Boosting can explain patterns of fluctuations of ratios of inapparent to symptomatic dengue virus infections

Indian Ocean temperature anomalies predict long-term global dengue trends

Despite identifying El Niño events as a factor in dengue dynamics, predicting the oscillation of global dengue epidemics remains challenging. Here, we investigate climate indicators and worldwide dengue incidence from 1990 to 2019 using climate-driven mechanistic models. We identify a distinct indicator, the Indian Ocean basin-wide (IOBW) index, as representing the regional average of sea surface temperature anomalies in the tropical Indian Ocean. IOBW is closely associated with dengue epidemics for both the Northern and Southern hemispheres. The ability of IOBW to predict dengue incidence likely arises as a result of its effect on local temperature anomalies through teleconnections. These findings indicate that the IOBW index can potentially enhance the lead time for dengue forecasts, leading to better-planned and more impactful outbreak responses.

Seroprevalence of dengue IgG and associated risk factors in symptomatic and asymptomatic adults in Posadas (Misiones, Argentina), 2017-2019

BACKGROUND

Dengue represents a major public health problem in the Americas in general, and in Posadas (Misiones, Argentina) in particular. This study aimed to assess the seroprevalence of dengue virus infection, analyze associated factors, and determine the proportion of asymptomatic cases.

METHODS

This prospective cross-sectional study took place from November 2017 to April 2019 in the High Complexity Laboratory of Misiones, at the School Hospital Dr. Ramón Madariaga in Posadas. A random sample of 301 adults (≥ 15 years) was selected from the electoral registry and stratified by geographical area of residence. Sociodemographic, clinical, and laboratory data were collected by means of a survey and serology. Results were analyzed using multivariable logistic regression.

RESULTS

The median age of the sample was 33 years; 66% were women, and 46.5% had completed at least secondary school. Anti-dengue IgG antibodies were present in 40.2% of the sample (95% confidence interval [CI] 34.5-45.9%), including 90% of those who reported dengue and 20.5% who did not (odds ratio [OR] 33.25, 95% CI 15.46-71.51, p < 0.001). In the multivariable analysis, adjusted for age, group, gender, and vaccination against yellow fever, seropositivity was associated with having relatives with dengue (adjusted OR 3.96, 95% CI 2.18-7.23; p < 0.001).

CONCLUSION

Seroprevalence for dengue in Posadas was higher than estimates based on the notification records, and there was a high proportion of asymptomatic cases. Educational level and having a family member who had suffered from dengue were associated with positive serology.

Effects of boosting and waning in highly exposed populations on dengue epidemic dynamics

Sequential infection with multiple dengue virus (DENV) serotypes is thought to induce enduring protection against dengue disease. However, long-term antibody waning has been observed after repeated DENV infection. Here, we provide evidence that highly immune Nicaraguan children and adults (n = 4478) experience boosting and waning of antibodies during and after major Zika and dengue epidemics. We develop a susceptible-infected-recovered-susceptible (SIRS-type) model that tracks immunity by titer rather than number of infections to show that boosts in highly immune individuals can contribute to herd immunity, delaying their susceptibility to transmissible infection. In contrast, our model of lifelong immunity in highly immune individuals, as previously assumed, results in complete disease eradication after introduction. Periodic epidemics under this scenario can only be sustained with a constant influx of infected individuals into the population or a high basic reproductive number. We also find that Zika virus infection can boost DENV immunity and produce delays and then surges in dengue epidemics, as observed with real epidemiological data. This work provides insight into factors shaping periodicity in dengue incidence and may inform vaccine efforts to maintain population immunity.

Insights into dengue immunity from vaccine trials

The quest for an effective dengue vaccine has culminated in two approved vaccines and another that has completed phase 3 clinical trials. However, shortcomings exist in each, suggesting that the knowledge on dengue immunity used to develop these vaccines was incomplete. Vaccine trial findings could refine our understanding of dengue immunity, because these are experimentally derived, placebo-controlled data. Results from these trials suggest that neutralizing antibody titers alone are insufficient to inform protection against symptomatic infection, implicating a role for cellular immunity in protection. These findings have relevance for both future dengue vaccine development and application of current vaccines for maximal public health benefit.

Inapparent infections shape the transmission heterogeneity of dengue

Transmission heterogeneity, whereby a disproportionate fraction of pathogen transmission events result from a small number of individuals or geographic locations, is an inherent property of many, if not most, infectious disease systems. For vector-borne diseases, transmission heterogeneity is inferred from the distribution of the number of vectors per host, which could lead to significant bias in situations where vector abundance and transmission risk at the household do not correlate, as is the case with dengue virus (DENV). We used data from a contact tracing study to quantify the distribution of DENV acute infections within human activity spaces (AS), the collection of residential locations an individual routinely visits, and quantified measures of virus transmission heterogeneity from two consecutive dengue outbreaks (DENV-4 and DENV-2) that occurred in the city of Iquitos, Peru. Negative-binomial distributions and Pareto fractions showed evidence of strong overdispersion in the number of DENV infections by AS and identified super-spreading units (SSUs): i.e. AS where most infections occurred. Approximately 8% of AS were identified as SSUs, contributing to more than 50% of DENV infections. SSU occurrence was associated more with DENV-2 infection than with DENV-4, a predominance of inapparent infections (74% of all infections), households with high Aedes aegypti mosquito abundance, and high host susceptibility to the circulating DENV serotype. Marked heterogeneity in dengue case distribution, and the role of inapparent infections in defining it, highlight major challenges faced by reactive interventions if those transmission units contributing the most to transmission are not identified, prioritized, and effectively treated.

Development of inapparent dengue associated with increased antibody levels to Aedes aegypti salivary proteins: a longitudinal dengue cohort in Cambodia

BACKGROUND

We established the first prospective cohort to understand how infection with dengue virus is influenced by vector-specific determinants like humoral immunity to Aedes aegypti salivary proteins.

METHODS

Children aged two to nine years old enrolled in the PAGODAS (Pediatric Assessment Group of Dengue and Aedes Saliva) cohort with informed consent by their guardians. Children were followed semi-annually for antibodies to dengue and to proteins in Ae. aegypti salivary gland homogenate using enzyme-linked immunosorbent assays and dengue-specific neutralization titers. Children presented with fever at any time for dengue testing.

RESULTS

From July 13 to August 30, 2018, we enrolled 771 children. At baseline, 22% (173/770) had evidence of neutralizing antibodies to one or more dengue serotypes. By April 2020, 51 children had symptomatic dengue while 148 dengue-naïve children had inapparent dengue defined by neutralization assays. In a multivariate model, individuals with higher antibodies to Ae. aegypti salivary proteins were 1.5x more likely to have dengue infection (HR 1.47 95% CI 1.05-2.06; p=0.02), particularly individuals with inapparent dengue (HR 1.64 95% CI 1.12-2.41; p=0.01).

CONCLUSIONS

High levels of seropositivity to Ae. aegypti salivary proteins are associated with future development of dengue infection, primarily inapparent, in dengue-naïve Cambodian children.

Dengue and Zika virus infections in children elicit cross-reactive protective and enhancing antibodies that persist long term

[Figure: see text].