Mass vaccination: when and why.

With increased demand for smallpox vaccination during the nineteenth century, vaccination days--early mass vaccination campaigns--were conducted over time-limited periods to rapidly and efficiently protect maximum numbers of susceptible persons. Two centuries later, the challenge to rapidly and efficiently protect populations by mass vaccintion continues, despite the strengthening of routine immunization services in many countries through the Expanded Programme on Immunization strategies and GAVI support. Perhaps the most widely accepted reason for mass vaccination is to rapidly increase population (herd) immunity in the setting of an existing or potential outbreak, thereby limiting the morbidity and mortality that might result, especially when there has been no routine vaccination, or because populations have been displaced and routine immunization services disrupted. A second important use of mass vaccination is to accelerate disease control to rapidly increase coverage with a new vaccine at the time of its introduction into routine immunization programmes, and to attain the herd immunity levels required to meet international targets for eradication and mortality reduction. In the twenty-first century, mass vaccination and routine immunization remain a necessary alliance for attaining both national and international goals in the control of vaccine preventable disease.

Early detection and response to meningococcal disease epidemics in sub-Saharan Africa: appraisal of the WHO strategy.

OBJECTIVE: To assess the sensitivity, specificity and predictive value positive of the WHO threshold strategy for detecting meningococcal disease epidemics in sub-Saharan Africa and to estimate the impact of the strategy on an epidemic at district level. METHODS: Data on meningitis cases at the district level were collected weekly from health ministries, WHO country and regional offices, and nongovernmental organizations in countries where there were epidemics of meningococcal disease in 1997. An epidemic was defined as a cumulative district attack rate of at least 100 cases per 100,000 population from January to May, the period of epidemic risk. The sensitivity, specificity and predictive value positive of the WHO threshold rate were calculated, and curves of sensitivity against (1 - specificity) were compared with alternatively defined threshold rates and epidemic sizes. The impact of the WHO strategy on a district epidemic was estimated by comparing the numbers of epidemic cases with cases estimated to have been prevented by vaccination. FINDINGS: An analysis was made of 48 198 cases reported in 174 districts in Benin, Burkina Faso, the Gambia, Ghana, Mali, Niger, and Togo. These cases were 80.3% of those reported from Africa to WHO during the 1997 epidemic period. District populations ranged from 10,298 to 573,908. The threshold rate was crossed during two consecutive weeks in 69 districts (39.7%) and there were epidemics in 66 districts (37.9%). Overall, the sensitivity of the threshold rate for predicting epidemics was 97%, the specificity was 95%, and the predictive value positive was 93%. Taken together, these values were equivalent or better than the sensitivity, specificity and predictive value positive of alternatively defined threshold rates and epidemics, and remained high regardless of district size. The estimated number of potential epidemic cases decreased by nearly 60% in the age group targeted for vaccination in one district where the guidelines were followed in a timely manner. CONCLUSION: The use of the WHO strategy was sensitive and specific for the early detection of meningococcal disease epidemics in countries of sub-Saharan Africa during 1997 and had a substantial impact on a district epidemic. Nevertheless, the burden of meningococcal disease in these countries remains formidable and additional control measures are needed.

Hot spots in a wired world: WHO surveillance of emerging and re-emerging infectious diseases.

The resurgence of the microbial threat, rooted in several recent trends, has increased the vulnerability of all nations to the risk of infectious diseases, whether newly emerging, well-established, or deliberately caused. Infectious disease intelligence, gleaned through sensitive surveillance, is the best defence. The epidemiological and laboratory techniques needed to detect, investigate, and contain a deliberate outbreak are the same as those used for natural outbreaks. In April 2000, WHO formalised an infrastructure (the Global Outbreak Alert and Response Network) for responding to the heightened need for early awareness of outbreaks and preparedness to respond. The Network, which unites 110 existing networks, is supported by several new mechanisms and a computer-driven tool for real time gathering of disease intelligence. The procedure for outbreak alert and response has four phases: systematic detection, outbreak verification, real time alerts, and rapid response. For response, the framework uses different strategies for combating known risks and unexpected events, and for improving both global and national preparedness. New forces at work in an electronically interconnected world are beginning to break down the traditional reluctance of countries to report outbreaks due to fear of the negative impact on trade and tourism. About 65% of the world's first news about infectious disease events now comes from informal sources, including press reports and the internet.

Rumors of disease in the global village: outbreak verification.

Emerging infectious diseases and the growth of information technology have produced new demands and possibilities for disease surveillance and response. Increasing numbers of outbreak reports must be assessed rapidly so that control efforts can be initiated and unsubstantiated reports can be identified to protect countries from unnecessary economic damage. The World Health Organization has set up a process for timely outbreak verification to convert large amounts of data into accurate information for suitable action. We describe the context and processes of outbreak verification and information dissemination.

Considerations regarding mass vaccination against typhoid fever as an adjunct to sanitation and public health measures: potential use in an epidemic in Tajikistan.

We report on the ongoing epidemic of typhoid fever in Tajikistan that started in 1996. It has involved more than 24,000 cases to date, and is characterized by multiple point sources, overflow of sewage, contaminated municipal water, and person-to-person spread. Of the Salmonella typhi isolates available for testing in western laboratories, more than 90% are multidrug-resistant (MDR). Most recently, 28 (82%) of 34 isolates are resistant to ciprofloxacin, representing the first reported epidemic of quinolone-resistant typhoid fever. In the past, mass immunization during typhoid fever epidemics has been discouraged. A review of this policy is recommended in light of the alarming emergence of quinolone-resistant strains of S. typhi, the availability of improved vaccines, and the ongoing epidemic in Tajikistan. Mass immunization may be a useful measure for the control of prolonged MDR typhoid fever epidemics, as an adjunct to correction of municipal infrastructure and public health intervention.

Ebola hemorrhagic fever: lessons from Kikwit, Democratic Republic of the Congo.

The outbreak of Ebola hemorrhagic fever in Kikwit, Democratic Republic of the Congo, clearly signaled an end to the days when physicians and researchers could work in relative obscurity on problems of international importance, and it provided many lessons to the international public health and scientific communities. In particular, the outbreak signaled a need for stronger infectious disease surveillance and control worldwide, for improved international preparedness to provide support when similar outbreaks occur, and for accommodating the needs of the press in providing valid information. A need for more broad-based international health regulations and electronic information systems within the World Health Organization also became evident, as did the realization that there are new and more diverse partners able to rapidly respond to international outbreaks. Finally, a need for continued and coordinated Ebola research was identified, especially as concerns development of simple and valid diagnostic tests, better patient management procedures, and identification of the natural reservoir.

Ebola between outbreaks: intensified Ebola hemorrhagic fever surveillance in the Democratic Republic of the Congo, 1981-1985.

Surveillance for Ebola hemorrhagic fever was conducted in the Democratic Republic of the Congo from 1981 to 1985 to estimate the incidence of human infection. Persons who met the criteria of one of three different case definitions were clinically evaluated, and blood was obtained for antibody confirmation by IFA. Contacts of each case and 4 age- and sex-matched controls were also clinically examined and tested for immunofluorescent antibody. Twenty-one cases of Ebola infection (persons with an antibody titer of > or = 1:64, or lower if they fit the clinical case definition) were identified, with a maximum 1-year incidence of 9 and a case fatality rate of 43%. Cases occurred throughout the year, but most (48%) occurred early in the rainy season. Fifteen percent of contacts had antibody titers > or =1:64 to Ebola virus, compared with 1% of controls (P < .0001). Results suggest that Ebola virus periodically emerges from nature to infect humans, that person-to-person transmission is relatively limited, and that amplification to large epidemics is unusual.

The reemergence of Ebola hemorrhagic fever, Democratic Republic of the Congo, 1995. Commission de Lutte contre les Epidémies à Kikwit.

In May 1995, an international team characterized and contained an outbreak of Ebola hemorrhagic fever (EHF) in Kikwit, Democratic Republic of the Congo. Active surveillance was instituted using several methods, including house-to-house search, review of hospital and dispensary logs, interview of health care personnel, retrospective contact tracing, and direct follow-up of suspect cases. In the field, a clinical case was defined as fever and hemorrhagic signs, fever plus contact with a case-patient, or fever plus at least 3 of 10 symptoms. A total of 315 cases of EHF, with an 81% case fatality, were identified, excluding 10 clinical cases with negative laboratory results. The earliest documented case-patient had onset on 6 January, and the last case-patient died on 16 July. Eighty cases (25%) occurred among health care workers. Two individuals may have been the source of infection for >50 cases. The outbreak was terminated by the initiation of barrier-nursing techniques, health education efforts, and rapid identification of cases.

A search for Ebola virus in animals in the Democratic Republic of the Congo and Cameroon: ecologic, virologic, and serologic surveys, 1979-1980. Ebola Virus Study Teams.

More than 30 years after the first outbreak of Marburg virus disease in Germany and Yugoslavia and 20 years after Ebola hemorrhagic fever first occurred in central Africa, the natural history of filoviruses remains unknown. In 1979 and 1980, animals in the Democratic Republic of the Congo and Cameroon were collected during the dry season near the site of the 1976 Ebola hemorrhagic fever epidemic. The study objectives were to identify local animals and search for evidence of Ebola virus in their tissues. A total of 1664 animals representing 117 species was collected, including >400 bats and 500 rodents. Vero and CV-1 cells and IFA and RIA were used for virus and antibody detection, respectively. No evidence of Ebola virus infection was found. This study was limited in time and animal collections and excluded insects and plants. Long-term, prospective, multidisciplinary comparative studies will yield more information than will repeat short forays on the ecology of filoviruses.

Diagnosis of sexually transmitted infections in female prostitutes in Dakar, Senegal.

OBJECTIVE: To study the validity and performance of a number of rapid indicators for the diagnosis of sexually transmitted infections (STIs) in female prostitutes in Dakar, Senegal; characteristics of these indicators were rapidly obtainable, easy to perform, accurate, useful at district level, and reasonable cost. METHODS: An STI prevalence study in female prostitutes (n = 374) seen at the STD clinic in Dakar, Senegal was done; a history, clinical examination, simple laboratory tests, and "gold standard" microbiological tests were performed. For a number of sociodemographic data, actual or past symptoms of STI, clinical signs, and rapid laboratory tests, validity variables, performance characteristics, and likelihood ratios for detection of gonococcal or chlamydial cervical infection were determined. RESULTS: Cervical infection (chlamydial or gonococcal) was present in 24.9% of prostitutes; 46% had trichomoniasis and 29.4% had syphilis. Young age, abnormal vaginal discharge, endocervical mucopus, a positive leucocyte esterase test on urine, and 10 or more leucocytes in Gram stained smears of vaginal, cervical, or urine samples were significantly associated with cervical STI. Some of the rapid indicators had high sensitivity, others high specificity but none had acceptable overall validity. None of the indicators had at the same time a sensitivity above 50% and a positive predictive value above twice the background prevalence of cervical infection. 10 or more leucocytes in the cervical smear had a likelihood ratio of 1.83 increasing pretest probability of 24.9% to post-test probability of 38%, the best result obtained by any of the rapid indicators. CONCLUSIONS: Rapid indicators of cervical STIs are insufficiently valid, which largely restricts their usefulness to high STI prevalence situations for instance, in prostitute populations and in STD patient management.

Re-emergence of monkeypox in Africa: a review of the past six years.

Human monkeypox was first identified in 1970 in the Democratic Republic of the Congo. Extensive studies of this zoonotic infection in the 1970s and 1980s indicated a largely sporadic disease with a minority of cases resulting from person-to-person transmission, rarely beyond two generations. In August 1996, an unusually large outbreak of human monkeypox was reported, and cases continued through 1997 with peak incidence in August 1996, March 1997 and August 1997. Preliminary results from the field investigations in 1997 suggest a new epidemiological pattern where a majority of secondary cases result from person-to-person transmission, and a clinically milder disease. But there is preliminary laboratory evidence of a simultaneous outbreak of varicella in the same geographic region which will undoubtedly modify these preliminary results. Since smallpox was eradicated and vaccinia vaccination terminated in this region, the population of susceptible individuals has grown. The use of vaccination to protect the population at risk, however, must take into account HIV prevalence and the risk of generalized vaccinia when using vaccinia vaccine in populations where HIV is known to be present.