an actual increase or the result of increased recognition and changes in diagnostic criteria. The apparent onset of autism in close proximity to vaccination may be a coincidental temporal association. The clinical evidence in support of an association derives from a series of 12 patients with inflammatory bowel conditions and regressive developmental disorders, mostly autism. The possibility that measles vaccine may cause autism through a persistent bowel infection has generated much interest, since it provides a possible biological mechanism. Epidemiological studies, however, have not found an association between MMR vaccination and autism. The epidemiological findings are consistent with current understanding of the pathogenesis of autism, which has a strong genetic component and in which the neurological defects probably occur early in embryonic development. It seems unlikely that a vaccination that is given after birth could cause autism. A minority of cases of autism may have onset after 1 year of age (regressive autism), but the single epidemiological study that included such cases did not find an association with MMR vaccination. Currently, the weight of the available epidemiological and related evidence does not support a causal association between MMR vaccine, or any other vaccine or vaccine constituent, and autism.

Childhood vaccinations, vaccination timing, and risk of type 1 diabetes mellitus

OBJECTIVES

To evaluate suggested associations between childhood vaccinations, particularly against hepatitis B and Haemophilus influenzae type b, and risk of developing type 1 diabetes; and to determine whether timing of vaccination influences risk.

METHODS

We conducted a case-control study within 4 health maintenance organizations (HMOs) that participate in the Vaccine Safety Datalink project of the Centers for Disease Control and Prevention. Study eligibility was restricted to children who met the following criteria: 1) born during 1988 through 1997; 2) HMO member since birth; 3) continuously enrolled for first 6 months of life; and 4) at least 12 months of HMO membership before diabetes incidence date (or index date for controls) unless incidence date was before 12 months of age. All 4 HMOs maintain registries of their members who have diabetes, and we used the registries to identify potential cases of diabetes. We conducted chart reviews to verify that potential cases met the World Health Organization epidemiologic case definition for type 1 diabetes mellitus (ie, a physician's diagnosis of diabetes plus treatment with daily insulin injections). We defined the incidence date of diabetes as the first date that the child received a diagnosis of diabetes. We attempted to match 3 controls to each case. Controls had the same eligibility criteria as cases and were matched to individual cases on HMO, sex, date of birth (within 7 days), and length of health plan enrollment (up to the incidence or index date). The index date for controls was defined as the incidence date of the case to which the control was matched. Chart abstraction was performed by trained chart abstractors using standardized forms. In addition to complete vaccination histories, the chart abstraction forms for both cases and controls included information on sociodemographic characteristics, selected medical conditions, history of breastfeeding, and family medical history. We used conditional logistic regression to estimate the odds ratio (OR) of diabetes associated with vaccination, with vaccine exposure defined as before the diabetes incidence date (or index date for controls).

RESULTS

Two hundred fifty-two confirmed cases of diabetes and 768 matched controls met the study eligibility criteria. The OR (95% confidence interval) for the association with type 1 diabetes was 0.28 (0.07-1.06) for whole cell pertussis vaccine (predominantly in combination as diphtheria, tetanus toxoids and pertussis vaccine), 1.36 (0.70-2.63) for measles-mumps-rubella, 1.14 (0.51-2.57) for Haemophilus influenzae type b, 0.81 (0.52-1.27) for hepatitis B vaccine, 1.16 (0.72-1.89) for varicella vaccine, and 0.92 (0.53-1.57) for acellular pertussis-containing vaccines. Compared with children who had not received hepatitis B vaccine, the OR of diabetes was 0.51 (0.23-1.15) for children vaccinated at birth and 0.86 (0.54-1.35) for those first vaccinated against hepatitis B at 2 months of age or later. Race and ethnicity and family history of diabetes were independently associated with risk of type 1 diabetes, but adjustment for these factors did not materially alter the ORs for any of the vaccines.

CONCLUSIONS

In this large, population-based, case-control study, we did not find an increased risk of type 1 diabetes associated with any of the routinely recommended childhood vaccines. Our study adds to previous research by providing data on newer vaccines, including hepatitis B, acellular pertussis, and varicella vaccines. For the older vaccines, our results are generally in agreement with previous studies in not finding any increased risks. Ours is the first epidemiologic study to evaluate the possibility that timing of vaccination is related to risk of clinical diabetes in children. Our results on hepatitis B vaccine do not support the hypothesis; risk of type 1 diabetes was not different between infants vaccinated at birth and those who received their first vaccination later in life. The results of our study and the preponderance of epidemiologic evidence do not support an association between any of the recommended childhood vaccines and an increased risk of type 1 diabetes. Suggestions that diabetes risk in humans may be altered by changes in the timing of vaccinations also are unfounded.

Ability to trace migrant farmworkers ten years after initial identification in a Northern State (Wisconsin)

BACKGROUND

Migrant farmworkers have rarely been included in epidemiologic studies. To assess the feasibility of following farmworkers over extended periods, a critical feature of many study designs, we attempted to trace a sample of Mexican-American farmworkers identified in a clinic in Wisconsin.

METHODS

We randomly chose 100 farmworkers from a migrant health center registration list for 1984-85. In 1995, we searched recent clinic records, made telephone calls, and visited migrant camps to find these farmworkers in Wisconsin during the growing season. We also attempted to find 46 farmworkers at their homes in southwest Texas over a two-week period in 1996 using the address listed in the clinic records, local phone books, and conversations with next-door neighbors.

RESULTS

Although 25 farmworkers had reregistered at the clinic in recent years, we found only 6 of them in Wisconsin in 1995. In southwest Texas, we either located or ascertained information about the vital status of 25 of the 46 farmworkers (54%).

CONCLUSIONS

Tracing efforts must include extensive contacts in farmworkers' home states and must incorporate a variety of information sources. Tracing farmworkers in epidemiologic studies appears to be feasible but requires more intensive methods over longer periods of time than those used in this study.

Safety and immunogenicity of varying dosages of trivalent inactivated influenza vaccine administered by needle-free jet injectors

To evaluate the perceived pain, other adverse events, and immunogenicity of influenza virus vaccine administered by needle-free jet injector (JI) compared with that of vaccine administered by needle and syringe (N&S), we randomly assigned 304 healthy young adults to receive one of three dosages (0.5, 0.3, or 0.2 ml) of the 1998-1999 season vaccine administered by either of two JI devices or by N&S. In multivariate analysis, female gender and JI administration were associated with higher levels of pain reported at the time of vaccination as well as with the occurrence of local injection site reactions following vaccination. Immune response did not vary significantly by dosage but administration by one JI device was associated with higher post-vaccination H1N1 antibody titers.

The risk of seizures after receipt of whole-cell pertussis or measles, mumps, and rubella vaccine

BACKGROUND

The administration of the diphtheria and tetanus toxoids and whole-cell pertussis (DTP) vaccine and measles, mumps, and rubella (MMR) vaccine has been associated with adverse neurologic events, including seizures. We studied the relation between these vaccinations and the risk of a first seizure, subsequent seizures, and neurodevelopmental disability in children.

METHODS

This cohort study was conducted at four large health maintenance organizations and included reviews of the medical records of children with seizures. We calculated the relative risks of febrile and nonfebrile seizures among 679,942 children after 340,386 vaccinations with DTP vaccine, 137,457 vaccinations with MMR vaccine, or no recent vaccination. Children who had febrile seizures after vaccination were followed to identify the risk of subsequent seizures and other neurologic disabilities.

RESULTS

Receipt of DTP vaccine was associated with an increased risk of febrile seizures only on the day of vaccination (adjusted relative risk, 5.70; 95 percent confidence interval, 1.98 to 16.42). Receipt of MMR vaccine was associated with an increased risk of febrile seizures 8 to 14 days after vaccination (relative risk, 2.83; 95 percent confidence interval, 1.44 to 5.55). Neither vaccination was associated with an increased risk of nonfebrile seizures. Analyses of automated data alone gave results similar to the analyses of the data from medical-record reviews. The number of febrile seizures attributable to the administration of DTP and MMR vaccines was estimated to be 6 to 9 and 25 to 34 per 100,000 children, respectively. As compared with other children with febrile seizures that were not associated with vaccination, the children who had febrile seizures after vaccination were not found to be at higher risk for subsequent seizures or neurodevelopmental disabilities.

CONCLUSIONS

There are significantly elevated risks of febrile seizures on the day of receipt of DTP vaccine and 8 to 14 days after the receipt of MMR vaccine, but these risks do not appear to be associated with any long-term, adverse consequences.

The Vaccine Safety Datalink project

The Vaccine Safety Datalink (VSD) is a collaborative project between the National Immunization Program of the Centers for Disease Control and Prevention (CDC) and several large health maintenance organizations (HMOs) in the United States. The project began in 1990 with the primary purpose of rigorously evaluating concerns about the safety of vaccines. Computerized data on vaccination, medical outcome (e.g. hospital discharge, outpatient visits, emergency room visits, and deaths), and covariate data (e.g. birth certificates and census) are prospectively collected at multiple HMOs (initially four) and linked under joint protocol for analyses. Approximately 6 million people (2% of the US population) are members of HMOs participating in the VSD. The VSD has proven to be a valuable resource that has provided important information on a number of vaccine safety issues. The databases and infrastructure created for the VSD have also provided opportunities to address other immunization questions including vaccination coverage and cost-effectiveness. In a recent investigation of intussusception following rotavirus vaccination, the VSD methodology was expanded to include 10 managed care organizations. A cohort study was conducted that allowed estimation of incidence rates of intussusception and attributable risks associated with rotavirus vaccine.

Challenges and controversies in immunization safety

No vaccine is perfectly safe or effective. As diseases such as diphtheria and polio fade, vaccine safety concerns, especially alleged links between vaccinations and several chronic illnesses, have become increasingly prominent in the media and to the public. This article reviews the current scientific evidence on several recent vaccine safety controversies. It also provides information on how various safety research is conducted, some of the concurrent challenges, and finally, some guidance on communicating with patients on vaccine risks.

Short- and long-term survival after cardiopulmonary resuscitation

BACKGROUND

The objective was to evaluate the effect of patient characteristics and other factors on cardiopulmonary resuscitation (CPR) survival, hospital discharge survival and function, and long-term survival.

METHODS

All patients 18 years and older experiencing in-hospital CPR from December 1983 through November 1991 at Marshfield Medical Center (Marshfield Clinic and adjoining St Joseph's Hospital), Marshfield, Wis, were selected. We performed a retrospective medical record review and augmented these data with updated vital status information.

MAIN OUTCOME MEASURES

Cardiopulmonary resuscitation survival, hospital discharge survival and function, and long-term survival.

RESULTS

Of 948 admissions during which CPR was performed, 61.2% of patients survived the arrest and 32.2% survived to hospital discharge. Mechanism of arrest was the most important variable associated with hospital discharge. Patients with pulseless electrical activity had the worst chance of hospital discharge, followed by those with asystole and bradycardia. Follow-up information was available for 298 patients who survived to discharge. One year after hospital discharge, 24.5% of patients, regardless of age, had died. Survival was 18.5% at 7 years in those 70 years or older, compared with 45.4% in those aged 18 to 69 years. Heart rhythm at the time of arrest strongly influenced long-term survival. Bradyarrhythmias produced a nearly 2-fold increased mortality risk compared with normal sinus rhythm.

CONCLUSIONS

Survival until hospital discharge after CPR at our institution during an 8-year period was higher than previously reported for other institutions. Long-term survival after discharge was equal to or higher than reported estimates from other institutions. Hospital admission practices and selection of patients receiving CPR may account for these findings.

Does influenza vaccination exacerbate asthma? Analysis of a large cohort of children with asthma. Vaccine Safety Datalink Team

CONTEXT

Although influenza vaccination is recommended for children with asthma, only a minority are vaccinated. One reason for low influenza vaccine coverage among children with asthma may be concern that influenza vaccination may induce an exacerbation of asthma.

OBJECTIVE

To evaluate the safety of influenza vaccination in children with asthma, we studied the incidence of hospitalizations and emergency department visits for asthma following influenza vaccination.

DESIGN

Retrospective cohort study-analysis of population-based computerized medical and vaccination records.

SETTING

: Four large health maintenance organizations on the West Coast of the United States.

SUBJECTS

Children with asthma 1 through 6 years of age, identified by search of computerized databases of medical encounters and pharmacy prescriptions.

MAIN OUTCOME MEASURES

Exacerbations of asthma.

RESULTS

In unadjusted analyses vaccination was associated with high rates of asthma exacerbations. However, after adjusting for asthma severity using a self-control method, the incidence rate ratios of asthma exacerbations after vaccination were 0.58 (95% confidence interval, 0.36-0.95), 0.74 (95% confidence interval, 0.47-1.17), and 0.98 (95% confidence interval, 0.76-1.27) during the 3 influenza seasons.

CONCLUSIONS

After controlling for asthma severity, we found that influenza vaccination does not result in acute asthma exacerbations in children. Concern about possible exacerbation of asthma is not a valid reason to not vaccinate children with asthma against influenza.

Safety of routine childhood vaccinations. An epidemiological review

Immunisations have been one of the most cost-effective public health interventions in human history. Despite remarkable progress, several challenges face immunisation programs worldwide. Paradoxically, despite vaccines' clear effectiveness in reducing risks of diseases that were previously widely prevalent and caused substantial morbidity and mortality, current vaccination policies have become increasingly controversial due to concerns about vaccine safety. Vaccines, like other pharmaceutical products, are not entirely risk-free. While most known adverse effects are minor and self-limited, some vaccines have been associated with very rare but serious adverse effects. Because such rare effects are often not evident until vaccines come into widespread use, ongoing surveillance programs to monitor vaccine safety are needed. Such monitoring will be essential if the public is to accept the increasing number of new vaccines made possible by biotechnology. The interpretation of data from vaccine safety research is complex and is associated with some uncertainty. Effectively communicating this uncertainty and continuing to improve understanding of rare risks and risk factors are essential for "mature" immunisation programs to maintain public confidence in immunisations.

Influenza vaccination in children with asthma in health maintenance organizations. Vaccine Safety Datalink Team

We assessed vaccination coverage and predictors of influenza vaccination in asthmatic children in four large Health Maintenance Organizations. We studied 68,839 children with asthma at four Health Maintenance Organizations (HMOs) in the 1995-1996 influenza season and 34,032 children at two HMOs in the 1996-1997 influenza season. In both seasons only 9-10% were vaccinated against influenza. Children who were hospitalized, had an emergency department visit for asthma or a prescription for a beta-agonist prior to the influenza season, were more likely to be vaccinated. Overall, 61% of the unvaccinated asthmatic children had made an outpatient clinic visit during months when influenza vaccination would have been appropriate. Vaccination coverage could be increased by taking advantage of all opportunities to vaccinate children with asthma whenever they make clinic visits in the fall and early winter.

Influenza and the rates of hospitalization for respiratory disease among infants and young children

BACKGROUND

Young children may be at increased risk for serious complications from influenzavirus infection. However, in population-based studies it has been difficult to separate the effects of influenzavirus from those of respiratory syncytial virus. Respiratory syncytial virus often circulates with influenzaviruses and is the most frequent cause of hospitalization for lower respiratory tract infections in infants and young children. We studied the rates of hospitalization for acute respiratory-disease among infants and children during periods when the circulation of influenzaviruses predominated over the circulation of respiratory syncytial virus.

METHODS

For each season from October to May during the period from 1992 to 1997, we used local viral surveillance data to define periods in Washington State and northern California when the circulation of influenzaviruses predominated over that of respiratory syncytial virus. We calculated the rates of hospitalization for acute respiratory disease, excess rates attributable to influenzavirus, and incidence-rate ratios for all infants and children younger than 18 years of age who were enrolled in either the Kaiser Permanente Medical Care Program of Northern California or the Group Health Cooperative of Puget Sound.

RESULTS

The rates of hospitalization for acute respiratory disease among children who did not have conditions that put them at high risk for complications of influenza (e.g., asthma, cardiovascular diseases, or premature birth) and who were younger than two years of age were 231 per 100,000 person-months at Northern California Kaiser sites (from 1993 to 1997) and 193 per 100,000 person-months at Group Health Cooperative sites (from 1992 to 1997). These rates were approximately 12 times as high as the rates among children without high-risk conditions who were 5 to 17 years of age (19 per 100,000 person-months at Northern California Kaiser sites and 16 per 100,000 person-months at Group Health Cooperative sites) and approached the rates among children with chronic health conditions who were 5 to 17 years of age (386 per 100,000 person-months and 216 per 100,000 person-months, respectively).

CONCLUSIONS

Infants and young children without chronic or serious medical conditions are at increased risk for hospitalization during influenza seasons. Routine influenza vaccination should be considered in these children.

The Vaccine Safety Datalink: immunization research in health maintenance organizations in the USA

The Vaccine Safety Datalink is a collaborative project involving the National Immunization Program of the Centers for Disease Control and Prevention and several large health maintenance organizations in the USA. The project began in 1990 with the primary purpose of rigorously evaluating concerns about the safety of vaccines. Computerized data on vaccination, medical outcome (e.g. outpatient visits, emergency room visits, hospitalizations, and deaths) and covariates (e.g. birth certificates, census data) are prospectively collected and linked under joint protocol at multiple health maintenance organizations for analysis. Approximately 6 million persons (2% of the population of the USA) are now members of health maintenance organizations participating in the Vaccine Safety Datalink, which has proved to be a valuable resource providing important information on a number of vaccine safety issues. The databases and infrastructure created for the Vaccine Safety Datalink have also provided opportunities to address vaccination coverage, cost-effectiveness and other matters connected with immunization as well as matters outside this field.