Seroconversions in unvaccinated infants: further evidence for subclinical measles from vaccine trials in Niakhar, Senegal

Improving the quality of adult mortality data collected in demographic surveys: validation study of a new siblings' survival questionnaire in Niakhar, Senegal

BACKGROUND

In countries with limited vital registration, adult mortality is frequently estimated using siblings' survival histories (SSHs) collected during Demographic and Health Surveys (DHS). These data are affected by reporting errors. We developed a new SSH questionnaire, the siblings' survival calendar (SSC). It incorporates supplementary interviewing techniques to limit omissions of siblings and uses an event history calendar to improve reports of dates and ages. We hypothesized that the SSC would improve the quality of adult mortality data.

METHODS AND FINDINGS

We conducted a retrospective validation study among the population of the Niakhar Health and Demographic Surveillance System in Senegal. We randomly assigned men and women aged 15-59 y to an interview with either the DHS questionnaire or the SSC. We compared SSHs collected in each group to prospective data on adult mortality collected in Niakhar. The SSC reduced respondents' tendency to round reports of dates and ages to the nearest multiple of five or ten ("heaping"). The SSC also had higher sensitivity in recording adult female deaths: among respondents whose sister(s) had died at an adult age in the past 15 y, 89.6% reported an adult female death during SSC interviews versus 75.6% in DHS interviews (p = 0.027). The specificity of the SSC was similar to that of the DHS questionnaire, i.e., it did not increase the number of false reports of deaths. However, the SSC did not improve the reporting of adult deaths among the brothers of respondents. Study limitations include sample selectivity, limited external validity, and multiple testing.

CONCLUSIONS

The SSC has the potential to collect more accurate SSHs than the questionnaire used in DHS. Further research is needed to assess the effects of the SSC on estimates of adult mortality rates. Additional validation studies should be conducted in different social and epidemiological settings.

TRIAL REGISTRATION

Controlled-Trials.com ISRCTN06849961

The importance of biological samples in longitudinal birth cohort studies

Here we describe how measurements in biological samples can be used to provide direct measures of exposures to environmental pollutants, nutrients, infectious organisms and drugs of abuse, and to validate other less direct measures of exposure such as questionnaire responses. They can also be used as measures of outcome traits or intermediate phenotypes which may lie on the pathways to disease.

Emergence of infectious diseases: when hidden pathogens break out

Our understanding of disease emergence is largely limited by the assumption that disease emergence is the result of increased exposure to pathogenic agents. Pathogen exposure is thought to arise through an increase in the number of interactions between humans and their natural environment, changes in demography and mobility, or through genetic variation in the infectious agents which may alter virulence or ability to infect new host species. The study of new diseases (which are often revealed by unusually severe symptoms or atypical epidemiological patterns) applies the most effort to the research of new pathogens. Here, using examples, we discuss alternative but non-exclusive mechanisms that may either reveal the presence of long-term circulating pathogens or explain changes in their nosologic properties in relation to their pattern of circulation and infection conditions. A better understanding of the ecology of pathogenic organisms and their host populations should help to define more efficient health management strategies.

The role of maternal antibodies in the emergence of severe disease as a result of fragmentation

Population fragmentation is a major problem for the conservation of mammalian species. Since the spread of an infectious disease is related to the intensity of contacts between individuals, fragmentation destabilizes the way the parasites circulate in their host population. Recently, Zinkernagel has proposed that a reduction in the frequency of infections by a parasite could lead to the emergence of severe forms of the disease, previously avoided because the disease was contracted early in life and attenuated by maternal antibodies. However, it is still unclear whether this change in disease expression increases the global mortality it induces because the disease becomes more severe and also less frequent. Here, we use a mathematical model to link population fragmentation with the hypothesis of Zinkernagel. Firstly, we show that there is a change in the severity of the disease during the fragmentation process, especially at a local scale, suggesting that host population fragmentation could be a widespread mechanism of disease emergence. Secondly, we show that the emergence of the severe form of the disease can lead to a significant increase in its induced mortality. Finally, we determine the types of interactions for which the fragmentation of the host population could be the most dangerous.

Waning of maternal immunity and the impact of diseases: The example of myxomatosis in natural rabbit populations

Myxomatosis is a leporipoxvirus that infects the european rabbit, inducing a high mortality rate. Observations lead us to hypothesize that a rabbit carrying maternal antibodies (or having recovered) can be infected (or re-infected) upon being exposed (or re-exposed) to the virus. Infection will lead to mild disease, boosting host immune protection. Using a modelling approach we show that this phenomenon may lead to a difference of impact of myxomatosis according to its transmission rate. Young are exposed when they still carry maternal antibodies and develop a mild disease in high transmission populations. Our results show that the impact of myxomatosis is generally higher in epidemic situations compared to populations where the virus circulates all the year. As a consequence, waning of acquired immunity and the continuous supply of newborn along the year may reduce the impact of the disease.

The clinical significance of measles: a review

Forty years after effective vaccines were licensed, measles continues to cause death and severe disease in children worldwide. Complications from measles can occur in almost every organ system. Pneumonia, croup, and encephalitis are common causes of death; encephalitis is the most common cause of long-term sequelae. Measles remains a common cause of blindness in developing countries. Complication rates are higher in those <5 and >20 years old, although croup and otitis media are more common in those <2 years old and encephalitis in older children and adults. Complication rates are increased by immune deficiency disorders, malnutrition, vitamin A deficiency, intense exposures to measles, and lack of previous measles vaccination. Case-fatality rates have decreased with improvements in socioeconomic status in many countries but remain high in developing countries.

Experimental vaccines against measles in a world of changing epidemiology

Vaccination with the current live attenuated measles vaccine is one of the most successful and cost-effective medical interventions. However, as a result of persisting maternal antibodies and immaturity of the infant immune system, this vaccine is poorly immunogenic in children <9 months old. Immunity against the live vaccine is less robust than natural immunity and protection less durable. There may also be some concern about (vaccine) virus spread during the final stage of an eventual measles eradication program. Opinions may differ with respect to the potential threat that some of these concerns may be to the World Health Organisation goal of measles elimination, but there is a consensus that the development of new measles vaccines cannot wait. Candidate vaccines are based on viral or bacterial vectors expressing recombinant viral proteins, naked DNA, immune stimulating complexes or synthetic peptides mimicking neutralising epitopes. While some of these candidate vaccines have proven their efficacy in monkey studies, aerosol formulated live attenuated measles vaccine are evaluated in clinical trials.

Antibody dynamics in childhood diseases: waning and boosting of immunity and the impact of vaccination

The introduction of vaccination against acute diseases such as measles induced a dramatic decline in the prevalence of the disease, and a more gradual rise in the proportion of the population whose immunity is derived solely from vaccination. These two factors combine to constitute an important shift in the dynamics of immunity, especially in highly vaccinated populations. We develop a general model to describe both loss of immunity in the absence of disease, and boosting of immunity corresponding to subclinical infection in individuals whose immunity has waned. We consider the interaction between infection and immunity and identify the key parameters that determine the eradication threshold. We explore the dynamics in the years following the introduction of vaccination using a stochastic version of the model, and consider the effect of different assumptions concerning the nature of immunity. A comparison of the model results with recently published data suggests that heterogeneity in host immune response is an important feature of the antibody dynamics.

The challenge of improving the efficacy of measles vaccine

Despite a safe and effective measles vaccine, measles still claims an estimated 800,000 lives per year mostly among children in developing countries. This paper deals with strategies to improve vaccine efficacy and prevent unnecessary deaths, including considerations of one dose at 9 months strategy for developing countries, strain of vaccine, potency and number of doses of measles vaccine. After more than 20 years of measles immunisation in the developing world, the epidemiology of measles is radically changed, and the absence of measles epidemics might lead to waning immunity due to less clinical and subclinical infections boosting the antibody level. An increasing proportion of mothers are vaccinated, thus transferring a lower maternal antibody level to their infants who will be susceptible to measles at a younger age. The strategies to limit nosocomial measles infection and spread of measles epidemics are reviewed. Though the measles elimination programmes have been very effective in the Americas, it seems unlikely that they will be equally effective in the rest of the world. Even if eradication should be possible, it might be unwise to stop measles vaccination because the vaccine apparently has beneficial effects and because it would make measles a likely weapon for bio-terrorism. If we are unlikely to get rid of measles and measles vaccine, it might be wise to study further some of the many unanswered questions regarding the long-term effects of measles and measles vaccination.

Effect of subclinical infection on maintaining immunity against measles in vaccinated children in West Africa

BACKGROUND

Despite a high coverage with measles vaccines in parts of west Africa, epidemics of measles occur with reduced severity in an increasing proportion of older children who have been vaccinated. We examined the effect of exposure to natural measles on immunity in vaccinated children.

METHODS

Our study was carried out in 1992 during an epidemic of measles in Niakhar, a rural area of Senegal with about 27,000 inhabitants who mostly live in compounds that include several households; within each household people live in different huts. Vaccine coverage in Niakhar was 81% at the time of our study. We measured haemagglutinin-inhibiting antibody at exposure and twice thereafter (after 4-5 weeks and at 6 months) in 36 vaccinated and 87 unvaccinated children. The frequency of measles and subclinical measles--defined as a four-fold or greater rise in antibody titre without clinical signs or symptoms--was related to intensity of exposure according to whether the index case was in the same hut, household, or compound.

FINDINGS

Clinical measles occurred in 20 (56%) of 36 unvaccinated children and in one (1%) of 87 vaccinated children. Subclinical measles occurred in 39 (45%) of 86 vaccinated children who were exposed to measles and in four (25%) of 16 unvaccinated children. The frequency was inversely related to pre-exposure antibody concentration (p<0.001 for trend) and directly related to intensity of exposure (p=0.002 for trend). Antibody concentrations in subclinical cases increased on average by 45-fold and remained raised for at least 6 months.

INTERPRETATION

Increased antibody titre after subclinical measles may be common in vaccinated children in West Africa where the intensity of exposure is high. As measles vaccination coverage increases, the circulation of wild measles will decrease, and vaccine-induced antibody is less likely to be boosted. Thus, new epidemics, albeit milder in form, may occur in vaccinated areas which should be recognised in campaigns to eradicate measles.