The role of the Vaccine Adverse Event Reporting system (VAERS) in monitoring vaccine safety

Myocarditis and Pericarditis Post-mRNA COVID-19 Vaccination: Insights from a Pharmacovigilance Perspective

Concerns remain regarding the rare cardiovascular adverse events, myocarditis and pericarditis (myo/pericarditis), particularly in younger individuals following mRNA COVID-19 vaccination. Our study aimed to comprehensively assess potential safety signals related to these cardiac events following the primary and booster doses, with a specific focus on younger populations, including children as young as 6 months of age. Using the Vaccine Adverse Events Reporting System (VAERS), the United States national passive surveillance system, we conducted a retrospective pharmacovigilance study analyzing spontaneous reports of myo/pericarditis. We employed both frequentist and Bayesian methods and conducted subgroup analyses by age, sex, and vaccine dose. We observed a higher reporting rate of myo/pericarditis following the primary vaccine series, particularly in males and mainly after the second dose. However, booster doses demonstrated a lower number of reported cases, with no significant signals detected after the fourth or fifth doses. In children and young adults, we observed notable age and sex differences in the reporting of myo/pericarditis cases. Males in the 12-17 and 18-24-year-old age groups had the highest number of cases, with significant signals for both males and females after the second dose. We also identified an increased reporting for a spectrum of cardiovascular symptoms such as chest pain and dyspnea, which increased with age, and were reported more frequently than myo/pericarditis. The present study identified signals of myo/pericarditis and related cardiovascular symptoms after mRNA COVID-19 vaccination, especially among children and adolescents. These findings underline the importance for continued vaccine surveillance and the need for further studies to confirm these results and to determine their clinical implications in public health decision-making, especially for younger populations.

Opportunities and challenges of leveraging COVID-19 vaccine innovation and technologies for developing sustainable vaccine manufacturing capabilities in Africa

The COVID-19 pandemic heralded unprecedented resource mobilisation and global scientific collaboration to rapidly develop effective vaccines. Regrettably, vaccine distribution has been inequitable, particularly in Africa where manufacturing capacity remains nominal. To address this, several initiatives are underway to develop and manufacture COVID-19 vaccines in Africa. Nevertheless, diminishing demand for COVID-19 vaccines, the cost competitiveness of producing goods locally, intellectual property rights issues, and complex regulatory environments among other challenges can undermine these ventures. We outline how extending COVID-19 vaccine manufacturing in Africa to include diverse products, multiple vaccine platforms, and advanced delivery systems will ensure sustainability. Possible models, including leveraging public-academic-private partnerships to enhance success of vaccine manufacturing capacity in Africa are also discussed. Intensifying research in vaccine discovery on the continent could yield vaccines that further bolster sustainability of local production, ensuring greater pandemic preparedness in resource-constrained environments, and long-term health systems security.

Risk of Myocarditis and Pericarditis among Young Adults following mRNA COVID-19 Vaccinations

There have been reports of cases of myocarditis and pericarditis as rare complications following mRNA COVID-19 vaccinations among young adults. While most reported cases are mild, this potential vaccine safety signal should be closely monitored. Using data from the CDC and the Vaccine Adverse Event Reporting System (VAERS), we calculated the combined reporting rate of myocarditis and pericarditis stratified by age group, sex, vaccine dose, and manufacturer, and compared these rates to the crude background incidence rates. Compared to the general population prior to the administration of the first COVID-19 vaccines in December 2020, we identified a higher-than-expected reporting rate of myocarditis and pericarditis following mRNA vaccination; the risk was higher after a second vaccine dose, higher in males than in females, and decreased with age. The highest risk was seen in males 12–17 years of age with approximately 6 cases per 100,000 second doses. Our findings suggest an increased risk of myocarditis and pericarditis in young males following a second dose of an mRNA COVID-19 vaccine. Since these findings are based on safety signals derived from passive surveillance data, confirmatory epidemiological studies should be undertaken.

Parental COVID-19 Vaccine Hesitancy in Diverse Communities: A National Survey

OBJECTIVE

We surveyed a diverse group of US participants to understand parental coronavirus disease 2019 (COVID-19) vaccine hesitancy.

METHODS

We administered a telephone and online survey from May 7 to June 7, 2021 using stratified sampling to ensure robust sample sizes of racial and ethnic minorities. Of the 20,280 contacted, 12,288 respondents completed the survey (response rate 61%). We used chi-square tests and adjusted risk ratios to compare results by racial/ethnic group.

RESULTS

Overall, 23% of parents stated that they plan to (or have) vaccinated their children; 30% said that they would not vaccinate their children, and 25% were unsure. Latino/a, Native American, and Asian American-Pacific Islander (AAPI) parents were generally more likely to vaccinate their children than Black or White parents. After adjusting for demographic factors, AAPI parents were significantly more likely to vaccinate their children than were others. Of parents who said that they would not vaccinate their child, 55% stated it was due to insufficient research. However, over half of parents stated that they would follow their child's health care provider's recommendations. After adjusting for demographic factors, trust in their primary care doctor was significantly lower among AAPI, Black, and Native American parents than White parents.

CONCLUSIONS

Parental vaccine hesitancy was similar overall, but drivers of hesitancy varied by racial/ethnic groups. While the perception that vaccines had been "insufficiently researched" was a major concern among all groups, we found that parents are generally inclined to follow health providers' recommendations. Health professionals can play an important role in COVID-19 vaccine education and should provide access to vaccines.

Thrombotic events and COVID-19 vaccines

COVID-19 vaccines are considered promising agents in the control of the pandemic. Although their safety was assessed in randomised clinical trials, severe adverse events (AEs) have been reported after large-scale administration. This study aims to evaluate thromboembolic AEs reported after vaccination in a real-world context and how they led to the interruption of vaccination campaigns. We also review the benefits and risks of the vaccines approved in the European Union and provide recommendations. A review of the literature was performed using Medline/PubMed electronic database as well as institutional and pharmacovigilance official reports. Our findings show that vaccine-induced prothrombotic immune thrombocytopenia has been suggested as a very rare AE associated with viral vector vaccines. Unusual thrombotic events combined with moderate-to-severe thrombocytopenia were reported mainly in women under 60 years of age. As safety signals emerged, Vaxzevria and Janssen´s COVID-19 vaccine campaigns have been paused while investigations proceed. On the other hand, the number of deep vein thrombosis and pulmonary embolism reports have not increased. Post-marketing surveillance indicated that mRNA vaccines are safe and should continue to be used. The thrombotic events report rate is not increased in people over 60 years. As they are at greater risk for COVID-19 complications and death, no vaccine restrictions are recommended in this group. Risk factors for vaccine-induced prothrombotic immune thrombocytopenia should be established so that evidence-based decisions can be made. Systematic monitoring of COVID-19 vaccine safety is essential to ensure that the benefits of vaccination outweigh the risks.

From program suspension to the pandemic: A qualitative examination of Australia's vaccine pharmacovigilance system over 10 years

BACKGROUND

In 2010, the Australian seasonal influenza vaccination program for children under 5 years of age was suspended due to an unexpected increase in fever and febrile convulsions causally associated with one particular influenza vaccine brand. A subsequent national review made seven recommendations to improve vaccine pharmacovigilance. Ten years on, in advance of implementing the COVID-19 immunisation program, we evaluated views on the capacity of Australia's vaccine pharmacovigilance system to promptly detect, examine and communicate a signal.

METHODS

Semi-structured interviews were conducted between July and October 2020 with individuals with expertise in vaccine safety in Australia using an interview guide informed by key Australian and international frameworks. Interviews were digitally recorded and transcribed verbatim. Thematic analysis was used to code data using a deductive approach.

RESULTS

Interviews with seventeen participants enabled six themes to be identified. Participants described improvement and significant innovation within Australia's vaccine pharmacovigilance system over the decade since 2010, particularly through establishment of a new active, cohort event monitoring system using short message service surveys. Participants thought Australia had a good foundation for COVID-19 vaccine safety surveillance; implementation of the COVID-19 immunisation program was seen as a potential driver for ongoing enhancement through: a) improved integration of the active surveillance and spontaneous reporting systems, and; b) development of population-level active surveillance, including through data linkage. Transparent communication was considered essential to address the unprecedented challenges of COVID-19 and broader vaccine safety concerns.

CONCLUSIONS

Vaccine safety experts in Australia convey confidence in the innovative pharmacovigilance systems implemented over the past 10 years. While Australia has a multifaceted system incorporating both active surveillance and spontaneous reporting systems, COVID-19 vaccine implementation represents an opportunity to enhance current systems and to develop new, systematic approaches to vaccine pharmacovigilance that should make both a local and global contribution.

Characterizing the incidence of adverse events of special interest for COVID-19 vaccines across eight countries: a multinational network cohort study

BACKGROUND

As large-scale immunization programs against COVID-19 proceed around the world, safety signals will emerge that need rapid evaluation. We report population-based, age- and sex-specific background incidence rates of potential adverse events of special interest (AESI) in eight countries using thirteen databases.

METHODS

This multi-national network cohort study included eight electronic medical record and five administrative claims databases from Australia, France, Germany, Japan, Netherlands, Spain, the United Kingdom, and the United States, mapped to a common data model. People observed for at least 365 days before 1 January 2017, 2018, or 2019 were included. We based study outcomes on lists published by regulators: acute myocardial infarction, anaphylaxis, appendicitis, Bell's palsy, deep vein thrombosis, disseminated intravascular coagulation, encephalomyelitis, Guillain-Barre syndrome, hemorrhagic and non-hemorrhagic stroke, immune thrombocytopenia, myocarditis/pericarditis, narcolepsy, pulmonary embolism, and transverse myelitis. We calculated incidence rates stratified by age, sex, and database. We pooled rates across databases using random effects meta-analyses. We classified meta-analytic estimates into Council of International Organizations of Medical Sciences categories: very common, common, uncommon, rare, or very rare.

FINDINGS

We analysed 126,661,070 people. Rates varied greatly between databases and by age and sex. Some AESI (e.g., myocardial infarction, Guillain-Barre syndrome) increased with age, while others (e.g., anaphylaxis, appendicitis) were more common in young people. As a result, AESI were classified differently according to age. For example, myocardial infarction was very rare in children, rare in women aged 35-54 years, uncommon in men and women aged 55-84 years, and common in those aged ≥85 years.

INTERPRETATION

We report robust baseline rates of prioritised AESI across 13 databases. Age, sex, and variation between databases should be considered if background AESI rates are compared to event rates observed with COVID-19 vaccines.

Safety profile of the meningococcal conjugate vaccine (Menafrivac™) in clinical trials and vaccination campaigns: a review of published studies

The study aimed to assess the capacity of AEFI surveillance during vaccination campaigns with the new conjugate meningitis vaccine (MenAfrivac). A systematic review of studies on MenAfrivac™ published in English during 2001-2016 was done.AEFIs incidence (I) was estimated and compared between MenAfrivac™ clinical trials and immunization campaigns using incidence difference (Id). Nine studies were included with an overall local AEFI I of 11,496/100,000 doses administered per week in clinical trials and 0.72/100,000 doses in immunization campaigns. An Id of 11,497.92 [11,497.91-11,497.93] and 17,243.20 [17,241.80-17,245.90] per 100,000 doses administered per week for overall local and systemic AEFI, respectively, were observed with highest from clinical trials. The incidence of AEFIs after MenAfrivac™ vaccination was far lower in campaigns than in clinical trial studies. Current capacity of AEFI surveillance during vaccination campaigns requires extensive re-assessment of its structure and capacity.

Effectiveness of a Census-Based Management Information System for Guiding Polio Eradication and Routine Immunization Activities: Evidence from the CORE Group Polio Project in Uttar Pradesh, India

Census-based management information systems (CB-MISs) bring strength and power to public health programs by providing current information about everyone in the population covered by the program. Such a system has been developed by the CORE Group Polio Project (CGPP) in India. This article assesses the effectiveness of the CGPP CB-MIS in the management of social and behavioral change interventions. It also assesses the feasibility of the CB-MIS for vital events registration. We describe the procedures of the CB-MIS and measure the outcomes of the CGPP by observing the trends of vaccination coverage in CGPP catchment areas over time. We also compute vital statistics from births and deaths registered through the CGPP CB-MIS and compare them with the estimates from the Civil Registration System of India using statistics from India's Sample Registration System in Uttar Pradesh as the "gold standard." The CB-MIS has helped the CGPP to manage its social and behavior change communication interventions effectively, and it has contributed to the increase in polio vaccine coverage facilitated by the CGPP. We also estimate that the CGPP's CB-MIS has registered 86% of births and 98% of infant deaths, a much higher level of registration than has been achieved by the Civil Registration System for the entire state of Uttar Pradesh. The CB-MIS has helped to make it possible for community-based health workers to make behavioral diagnoses of barriers to immunization and to overcome them. The CB-MIS also provides a robust platform for community-based health workers to register vital events.

Assessment of Quadrivalent Human Papillomavirus Vaccine Safety Using the Self-Controlled Tree-Temporal Scan Statistic Signal-Detection Method in the Sentinel System

The self-controlled tree-temporal scan statistic-a new signal-detection method-can evaluate whether any of a wide variety of health outcomes are temporally associated with receipt of a specific vaccine, while adjusting for multiple testing. Neither health outcomes nor postvaccination potential periods of increased risk need be prespecified. Using US medical claims data in the Food and Drug Administration's Sentinel system, we employed the method to evaluate adverse events occurring after receipt of quadrivalent human papillomavirus vaccine (4vHPV). Incident outcomes recorded in emergency department or inpatient settings within 56 days after first doses of 4vHPV received by 9- through 26.9-year-olds in 2006-2014 were identified using International Classification of Diseases, Ninth Revision, diagnosis codes and analyzed by pairing the new method with a standard hierarchical classification of diagnoses. On scanning diagnoses of 1.9 million 4vHPV recipients, 2 statistically significant categories of adverse events were found: cellulitis on days 2-3 after vaccination and "other complications of surgical and medical procedures" on days 1-3 after vaccination. Cellulitis is a known adverse event. Clinically informed investigation of electronic claims records of the patients with "other complications" did not suggest any previously unknown vaccine safety problem. Considering that thousands of potential short-term adverse events and hundreds of potential risk intervals were evaluated, these findings add significantly to the growing safety record of 4vHPV.

No signal of interactions between influenza vaccines and drugs used for chronic diseases: a case-by-case analysis of the vaccine adverse event reporting system and vigibase

BACKGROUND

An increasing number of reports indicates that vaccines against influenza may interact with specific drugs via drug metabolism. To date, actual impact of vaccine-drug interactions observed in the real world clinical practice has not been investigated.

METHODS

From VAERS and VigiBase, we collected Adverse Event Following Immunization (AEFI) reports for individuals receiving vaccines against influenza recorded as suspect and selected cases where predictable toxicity was recorded with oral anticoagulants, antiepileptics and statins (i.e. hemorrhages, overdosage and rhabdomyolysis, respectively). We applied AEFI and Drug Interaction Probability Scale (DIPS) Algorithms to assess causality of drug-vaccine interactions.

RESULTS

116 AEFI reports submitted to VAERS and 83 from Vigibase were included in our analysis; antiepileptics and statins were related to the highest number of indeterminate/consistent (93.7%; 65.3%) and possible/probable (50%; 57.7%) cases according to the AEFI and DIPS, respectively. The majority of cases occurred within the first week after vaccine administration (5-7 days).

CONCLUSION

The relative paucity of detected interactions does not impact on the benefit of the vaccination against influenza, which remains strongly recommended; this does not exclude that closer monitoring for selected patients exposed to concomitant chronic pharmacological therapies and affected by predisposing factors may be useful.

Postmarketing safety surveillance of trivalent recombinant influenza vaccine: Reports to the Vaccine Adverse Event Reporting System

On January 16, 2013, the Food and Drug Administration approved recombinant hemagglutinin influenza vaccine (RIV3) (Spodoptera frugiperda cell line; Flublok), which is the first completely egg-free flu vaccine licensed in the United States. To improve our understanding of the safety profile of this vaccine, we reviewed and summarized reports to the Vaccine Adverse Event Reporting System (VAERS) following RIV3. Through June 30, 2016, VAERS received 88 reports. Allergic reactions, including anaphylaxis, were the most common type of adverse event. Based on medical review, 10 cases met the Brighton Collaboration case definition of anaphylaxis, 21 reports described allergic reactions other than anaphylaxis, and 11 reports described signs and symptoms that suggested hypersensitivity. Other adverse events included injection site reactions, fatigue, myalgia, headache, and fever. The occurrence of anaphylaxis and other allergic reactions in some individuals may reflect an underlying predisposition to atopy that may manifest itself after an exposure to any drug or vaccine, and it does not necessarily suggest a causal relationship with the unique constituents that are specific to the vaccine product administered. Further research may elucidate the mechanism of allergic reactions following influenza vaccination: it is possible that egg proteins and influenza hemagglutinin play little or no role. Vaccination remains the single best defense against influenza and its complications. The information summarized here may enable policy makers, health officials, clinicians, and patients to make a more informed decision regarding vaccination strategies.

The relationship between mercury and autism: A comprehensive review and discussion

The brain pathology in autism spectrum disorders (ASD) indicates marked and ongoing inflammatory reactivity with concomitant neuronal damage. These findings are suggestive of neuronal insult as a result of external factors, rather than some type of developmental mishap. Various xenobiotics have been suggested as possible causes of this pathology. In a recent review, the top ten environmental compounds suspected of causing autism and learning disabilities were listed and they included: lead, methyl-mercury, polychorinated biphenyls, organophosphate pesticides, organochlorine pesticides, endocrine disruptors, automotive exhaust, polycyclic aromatic hydrocarbons, polybrominated diphenyl ethers, and perfluorinated compounds. This current review, however, will focus specifically on mercury exposure and ASD by conducting a comprehensive literature search of original studies in humans that examine the potential relationship between mercury and ASD, categorizing, summarizing, and discussing the published research that addresses this topic. This review found 91 studies that examine the potential relationship between mercury and ASD from 1999 to February 2016. Of these studies, the vast majority (74%) suggest that mercury is a risk factor for ASD, revealing both direct and indirect effects. The preponderance of the evidence indicates that mercury exposure is causal and/or contributory in ASD.

Countering Vaccine Hesitancy

Immunizations have led to a significant decrease in rates of vaccine-preventable diseases and have made a significant impact on the health of children. However, some parents express concerns about vaccine safety and the necessity of vaccines. The concerns of parents range from hesitancy about some immunizations to refusal of all vaccines. This clinical report provides information about addressing parental concerns about vaccination.

Adverse Events After MMR or MMRV Vaccine in Infants Under Nine Months Old

BACKGROUND

In the United States, measles is resurging, with more than 700 confirmed cases since January 2014. During measles outbreaks, vaccination as early as at 6 months of age is sometimes recommended for infants who are at risk for exposure.

METHODS

We searched the Vaccine Adverse Event Reporting System for reports of measles, mumps and rubella vaccine combined or measles, mumps, rubella and varicella vaccine combined vaccination in children less than 9 months of age. We performed a clinical assessment of each report and summarized the frequency, range, onset time and severity of adverse events.

RESULTS

After excluding 346 reports because they were duplicates or because they contained insufficient information about the child's age or vaccine(s), we retained 204 reports in the analysis, including 35 (17%) that were serious. Among the 169 nonserious reports, more than half (88; 52%) described a vaccination error without any adverse event per se. Other nonserious reports described fever, injection reactions and gastrointestinal symptoms. Serious adverse events included developmental disorders, fever and fussiness. There were 44 reports of fever, but only 4 cases began 5-12 days after immunization, the peak risk window. The vast majority of fever reports listed concomitant vaccines, such as diphtheria and tetanus toxoids, acellular or whole-cell pertussis vaccine.

CONCLUSIONS

This review did not identify any major safety concerns. These findings may facilitate discussions about the risks and benefits of vaccinating infants who are potentially exposed to this life-threatening disease.

Adverse event reports following yellow fever vaccination, 2007-13

BACKGROUND

Yellow fever (YF) vaccines have been available since the 1930s and are generally considered safe and effective. However, rare reports of serious adverse events (SAE) following vaccination have prompted the Advisory Committee for Immunization Practices to periodically expand the list of conditions considered contraindications and precautions to vaccination.

METHODS

We describe adverse events following YF vaccination reported to the U.S. Vaccine Adverse Event Reporting System (VAERS) from 2007 through 2013 and calculate age- and sex-specific reporting rates of all SAE, anaphylaxis, YF vaccine-associated neurologic disease (YEL-AND) and YF vaccine-associated viscerotropic disease (YEL-AVD).

RESULTS

There were 938 adverse events following YF vaccination reported to VAERS from 2007 through 2013. Of these, 84 (9%) were classified as SAEs for a rate of 3.8 per 100 000 doses distributed. Reporting rates of SAEs increased with increasing age with a rate of 6.5 per 100 000 in persons aged 60-69 years and 10.3 for ≥70 years. The reporting rate for anaphylaxis was 1.3 per 100 000 doses distributed and was highest in persons ≤18 years (2.7 per 100 000). Reporting rates of YEL-AND and YEL-AVD were 0.8 and 0.3 per 100 000 doses distributed, respectively; both rates increased with increasing age.

CONCLUSIONS

These findings reinforce the generally acceptable safety profile of YF vaccine, but highlight the importance of continued physician and traveller education regarding the risks and benefits of YF vaccination, particularly for older travellers.

Quadrivalent HPV vaccine safety review and safety monitoring plans for nine-valent HPV vaccine in the United States

Quadrivalent human papillomavirus (4vHPV) vaccine was licensed for use in the United States in 2006 and through 2015 was the predominate HPV vaccine used. With the exception of syncope, a known preventable adverse event after any injected vaccination, both pre-licensure and post-licensure 4vHPV safety data have been reassuring with no confirmed safety signals identified. Nine-valent HPV vaccine (9vHPV) was licensed in 2014. This review includes post-licensure 4vHPV safety findings published to date that have informed the US vaccination program; these data will inform US safety monitoring and evaluation for 9vHPV.

An Overview of Quadrivalent Human Papillomavirus Vaccine Safety: 2006 to 2015

BACKGROUND

A quadrivalent human papillomavirus (HPV4) type 6/11/16/18 vaccine (GARDASIL/SILGARD®) has been licensed in many countries around the world for the prevention of cervical, vulvar, vaginal, and anal cancers and precancers, as well as external genital warts causally related to HPV types 6/11/16/18. Across 7 phase 3 clinical trials involving more than 29,000 males and females ages 9-45 years, vaccination was generally well tolerated. Because of its expected public health benefit in reducing cervical cancer and other HPV-related diseases, the vaccine has been implemented in the national vaccination programs of several countries, with over 178 million doses distributed worldwide.

METHODS

Extensive efforts to assess the safety of the vaccine in routine practice have been conducted over the past 9 years since licensure, including more than 15 studies in more than 1 million preadolescents, adolescents and adults from various countries. Most have been performed in the general population although there have been some in special populations (pregnant women, HIV-infected individuals and those with systemic lupus erythematosus).

RESULTS

We present a summary of the published, postlicensure safety data from active and passive surveillance. Only syncope, and possibly skin infections were associated with vaccination in the postlicensure setting. Serious adverse events, such as adverse pregnancy outcomes, autoimmune diseases (including Guillain-Barre Syndrome and multiple sclerosis), anaphylaxis, venous thromboembolism and stroke, were extensively studied, and no increase in the incidence of these events was found compared with background rates.

CONCLUSIONS

These results, along with the safety data from the prelicensure clinical trials, confirm that the HPV4 vaccine has a favorable safety profile. Key policy, medical and regulatory organizations around the world have independently reviewed these data and continue to recommend routine HPV vaccination.

Safety monitoring in the Vaccine Adverse Event Reporting System (VAERS)

The Centers for Disease Control and Prevention (CDC) and the U.S. Food and Drug Administration (FDA) conduct post-licensure vaccine safety monitoring using the Vaccine Adverse Event Reporting System (VAERS), a spontaneous (or passive) reporting system. This means that after a vaccine is approved, CDC and FDA continue to monitor safety while it is distributed in the marketplace for use by collecting and analyzing spontaneous reports of adverse events that occur in persons following vaccination. Various methods and statistical techniques are used to analyze VAERS data, which CDC and FDA use to guide further safety evaluations and inform decisions around vaccine recommendations and regulatory action. VAERS data must be interpreted with caution due to the inherent limitations of passive surveillance. VAERS is primarily a safety signal detection and hypothesis generating system. Generally, VAERS data cannot be used to determine if a vaccine caused an adverse event. VAERS data interpreted alone or out of context can lead to erroneous conclusions about cause and effect as well as the risk of adverse events occurring following vaccination. CDC makes VAERS data available to the public and readily accessible online. We describe fundamental vaccine safety concepts, provide an overview of VAERS for healthcare professionals who provide vaccinations and might want to report or better understand a vaccine adverse event, and explain how CDC and FDA analyze VAERS data. We also describe strengths and limitations, and address common misconceptions about VAERS. Information in this review will be helpful for healthcare professionals counseling patients, parents, and others on vaccine safety and benefit-risk balance of vaccination.

Preparation for global introduction of inactivated poliovirus vaccine: safety evidence from the US Vaccine Adverse Event Reporting System, 2000-12

BACKGROUND

Safety data from countries with experience in the use of inactivated poliovirus vaccine (IPV) are important for the global polio eradication strategy to introduce IPV into the immunisation schedules of all countries. In the USA, IPV has been included in the routine immunisation schedule since 1997. We aimed to analyse adverse events after IPV administration reported to the US Vaccine Adverse Event Reporting System (VAERS).

METHODS

We analysed all VAERS data associated with IPV submitted between Jan 1, 2000, and Dec 31, 2012, either as individual or as combination vaccines, for all age and sex groups. We analysed the number and event type (non-serious, non-fatal serious, and death reports) of individual reports, and explored the most commonly coded event terms to describe the adverse event. We classified death reports according to previously published body-system categories (respiratory, cardiovascular, neurological, gastrointestinal, other infectious, and other non-infectious) and reviewed death reports to identify the cause of death. We classified sudden infant death syndrome as a separate cause of death considering previous concerns about sudden infant syndrome after vaccines. We used empirical Bayesian data mining methods to identify disproportionate reporting of adverse events for IPV compared with other vaccines. Additional VAERS data from 1991 to 2000 were analysed to compare the safety profiles of IPV and oral poliovirus vaccine (OPV).

FINDINGS

Of the 41,792 adverse event reports submitted, 39,568 (95%) were for children younger than 7 years. 38,381 of the reports for children in this age group (97%) were for simultaneous vaccination with IPV and other vaccines (most commonly pneumococcal and acellular pertussis vaccines), whereas standalone IPV vaccines accounted for 0·5% of all reports. 34,880 reports were for non-serious events (88%), 3905 reports were for non-fatal serious events (10%), and 783 reports were death reports (2%). Injection-site erythema was the most commonly coded term for non-serious events (29%), and pyrexia for non-fatal serious events (38%). Most deaths (96%) were in children aged 12 months or younger; most (52%) had sudden infant death syndrome as the reported cause of death. The safely profiles of combined IPV and whole-cell pertussis vaccines, OPV and whole-cell pertussis vaccines, and OPV and acellular pertussis vaccines were similar. We noted no indication of disproportionate reporting of adverse events after immunisation with IPV-containing vaccines compared with other vaccines between 1990 and 2013.

INTERPRETATION

Fairly few adverse events were reported for the more than 250 million IPV doses distributed between 2000 and 2012. Sudden infant death syndrome reports after IPV were consistent with reporting patterns for other vaccines. No new or unexpected vaccine safety problems were identified for fatal, non-fatal serious, and non-serious reports in this assessment of adverse events after IPV.

FUNDING

None.

Military healthcare providers reporting of adverse events following immunizations to the vaccine adverse event reporting system

OBJECTIVES

We studied military health care provider (HCP) practices regarding reporting of adverse events following immunization (AEFI).

METHODS

A convenience sample of HCP was surveyed to assess familiarity with Vaccine Adverse Event Reporting System (VAERS), AEFI they were likely to report, methods used and preferred for reporting, and perceived barriers to reporting. We analyzed factors associated with HCP reporting AEFI to VAERS.

RESULTS

A total of 547 surveys were distributed with 487 completed and returned for an 89% response rate. The percentage of HCP aware of VAERS (54%) varied by occupation. 47% of respondents identified knowledge of at least one AEFI with only 34% of these indicating that they had ever reported to VAERS. More serious events were more likely to be reported. Factors associated with HCP reporting AEFIs in bivariate analysis included HCP familiarity with filing a paper VAERS report, HCP familiarity with filing an electronic VAERS report, HCP familiarity with VAERS, and time spent on immunization tasks. In a multivariable analysis, only HCP familiarity with filing a paper VAERS report was statistically significant (Odds ratio = 115.3; p < 0.001).

CONCLUSIONS

Specific educational interventions targeted to military HCP likely to see AEFIs but not currently filing VAERS reports may improve vaccine safety reporting practices.

Allergic reactions after egg-free recombinant influenza vaccine: reports to the US Vaccine Adverse Event Reporting System

The Vaccine Adverse Event Reporting System has received reports of allergic reactions following immunization with egg-free recombinant influenza vaccine, among patients with a self-reported egg allergy or previous allergic reaction to inactivated influenza vaccine. These results suggest that allergic reactions following influenza vaccination are not necessarily related to egg proteins.

Big Data Usage Patterns in the Health Care Domain: A Use Case Driven Approach Applied to the Assessment of Vaccination Benefits and Risks. Contribution of the IMIA Primary Healthcare Working Group

BACKGROUND

Generally benefits and risks of vaccines can be determined from studies carried out as part of regulatory compliance, followed by surveillance of routine data; however there are some rarer and more long term events that require new methods. Big data generated by increasingly affordable personalised computing, and from pervasive computing devices is rapidly growing and low cost, high volume, cloud computing makes the processing of these data inexpensive.

OBJECTIVE

To describe how big data and related analytical methods might be applied to assess the benefits and risks of vaccines.

METHOD

We reviewed the literature on the use of big data to improve health, applied to generic vaccine use cases, that illustrate benefits and risks of vaccination. We defined a use case as the interaction between a user and an information system to achieve a goal. We used flu vaccination and pre-school childhood immunisation as exemplars.

RESULTS

We reviewed three big data use cases relevant to assessing vaccine benefits and risks: (i) Big data processing using crowdsourcing, distributed big data processing, and predictive analytics, (ii) Data integration from heterogeneous big data sources, e.g. the increasing range of devices in the "internet of things", and (iii) Real-time monitoring for the direct monitoring of epidemics as well as vaccine effects via social media and other data sources.

CONCLUSIONS

Big data raises new ethical dilemmas, though its analysis methods can bring complementary real-time capabilities for monitoring epidemics and assessing vaccine benefit-risk balance.

Awareness and utilization of reporting pathways for adverse events following immunization: online survey among pediatricians in Russia and Germany

OBJECTIVES

Vaccine safety surveillance is highly dependent on accurate reporting of adverse events following immunization (AEFI). An online survey was conducted to assess the utilization of AEFI reporting standards and pathways among pediatricians in Germany, and in Russia where pediatric specialization begins in medical school.

METHODS

In May 2011, a 31-item online questionnaire was sent to members of the German Professional Association for Pediatricians (BVKJ) and the Union of Pediatricians of Russia (UPR), capturing information on vaccine safety training, awareness of AEFI reporting pathways, and use of standardized case definitions for the ascertainment of AEFI. A convenience sample of 1,632 completed online surveys was analyzed.

RESULTS

Participating pediatricians reported spending approximately 50 min per 8-hour workday on vaccine safety consultations, but only 42 % (56 % UPR, 26 % BVKJ) have ever received any formal vaccine safety training. Two-thirds reported having observed AEFI in their practice, but only one-third utilized standardized case definitions for case ascertainment. Only 35 % of participants named accurate AEFI reporting pathways. Every second pediatrician would report AEFI to institutions that are not primarily in charge of vaccine safety surveillance; the remaining reports would either be lost or delayed. Pediatricians who had received formal vaccine safety training were significantly more likely to apply international safety standards and to report adequately, both at the p < 0.05 level.

CONCLUSION

Pediatricians play a key role in the post-marketing surveillance of vaccine safety. The lack of training represents a missed opportunity. There may be a role for professional societies to improve vaccine safety training.

The logic of surveillance guidelines: an analysis of vaccine adverse event reports from an ontological perspective

BACKGROUND

When increased rates of adverse events following immunization are detected, regulatory action can be taken by public health agencies. However to be interpreted reports of adverse events must be encoded in a consistent way. Regulatory agencies rely on guidelines to help determine the diagnosis of the adverse events. Manual application of these guidelines is expensive, time consuming, and open to logical errors. Representing these guidelines in a format amenable to automated processing can make this process more efficient.

METHODS AND FINDINGS

Using the Brighton anaphylaxis case definition, we show that existing clinical guidelines used as standards in pharmacovigilance can be logically encoded using a formal representation such as the Adverse Event Reporting Ontology we developed. We validated the classification of vaccine adverse event reports using the ontology against existing rule-based systems and a manually curated subset of the Vaccine Adverse Event Reporting System. However, we encountered a number of critical issues in the formulation and application of the clinical guidelines. We report these issues and the steps being taken to address them in current surveillance systems, and in the terminological standards in use.

CONCLUSIONS

By standardizing and improving the reporting process, we were able to automate diagnosis confirmation. By allowing medical experts to prioritize reports such a system can accelerate the identification of adverse reactions to vaccines and the response of regulatory agencies. This approach of combining ontology and semantic technologies can be used to improve other areas of vaccine adverse event reports analysis and should inform both the design of clinical guidelines and how they are used in the future.

AVAILABILITY

Sufficient material to reproduce our results is available, including documentation, ontology, code and datasets, at http://purl.obolibrary.org/obo/aero.

Healthcare providers' knowledge, experience and challenges of reporting adverse events following immunisation: a qualitative study

Background

Healthcare provider spontaneous reporting of suspected adverse events following immunisation (AEFI) is central to monitoring post-licensure vaccine safety, but little is known about how healthcare professionals recognise and report to surveillance systems. The aim of this study was explore the knowledge, experience and attitudes of medical and nursing professionals towards detecting and reporting AEFI.

Methods

We conducted a qualitative study, using semi-structured, face to face interviews with 13 Paediatric Emergency Department consultants from a tertiary paediatric hospital, 10 General Practitioners, 2 local council immunisation and 4 General Practice nurses, recruited using purposive sampling in Adelaide, South Australia, between December 2010 and September 2011. We identified emergent themes related to previous experience of an AEFI in practice, awareness and experience of AEFI reporting, factors that would facilitate or impede reporting and previous training in vaccine safety. Thematic analysis was used to analyse the data.

Results

AEFI reporting was infrequent across all groups, despite most participants having reviewed an AEFI. We found confusion about how to report an AEFI and variability, according to the provider group, as to the type of events that would constitute a reportable AEFI. Participants’ interpretation of a “serious” or “unexpected” AEFI varied across the three groups. Common barriers to reporting included time constraints and unsatisfactory reporting processes. Nurses were more likely to have received formal training in vaccine safety and reporting than medical practitioners.

Conclusions

This study provides an overview of experience and beliefs of three healthcare professional groups in relation to identifying and reporting AEFI. The qualitative assessment reveals differences in experience and awareness of AEFI reporting across the three professional groups. Most participants appreciated the importance of their role in AEFI surveillance and monitoring the ongoing safety of vaccines. Future initiatives to improve education, such as increased training to health care providers, particularly, medical professionals, are required and should be included in both undergraduate curricula and ongoing, professional development.

Who is unlikely to report adverse events after vaccinations to the Vaccine Adverse Event Reporting System (VAERS)?

BACKGROUND

Healthcare provider (HCP) reporting to the Vaccine Adverse Event Reporting System (VAERS) is important to assuring the safety of U.S. licensed vaccines. HCP awareness of and practices regarding reporting of adverse events following immunization (AEFI) is understudied.

METHODS

A large, nationally representative sample of U.S. office-based HCP across three occupational groups (physicians, mid-level providers [physician assistants, advanced practice nurses] and nurses) and three primary care practice areas (pediatrics, family medicine, internal medicine) were surveyed utilizing standardized methodology. We assessed HCP familiarity with VAERS, the situations under which they were likely to report an AEFI, and the methods they used and preferred for reporting. We used logistic regression to determine factors associated with HCP not reporting AEFI to VAERS.

RESULTS

Our survey response rate was 54.9%. The percentage of HCP aware of VAERS (71%) varied by occupation and primary care practice area. About 37% of HCP had identified at least one AEFI with only 17% of these indicating that they had ever reported to VAERS. More serious events were more likely to be reported. Factors associated with HCP not reporting AEFI included: HCP not familiar versus very familiar with filing a paper VAERS report (OR=12.84; p<0.0001), primary care practice area of internal medicine versus pediatrics (OR=4.22; p=0.0005), and HCP not familiar versus very familiar with when it was required to file a VAERS report (OR=5.52; p=0.0013).

CONCLUSIONS

Specific educational interventions targeted to HCP likely to see AEFI but not currently reporting may improve vaccine safety reporting practices.

Invasive and in situ cervical cancer reported to the vaccine adverse event reporting system (VAERS)

The quadrivalent human papillomavirus (HPV) vaccine was recommended in 2006 for routine vaccination of 11 or 12-year-old girls, with catchup through age 26 years, for the prevention of genital HPV-related diseases. The Vaccine Adverse Event Reporting System (VAERS) is a national spontaneous surveillance system of adverse events following vaccination in the United States. The objective of this study was to identify and review VAERS reports of invasive and in situ cervical cancer in women immunized with the quadrivalent HPV vaccine. A VAERS database search was performed to identify such cases reported in the United States from January 1, 2006, through April 9, 2009. Medical Dictionary for Regulatory Activities (MedDRA) search terms used were "cervix carcinoma," "cervix carcinoma stage 0," "cervix carcinoma stage III," "carcinoma in situ," and "cervical dysplasia." Case inclusion required a report to contain a clear statement of a cervical carcinoma or carcinoma in situ diagnosis on any screening or diagnostic test after at least one dose of the HPV vaccine. All reports were reviewed by two investigators. Four VAERS reports for MedDRA term "cervix carcinoma," one for "cervix carcinoma stage 0," none for "cervix carcinoma stage III," three for "carcinoma in situ," and 53 for "cervical dysplasia" were identified. Of these, three cases of carcinoma in situ and one case of microinvasive cervical cancer met study inclusion criteria. Cases of cervical cancer and precancers are not unexpected in vaccinated women. Cervical cancer screening continues to be important, even for women who have received the HPV vaccine.

Considerations for controlling invasive meningococcal disease in high income countries

The development of conjugate vaccines has enabled the prevention and control of Neisseria meningitidis caused by serogroups A, C, W-135 and Y. Vaccines that provide protection against a broad number of serogroup B strains likely will be available soon to enable greater control of meningococcal disease in high income countries. We present an argument for adequate post-marketing surveillance to monitor epidemiological shifts and to provide a context for the safety and reactogenicity of serogroup B vaccines, including the newer recombinant vaccines. We also offer a series of recommendations to address possible concerns about vaccine safety.

Causality assessment of serious neurologic adverse events following 2009 H1N1 vaccination

BACKGROUND

Adverse events occurring after vaccination are routinely reported to the Vaccine Adverse Event Reporting System (VAERS). We studied serious adverse events (SAEs) of a neurologic nature reported after receipt of influenza A (H1N1) 2009 monovalent vaccine during the 2009-2010 influenza season. Investigators in the Clinical Immunization Safety Assessment (CISA) network sought to characterize these SAEs and to assess their possible causal relationship to vaccination.

METHODS

Centers for Disease Control and Prevention (CDC) and Food and Drug Administration (FDA) physicians reviewed all SAE reports (as defined by the Code of Federal Regulations, 21CFR§314.80) after receipt of H1N1 vaccine reported to VAERS between October 1, 2009 and March 31, 2010. Non-fatal SAE reports with neurologic presentation were referred to CISA investigators, who requested and reviewed additional medical records and clinical information as available. CISA investigators assessed the causal relationship between vaccination and the event using modified WHO criteria as defined.

RESULTS

212 VAERS reports of non-fatal serious neurological events were referred for CISA review. Case reports were equally distributed by gender (50.9% female) with an age range of 6 months to 83 years (median 38 years). The most frequent diagnoses reviewed were: Guillain-Barré Syndrome (37.3%), seizures (10.8%), cranial neuropathy (5.7%), and acute disseminated encephalomyelitis (3.8%). Causality assessment resulted in classification of 72 events as "possibly" related (33%), 108 as "unlikely" related (51%), and 20 as "unrelated" (9%) to H1N1 vaccination; none were classified as "probable" or "definite" and 12 were unclassifiable (6%).

CONCLUSION

The absence of a specific test to indicate whether a vaccine component contributes to the pathogenesis of an event occurring within a biologically plausible time period makes assessing causality difficult. The development of standardized protocols for providers to use in evaluation of adverse events following immunization, and rapid identification and follow-up of VAERS reports could improve causality assessment.

Early estimate of the effectiveness of quadrivalent meningococcal conjugate vaccine

BACKGROUND

In January 2005, a quadrivalent meningococcal conjugate vaccine (MenACWYD) was licensed for use in the United States. The Advisory Committee on Immunization Practices recommends MenACWYD for all adolescents 11 to 18 years of age and others at increased risk for meningococcal disease.

METHODS

Reports of breakthrough meningococcal disease after vaccination with MenACWYD were collected. A simulation approach was used to estimate the expected number of cases in vaccinated persons.

RESULTS

Between 2005 and 2008, 14 breakthrough cases, including 3 deaths occurred. At a vaccine effectiveness (VE) of 90%, 7 breakthrough cases would be expected (range, 1-17); at VE of 85%, 11 cases (range, 2-30); at VE of 80%, 15 cases (range, 5-28); and at VE of 75%, 18 cases (range, 7-32) would be expected. The probability of the ≥14 observed cases occurring was 2.9% at VE of 90%, 29.3% at VE of 85%, 66.1% at VE of 80%, and 83.0% at VE of 75%.

CONCLUSIONS

This report provides an early estimate of MenACWYD effectiveness within 3 to 4 years after vaccination, and suggests that MenACWYD effectiveness is 80% to 85%, similar to the VE reported for meningococcal polysaccharide vaccine.

Internet-based reporting to the vaccine adverse event reporting system: a more timely and complete way for providers to support vaccine safety

BACKGROUND

On March 22, 2002, Internet-based reports (IBRs) were added to the Vaccine Adverse Event Reporting System (VAERS) to allow rapid, expedited reporting of adverse events (AEs) in anticipation of wider use of counter-bioterrorism vaccines such as those against smallpox and anthrax.

OBJECTIVES

To evaluate the impact of IBRs on the timeliness and completeness of vaccine AE reporting.

METHODS

To evaluate timeliness and completeness, we compared the proportions of IBRs with non-Internet-based reports (NIBRs). Report interval was analyzed for timeliness and age at vaccination, birth date, and onset date for report completeness. To evaluate the impact of the smallpox vaccination program, we compared smallpox vaccine reports separately. Because influenza vaccine is the most widely used vaccine in adults each year, we compared influenza vaccine reports separately.

RESULTS

During the study period, VAERS received 54 364 NIBRs (85.8%) and 9008 IBRs (14.2%). Sixteen percent (1455) of IBRs followed smallpox vaccination. Overall, for all vaccines and for smallpox vaccine alone, IBRs had a greater proportion of completeness and a shorter report interval. The proportion of most frequently reported AEs did not differ between IBRs and NIBRs. A higher proportion of adults (18-64 years old) who received influenza vaccine chose to complete an IBR (62% vs 48%).

CONCLUSIONS

The improved timeliness and completeness of IBRs allow VAERS to more rapidly detect new or rare vaccine AEs. This important advantage is critical in times of increased public concern about vaccine safety. Clinical vaccine providers should be aware of VAERS and use IBRs whenever feasible to report vaccine AEs.

Adverse event reports following Japanese encephalitis vaccination in the United States, 1999-2009

We reviewed adverse events following receipt of inactivated mouse brain-derived Japanese encephalitis (JE) vaccine reported to the U.S. Vaccine Adverse Event Reporting System (VAERS) from 1999 to 2009. During this period, VAERS received 300 adverse event reports following JE vaccination (24 per 100,000 doses distributed); 106 (35%) were classified as hypersensitivity reactions (8.4 per 100,000 doses) and four (1%) were classified as neurologic events (0.3 per 100,000 doses). Twenty-three (8%) reports described serious adverse events (1.8 per 100,000 doses distributed). There were no reports of encephalitis, meningitis, or Guillain-Barré syndrome. As reported previously, hypersensitivity reactions were common among persons receiving inactivated mouse brain-derived JE vaccine.

Longitudinal myelitis associated with yellow fever vaccination

Severe adverse reaction to yellow fever (YF) vaccine includes the yellow fever vaccine-associated neurotropic disease. This terminology includes postvaccinal encephalitis, acute disseminated encephalomyelitis, and Guillain-Barré syndrome. The objective of this communication is to report a patient who received a YF vaccine in Argentina and subsequently developed longitudinal myelitis with a symptom that had previously gone unreported in the literature. A 56-year-old man began with progressive paraparesia, urinary retention, and constipation 48 h previous to admission. The patient received YF vaccine 45 days prior to the onset of the symptoms. There was no history of other immunization or relevant condition. MR of the spine showed longitudinal intramedullary hyperintense signal (D5-12) without gadolinium enhancement. A high concentration of YFV-specific IgM vaccine antibody was found in the cerebrospinal fluid (CSF). Serological tests for other flavivirus were negative. A diagnosis of longitudinal myelitis without encephalitis associated with YF vaccine was performed and symptoms improved 5 days later. This is the first report dealing with longitudinal myelitis as a serious adverse event associated with YF vaccination in which confirmation of the presence of antibodies in CSF was found. To date, it is also the first report with serological confirmation in Argentina and in South America. We consider that the present investigation will raise awareness in the region in the reporting of adverse events related to YF vaccine and improve our knowledge of adverse reactions to the vaccine.

Importance of background rates of disease in assessment of vaccine safety during mass immunisation with pandemic H1N1 influenza vaccines

Because of the advent of a new influenza A H1N1 strain, many countries have begun mass immunisation programmes. Awareness of the background rates of possible adverse events will be a crucial part of assessment of possible vaccine safety concerns and will help to separate legitimate safety concerns from events that are temporally associated with but not caused by vaccination. We identified background rates of selected medical events for several countries. Rates of disease events varied by age, sex, method of ascertainment, and geography. Highly visible health conditions, such as Guillain-Barré syndrome, spontaneous abortion, or even death, will occur in coincident temporal association with novel influenza vaccination. On the basis of the reviewed data, if a cohort of 10 million individuals was vaccinated in the UK, 21.5 cases of Guillain-Barré syndrome and 5.75 cases of sudden death would be expected to occur within 6 weeks of vaccination as coincident background cases. In female vaccinees in the USA, 86.3 cases of optic neuritis per 10 million population would be expected within 6 weeks of vaccination. 397 per 1 million vaccinated pregnant women would be predicted to have a spontaneous abortion within 1 day of vaccination.

Safety of trivalent inactivated influenza vaccines in adults: background for pandemic influenza vaccine safety monitoring

In preparation for pandemic vaccine safety monitoring, we assessed adverse events reported to the Vaccine Adverse Event Reporting System following receipt of trivalent inactivated influenza vaccines among adults from 1990 through 2005. We calculated reporting rates for nonserious, serious, and neurological adverse events. We reviewed reports of recurrent events and deaths, as well as reports identified through advanced signal detection. The most frequently reported events were local reactions and systemic symptoms. Guillain-Barré syndrome was the most frequently reported serious event (0.70 reports per million vaccinations). Adverse event reporting rates have been reasonably constant over time. No new safety concerns emerged after our review of 15 years of post-licensure surveillance data. These findings provide useful information if pandemic vaccine is rapidly distributed and pre-licensure data are limited.

A review of prophylactic human papillomavirus vaccines: recommendations and monitoring in the US

It has been estimated that genital human papillomavirus (HPV) is the most common sexually transmitted infection in the US. Nononcogenic types, such as HPV type 6 (HPV-6) and HPV-11, can cause benign or low-grade cervical cell changes, genital warts, and recurrent respiratory papillomatosis. Oncogenic types can cause cervical and other anogenital cancers; oncogenic HPV types are detected in 99% of cervical cancers worldwide. A quadrivalent HPV vaccine to prevent HPV-6, HPV-11, HPV-16, and HPV-18 was licensed for use in the US in June 2006 and an application for Food and Drug Administration licensure was submitted for a bivalent HPV vaccine to prevent HPV-16 and HPV-18 in March 2007. Currently in the US, the quadrivalent HPV vaccine is recommended for routine immunization of girls aged 11 and 12 years, and catch-up immunization is recommended through age 26 years. Monitoring the impact of prophylactic HPV vaccines will be useful for understanding the population level impact of vaccination. In this report, the authors provide a brief review of the epidemiology of HPV infection and an overview of prophylactic HPV vaccines and postvaccine licensure monitoring.

Adverse event reports following yellow fever vaccination

Yellow fever (YF) vaccine has been used for prevention of YF since 1937 with over 500 million doses administered. However, rare reports of severe adverse events following vaccination have raised concerns about the vaccine's safety. We reviewed reports of adverse events following YF vaccination reported to the U.S. Vaccine Adverse Event Reporting System (VAERS) from 2000 to 2006. We used estimates of age and sex distribution of administered doses obtained from a 2006 survey of authorized vaccine providers to calculate age- and sex-specific reporting rates of all serious adverse events (SAE), anaphylaxis, YF vaccine-associated neurotropic disease, and YF vaccine-associated viscerotropic disease. Reporting rates of SAEs were substantially higher in males and in persons aged > or =60 years. These findings reinforce the generally acceptable safety profile of YF vaccine, but highlight the importance of physician and traveler education regarding the risks and benefits of YF vaccination, particularly for travelers > or =60 years of age. Vaccination should be limited to persons traveling to areas where the risk of YF is expected to exceed the risk of serious adverse events after vaccination, or if not medically contraindicated, where national regulations require proof of vaccination to prevent introduction of YF.