The effect of maternal antibody on the serologic response and the incidence of adverse reactions after primary immunization with acellular and whole-cell pertussis vaccines combined with diphtheria and tetanus toxoids

OBJECTIVE

To evaluate the effect of maternally derived antibody on the immunogenicity and reactogenicity of acellular (DTaP) or whole-cell (DTP) pertussis vaccine with diphtheria and tetanus toxoids combined.

METHODS

A total of 2342 infants were randomized to receive one of 13 DTaP or 2 DTP vaccines at 2, 4, and 6 months of age. The correlation between preimmunization and postimmunization antibody after three doses of vaccine and the relation between preimmunization antibody and adverse reactions after the first immunization were modeled by linear regression.

RESULTS

After DTP but not DTaP, higher levels of preexisting antibody were associated with substantial (28% to 56%) reductions in the subsequent antibody response to pertussis toxin (PT). For other pertussis antibodies, modest inverse correlations were seen between preexisting antibody concentrations and most postimmunization antibody responses (resulting in 8% to 18% reductions in postimmunization antibody) for both DTP and DTaP. There was no consistent association in any DTP or DTaP group between adverse reactions and preimmunization antibody levels.

CONCLUSION

The PT antibody response to DTaP, unlike DTP, is not adversely affected by preexisting antibody to PT. Inhibitory effects with respect to other antibodies, seen with both DTP and DTaP, were relatively modest. Our data suggest that the use of acellular pertussis vaccines in adults, which could confer higher levels of antibody in women before pregnancy, would be unlikely to adversely affect pertussis antibody responses after DTaP among infants born to mothers with high antibody levels.

Comparison of 13 acellular pertussis vaccines: overview and serologic response

OBJECTIVE

To compare the immunogenicity of a licensed conventional whole-cell (WCL) and 13 diphtheria-tetanus-acellular pertussis (DTaP) vaccines that differed in source, method of manufacture, and included antigens; all vaccines included diphtheria and tetanus toxoids.

METHODS

Healthy infants were enrolled through six university-based vaccine and treatment evaluation units and were randomized to receive one of the study vaccines at 2, 4, and 6 months of age. Sera were obtained before the first immunization and 1 month after the third immunization and were analyzed for antibody to pertussis toxin (PT), filamentous hemagglutinin, fimbriae, pertactin, and diphtheria and tetanus toxins. Chinese hamster ovary cell toxin neutralization assays were performed, and levels of agglutinating antibodies were determined.

RESULTS

Of 2342 infants enrolled, 1942 contributed usable preimmunization and postimmunization serum specimens. Each vaccine produced significant increases in antibodies directed against the included antigens; postimmunization antibody titers differed significantly among the DTaP vaccines. For each evaluated antigen, the majority of DTaP vaccines produced antibody responses that equaled or exceeded those produced by WCL. For some antigens (eg, PT), mean antibody levels by vaccine correlated poorly with the quantity of antigen included in each vaccine; for others (eg., fimbriae), there was a close correlation.

CONCLUSION

Although serologic correlates of pertussis immunity are not defined, it is clear that DTaP vaccines can stimulate immune responses that exceed those of licensed whole-cell vaccine with respect to the measured antibodies. Particularly for PT, immunogenicity seems to depend on factors in addition to antigen concentration, possibly including antigen derivation and formulation. No DTaP was most or least immunogenic with respect to all included antigens.

Association of reactions after consecutive acellular or whole-cell pertussis vaccine immunizations

OBJECTIVE

To evaluate the relative frequency of adverse reactions after initial and subsequent immunizations among infants receiving primary immunization with acellular (DTaP) or whole-cell (DTP) pertussis vaccine with diphtheria and tetanus combined.

METHODS

We examined the occurrence of common reactions in 2127 infants within 48 hours after immunization at 2, 4, and 6 months with one of 13 DTaP or with Lederle DTP (WCL). Data on at least two consecutive immunizations were available for 357 WCL recipients and 1770 DTaP recipients. For these analyses, reactions evaluated included fever of 100.4 degrees F (38 degrees C) or greater, redness of 21 mm or larger, swelling of 21 mm or larger, moderate or severe pain, moderate or severe fussiness, loss of appetite, drowsiness, and vomiting.

RESULTS

With one exception, reactions were approximately 1.5 to 8 times more likely to occur in WCL recipients if the same reaction had been observed at the previous immunization (the single exception was redness after the second immunization). Both initial and repeated reactions were less likely in DTaP than in WCL recipients. As with WCL recipients, risks of repeated reactions in DTaP recipients were higher than the risks of initial reactions (from 2.5 to 24 times as high).

CONCLUSION

Reactions after a second or third immunization with either WCL or DTaP vaccine are more likely to occur in infants who had the same reaction after the preceding immunization. Absolute risks of repeated reactions tended to be lower after DTaP vaccine than after the WCL vaccine.

A randomized comparison of reactogenicity and immunogenicity of two whole-cell pertussis vaccines

OBJECTIVE

To compare prospectively the reactogenicity and immunogenicity of two licensed whole-cell pertussis vaccines.

METHODS

We conducted a prospective, randomized, double-blinded assessment of two licensed whole-cell pertussis vaccines with diphtheria and tetanus toxoids that were included in a multicenter trial evaluating 13 acellular pertussis vaccines. Infants were immunized at 2, 4, and 6 months of age with a single lot of Lederle (309 infants) or Massachusetts Public Health Biologic Laboratories (MPHBL; 94 infants) vaccine.

RESULTS

The group receiving the Lederle vaccine demonstrated significantly higher antibody titers to pertussis toxin by enzyme-linked immunosorbent assay (ELISA) and by the Chinese hamster ovary cell pertussis toxin neutralization assay, and to fimbrial antigens by ELISA, as well as higher mean agglutinin titers. In contrast, the group receiving the MPHBL vaccine demonstrated higher ELISA antibody levels to filamentous hemagglutinin and pertactin. Similar differences were observed in the proportions of vaccinees seroconverting to these antigens. Rates of systemic and local reactions were relatively low for both vaccines. Although the Lederle product had substantially lower reactogenicity in this study than previously reported for that vaccine, the MPHBL vaccine was significantly less reactogenic in nearly all clinical categories.

CONCLUSION

The two whole-cell vaccines demonstrated statistically significant differences in postimmunization antibody levels to all six evaluated pertussis antigens. Whether these statistically significant differences in antibody levels have clinical relevance is not clear. Rates of nearly all local and systemic reactions were significantly lower among the MPHBL group than the Lederle group. Licensed whole-cell diphtheria-tetanus-pertussis vaccines produced by different manufacturers cannot be assumed to be similar in reactogenicity or immunogenicity.

Defining the key parameters for comparing reactions among acellular and whole-cell pertussis vaccines

OBJECTIVE

To facilitate future vaccine reaction data collection and analysis, we sought to determine the minimum data set required to describe accurately and to compare common reactions after the administration of acellular (DTaP) or whole-cell (DTP) pertussis vaccine with diphtheria and tetanus toxoids combined.

METHODS

Thirteen DTaP and 2 DTP vaccines were studied in a multicenter trial involving 2342 infants who received a primary series of vaccinations at 2, 4, and 6 months of age. Temperature, fussiness, redness, swelling and pain at the injection site, antipyretic use, drowsiness, loss of appetite, and vomiting were evaluated. Reactions were assessed at 3 hours and (if not immunized in the evening) 6 hours after immunization, at bedtime each evening for 7 evenings, and on the 14th evening after immunization.

RESULTS

Two reaction assessment approaches were compared: (1) analysis of all reactions, regardless of the degree of severity; and (2) a condensation of the data to five key reactions (fever > 100 degrees F, moderate or more fussiness, any local redness, any local swelling, and moderate or more local pain). We found that the onset of reactions was infrequent beyond the second evening, and that collection and analysis of reaction data beyond that time did not further discriminate among the vaccines. Information regarding antipyretic use, loss of appetite, drowsiness, or vomiting did not assist in differentiating among these vaccines.

CONCLUSION

Monitoring the occurrence of fever greater than 100 degrees F, moderate or severe fussiness, injection site redness or swelling, and moderate or severe injection site pain occurring through the second evening after immunization will provide the minimum data set needed to discriminate among DTaP and DTP vaccines with respect to the common adverse reactions.

Simultaneous administration of Haemophilus influenzae type b vaccine with acellular or whole-cell pertussis vaccine: effects on reactogenicity and immune responses to pertussis vaccines

OBJECTIVE

To evaluate the effect of simultaneous Haemophilus influenzae type b conjugate (Hib) vaccination on the safety and immunogenicity of selected acellular (DTaP) and whole-cell (DTP) pertussis vaccines with diphtheria and tetanus toxoids combined.

METHODS

Enrollment of infants into a large multicenter study of the safety and immunogenicity of 13 DTaP and 2 DTP vaccines was partially completed when the first Hib vaccine, HbOC (Haemophilus b oligosaccharide conjugate vaccine), was licensed for use in infants. Thereafter, at each immunization most infants received HbOC simultaneously with DTaP (or DTP), administered in opposite thighs. Postvaccination geometric mean titers or concentrations (GMTs) of pertussis antibodies as measured by six different assays were compared pairwise among groups of infants receiving 0, 1, 2, or 3 simultaneous HbOC immunizations. The incidence of reactions was compared between infants who received only DTaP or DTP and those who received HbOC simultaneously.

RESULTS

Comparison of postvaccination GMTs was possible among groups of infants receiving different numbers of simultaneous immunizations for 10 of the 13 DTaP and both DTP vaccines. Increased HbOC exposure had no consistent dose-response effect on antibody titers for DTaP or DTP vaccines in any assay. Significant differences between groups in postvaccination GMTs were observed with 4 DTaP vaccines in 1 to 2 assays each; the GMTs were higher with increasing HbOC exposure for 2 DTaP vaccines and lower for 2 others. There was no significant increase in reactions with simultaneous HbOC and DTaP immunization.

CONCLUSIONS

Based on these retrospective analyses, there did not seem to be an interference in pertussis immunogenicity or alteration in reactogenicity associated with the simultaneous administration of HbOC and DTaP. These findings are encouraging with respect to the development of DTaP-Hib combination vaccines.

Comparison of 13 acellular pertussis vaccines: adverse reactions

OBJECTIVE

To compare the reactogenicity of a licensed conventional whole-cell (WCL) and 13 acellular pertussis vaccines that differed in the source, manufacture, and quantity of included antigens; all vaccines included diphtheria and tetanus toxoids.

METHODS

Healthy infants were enrolled through six university-based vaccine and treatment evaluation units and were randomized to receive one of the study vaccines at 2, 4, and 6 months of age. Parents recorded the occurrence of fever, redness, swelling, pain, fussiness, drowsiness, anorexia, and use of antipyretics for 2 weeks after each inoculation; nurses interviewed parents on the third day and at each succeeding visit; long-term follow-up information was collected from parents and medical records 1 year after the third immunization.

RESULTS

Of 2200 vaccinated infants, 2189 contributed reaction data after 6375 vaccinations. For every acellular vaccine, every monitored reaction except vomiting occurred at a significantly lower frequency and severity than was seen with WCL. The groups receiving acellular pertussis vaccines differed significantly with respect to redness, swelling, pain, and vomiting, but not with respect to fussiness, antipyretic use, drowsiness, or anorexia.

CONCLUSION

Although there were differences among the acellular vaccines, none was consistently the most or least reactogenic; all were associated with substantially fewer and less severe adverse reactions than a standard commercial whole-cell vaccine. Selection of acellular vaccines for further development and for introduction into efficacy trials can give priority to assessments of immunogenicity and purity, with comparative reactogenicity a secondary consideration.

OSHA inspections. Workplace inspections

The Occupational Safety and Health Act of 1970 requires that every worker be provided with a safe and healthful workplace and authorizes the Occupational Safety and Health Administration (OSHA) to conduct workplace inspections. OSHA conducts workplace inspections in hospitals and checks for compliance with the Bloodborne Pathogens Standard, the Enforcement Policy and Procedures for Occupational Exposure to Tuberculosis, and the Hazardous Chemicals Standards, among others. The hospital epidemiologist bears considerable responsibility for developing and implementing plans to protect employees from occupational exposures to infectious hazards such as bloodborne pathogens and tuberculosis. To prepare for an inspection, the hospital epidemiologist must understand the basis on which OSHA operates and must proceed in a thoughtful, coordinated manner.

Respiratory protection standard: comments on OSHA's proposed revision

On November 15, 1994, the Occupational Safety and Health Administration (OSHA) published in the Federal Register (59:58884-58956) the draft of a proposed revision of the Respiratory Protection Standard. One of OSHA's oldest standards, the Respiratory Protection Standard defines the conduct of the employer (eg, hospital) with respect to respirator training, fit testing, medical examinations, use, storage, and so on. The proposed revision appears to have been drafted with no consideration for its effect on healthcare workers or the healthcare industry. SHEA has prepared the following comments to OSHA, which have been submitted to the docket and will be presented at public hearings later this month.

Pertussis infection in adults with persistent cough

OBJECTIVE

To determine the prevalence of Bordetella pertussis infection in adult patients with persistent cough.

DESIGN

Prospective case series.

SETTING

Urban university hospital emergency department.

PATIENTS

Convenience sample of 75 patients aged 18 years or older with a cough lasting 2 weeks or longer. Serum specimens from 67 patients without respiratory complaints were used to develop reference values.

INTERVENTIONS

In patients with cough, nasopharyngeal culture and direct fluorescent antibody testing for B pertussis were performed and serum samples were obtained at the first visit and 1 month later. Serum specimens were assayed for antibody to pertussis toxin (PT) and filamentous hemagglutinin (FHA).

MAIN OUTCOME MEASURES

A subject with one or more of the following was defined as having a pertussis infection: a positive B pertussis culture result, a four-fold change in PT or FHA titer, and/or a single PT or FHA titer at least 2 SDs greater than the geometric mean of the control group.

RESULTS

No subject tested culture positive for B pertussis. Sixteen (21%) (95% confidence interval [CI], 13% to 32%) of 75 subjects met the serologic criteria for pertussis infection; for 13 (81%; 95% CI, 54% to 96%) of the 16, the criteria were met by the initial serum specimen. In contrast, the geometric mean levels of antibody to PT and FHA for the remaining 59 subjects with cough did not differ from those of the control group. Clinical symptoms and the lymphocyte count did not differentiate patients with pertussis from those without the disease.

CONCLUSION

Pertussis is a common cause of persistent cough in adults and should be considered in the differential diagnosis. Clinical symptoms, pertussis culture, direct fluorescent antibody testing, and lymphocytosis are of limited value in making the diagnosis.

Interchangeability of conjugated Haemophilus influenzae type b vaccines in infants

OBJECTIVE

To evaluate the safety and immunogenicity of two Haemophilus influenzae type b (Hib) conjugate vaccines when administered in serial combination. These vaccines consisted of Hib capsular polysaccharide polyribosyl-ribitol phosphate (PRP) conjugated to the meningococcal outer membrane protein (OMP) complex (PRP-OMP) and H influenzae oligosaccharide conjugated to a mutant toxin (CRM197) isolated from Corynebacterium diphtheriae (HbOC).

DESIGN

Randomized, double-blind, clinical trial evaluating five Hib vaccination regimens.

SETTING

Vaccine Treatment and Evaluation Units and affiliated private pediatric practices at Saint Louis (Mo) University, Vanderbilt University, Nashville, Tenn, and Baylor College of Medicine, Houston, Tex.

PATIENTS

A total of 497 healthy 2-month-old infants scheduled to receive routine immunization.

INTERVENTION

Participants received either PRP-OMP or HbOC given as recommended by the manufacturer, PRP-OMP at 2 and 6 months, HbOC at 2 months, then PRP-OMP at 4 and 6 months, or PRP-OMP at 2 months and then HbOC at 4 and 6 months. Unconjugated PRP was given at 15 months to evaluate priming.

RESULTS

Geometric mean antibody concentrations differed significantly among the groups following the second and third immunizations of the primary series and following booster immunization with unconjugated PRP. On each occasion, the groups receiving serial combinations of PRP-OMP and HbOC achieved mean antibody concentrations that equalled or exceeded those of the groups receiving a single product. Adverse reactions did not vary by group.

CONCLUSIONS

The studied sequential combinations of Hib vaccines were safe and at least as immunogenic as either vaccine alone.

Pertussis seroprevalence in emergency department staff

STUDY OBJECTIVE

The purpose of this study was to look at the prevalence of pertussis immunity in emergency department employees.

DESIGN

Prospective, observational study conducted in September 1992.

SETTING

A university hospital ED.

PARTICIPANTS

A convenience sample of 73 ED employees.

INTERVENTIONS

Blood was collected from ED staff and assayed for antibody to pertussis toxin and filamentous hemagglutinin.

RESULTS

Ninety-nine percent of the subjects reported childhood immunization. Geometric mean titers were 3.6 (geometric SD, 3.2) enzyme-linked immunosorbent assay (ELISA) units for pertussis toxin and 12.1 (geometric SD, 3.3) ELISA units for filamentous hemagglutinin. These titers are comparable to those previously reported in other adults not immunized since childhood and are substantially lower than the levels commonly seen in children or adults following immunization. Titers did not differ significantly between male and female subjects or between subjects with or without a recent prolonged cough, family members with a cough, or small children in the household.

CONCLUSION

Most ED staff members have low levels of antibody to pertussis and may be at risk for acquiring the disease from infected children or adults, with subsequent risk of transmission to susceptible patients. Booster immunization with an acellular pertussis vaccine has been shown to be safe and immunogenic in adults and may be appropriate for adult ED personnel to reduce these risks.

Enhanced antibody response in Venezuelan infants immunized with Haemophilus influenzae type b-tetanus toxoid conjugate vaccine

The safety and immunogenicity of primary immunization at 2, 4 and 6 months of age with Haemophilus influenzae type b capsular polysaccharide conjugated to tetanus toxoid (PRP-T; Act-HIB) were evaluated in infants in Valencia, Venezuela. In order better to assess reactions to PRP-T, subjects received their initial PRP-T vaccine a mean of 6.5 days after their initial diphtheria-tetanus-pertussis (DTP) vaccine. The PRP-T vaccine was well tolerated. Serum was obtained at ages 2 and 7 months (before the first and 1 month after the third PRP-T dose). Antibody responses were compared with those from Nashville infants who had received PRP-T and DTP simultaneously in a previous trial. The preimmunization titers in the Venezuelan and Nashville infants did not differ. The geometric mean postimmunization titer in the Venezuelan infants was 37.9 micrograms/ml, as compared with 3.63 micrograms/ml in the Nashville infants (P < 0.00001). Possible explanations for the exceptional antibody response of these Venezuelan infants to PRP-T include carrier priming caused by prior DTP immunization, synergy associated with the specific DTP vaccine used, preimmunization immunologic experience that differed from their United States counterparts and genetic differences that altered response to the vaccines. Further studies are proposed to evaluate these possibilities.

Serologic response to standard inactivated influenza vaccine in human immunodeficiency virus-infected children

We compared the serologic response of 46 human immunodeficiency virus (HIV)-infected children and adolescents and 61 age-matched controls to standard trivalent inactivated influenza vaccine (A/Taiwan (H1N1), A/Shanghai (H3N2), B/Yamagata). Children were immunized according to the package insert recommendations before the 1990 to 1991 influenza season. Serum antibody titers to influenza A were determined before and 1 month after each vaccination and compared for study and control subjects. Serologic responses of HIV-infected participants were correlated with absolute CD4 counts and stage of HIV disease. Regardless of age or HIV status, all groups responded with significant increases in antibody to the influenza A strains (range, 2.1-fold to 11.8-fold), with the exception that antibody to H3N2 rose only 1.5-fold (P = 0.058) among HIV-positive subjects > or = 9 years old. Pre- and postimmunization antibody titers were significantly higher for controls than for HIV-positive subjects. There was no correlation between serologic responses and CD4 counts among HIV-infected subjects, but those with Centers for Disease Control and Prevention-defined acquired immunodeficiency syndrome responded significantly less well to vaccine. We conclude that HIV-infected children and adolescents produce significant antibody rises after inactivated influenza A vaccination but that their absolute antibody concentrations are lower than those seen in age-matched controls.

Acellular and whole-cell pertussis vaccines as booster doses: a multicenter study

OBJECTIVE

To compare the safety and immunogenicity of a variety of acellular (AC) and whole-cell (WC) pertussis vaccines combined with diphtheria and tetanus toxoids.

METHODS

Standard enrollment and reaction forms were used at five sites, and serologic evaluation was performed at a single site. Nine AC (Massachusetts Public Health Laboratories, Biocine Sclavo recombinant pertussis toxoid [PT], Connaught/BIKEN, Lederle three-component, Biocine Sclavo recombinant three-component, SmithKline Beecham three-component, Porton three-component, Takeda-Wyeth, and Connaught multicomponent), and three WC (Connaught Laboratories, Lederle Laboratories, and Massachusetts Public Health Laboratories) were studied. All AC contained varying concentrations of PT; some vaccines also contained filamentous hemagglutinin (FHA), pertactin, and/or agglutinogens.

RESULTS

Two hundred forty children, aged 16 to 21 months and 4 to 6 years, were enrolled at five sites. Significantly less fever, redness, swelling, pain, limp, and use of pain medication were noted following AC compared with WC. Significant increases in antibody to PT were seen following all vaccines. Significant rises in FHA antibody were seen following all WC and the seven AC that contained FHA. Postbooster PT antibody levels were similar among the AC groups, regardless of the amount of PT administered (between 3.5 and 25 micrograms per dose). The dose of FHA did not affect PT antibody response. Infants primed with WC who were boosted with a monocomponent PT vaccine did not manifest a significant antibody response to FHA.

CONCLUSION

The rate of adverse reactions was not a function of the number of antigens or the antigen quantity in the acellular vaccines, and antibody responses following AC were similar or better than antibody responses following WC. These results support the further evaluation of these vaccines in a larger National Institute of Allergy and Infectious Diseases-sponsored study in infants.

Continuous quality improvement and hospital epidemiology: common themes

Recently, many have begun to explore the possibility of using analytical methods associated with continuous quality improvement in the efforts to improve health care. Central to this paradigm is the proposition that such analytical methods can be used to identify and react to variations in the processes and outcomes of care. These methods and principles are remarkably similar to those employed by hospital epidemiologists. This article explores the similarities between these fields in an attempt to bring together leading thinkers in both disciplines.

Responses of children to booster immunization with their primary conjugate Haemophilus influenzae type B vaccine or with polyribosylribitol phosphate conjugated with diphtheria toxoid

Children primed with one of four conjugate Haemophilus influenzae type b vaccines received booster immunization with their primary vaccine or with polyribosylribitol phosphate conjugated with diphtheria toxoid. The latter vaccine produced postbooster antibody levels that equaled or exceeded those produced by boosting with the original vaccine, and thus may be used as a booster irrespective of the original vaccine.

Adult immunization with acellular pertussis vaccine

OBJECTIVE

To evaluate the safety and immunogenicity in adults of several different concentrations of an acellular pertussis vaccine.

DESIGN

Double-blind, randomized, placebo-controlled trial.

SETTING

Medical center immunization clinic.

PARTICIPANTS

One hundred eighteen healthy adult volunteers.

INTERVENTIONS

Participants received standard adult tetanus-diphtheria vaccine alone or combined with full-strength, half-strength, or quarter-strength concentrations of a currently licensed acellular pertussis vaccine used for booster doses in young children. Full-strength vaccine contained 40 micrograms of pertussis proteins, consisting of 86% filamentous hemagglutinin, 8% pertussis toxin, 4% 69-kd outer-membrane protein, and 2% agglutinogens.

MAIN OUTCOME MEASURES

Local and systemic reactions were assessed for 14 days after vaccination. Serum samples for antibody assay were obtained before, 1 month after, and 1 year after immunization.

RESULTS

Adverse reactions were few and minor and did not differ in frequency or severity among the four study groups. The groups receiving acellular pertussis vaccine showed strong antibody responses to pertussis antigens, which did not significantly differ by concentration of vaccine. After 1 year, levels of antibody to pertussis had declined by approximately 50% but remained substantially higher than preimmunization levels. The four groups did not differ in antibody responses to tetanus or diphtheria toxoids.

CONCLUSIONS

Routine reimmunization of adults with a vaccine containing acellular pertussis antigens in addition to diphtheria and tetanus toxoids can substantially enhance pertussis antibody levels without an increase in adverse reactions or diminution in response to the diphtheria and tetanus components. Such a program might materially reduce respiratory illness among both adults and children.

Psychoactive drugs and the risk of injurious motor vehicle crashes in elderly drivers

To determine whether commonly used psychoactive drugs increase the risk of involvement in motor vehicle crashes for drivers > or = 65 years of age, the authors conducted a retrospective cohort study. Data were obtained from computerized files from the Tennessee Medicaid program, driver's license files, and police reports of injurious crashes. Cohort members were Medicaid enrollees 65-84 years of age who had a valid driver's license during the study period 1984-1988 and who met other criteria designed to exclude persons unlikely to be drivers and to ensure availability of necessary study data. There were 16,262 persons in the study cohort with 38,701 person-years of follow-up and involvement in 495 injurious crashes. For four groups of psychoactive drugs (benzodiazepines, cyclic antidepressants, oral opioid analgesics, and antihistamines), the risk of crash involvement was calculated with Poisson regression models that controlled for demographic characteristics and use of medical care as an indicator of health status. The relative risk of injurious crash involvement for current users of any psychoactive drug was 1.5 (95% confidence interval (CI) 1.2-1.9). This increased risk was confined to benzodiazepines (relative risk = 1.5; 95% CI 1.2-1.9) and cyclic antidepressants (relative risk = 2.2; 95% CI 1.3-3.5). For these drugs, the relative risk increased with dose and was substantial for high doses: 2.4 (95% CI 1.3-4.4) for > or = 20 mg of diazepam and 5.5 (95% CI 2.6-11.6) for > or = 125 mg of amitriptyline. Analysis of data for the crash-involved drivers suggested that these findings were not due to confounding by alcohol use or driving frequency.