Researchers at Sanofi Pasteur, have developed a “transmission model” for Dengue and assessed the population impact of vaccination in the 10 countries participating in the Phase III efficacy studies for CYD-TDV/Dengvaxia.
The viability of existing vaccines for Dengue virus may still be open to debate, but the impact of a safe and effective vaccine program in countries affected by the mosquito-borne infection no longer is.
Researchers at Sanofi Pasteur, the company that to date has developed the most viable vaccine candidate, assessed the value of CYD-TDV (or Dengvaxia) in reducing the incidence of the virus in countries where it has proved problematic. Their findings are “in press” with the journal Vaccine.
The authors developed a “transmission model” for Dengue and assessed the population impact of vaccination in the 10 countries participating in the Phase III efficacy studies for CYD-TDV/Dengvaxia: namely, Indonesia, Malaysia, Philippines, Thailand, Vietnam, Brazil, Colombia, Honduras, Mexico, and Puerto Rico. They used what they described as an “age-structured, host-vector, serotype-specific compartmental model” and estimated parameters related to vaccine efficacy and levels of Dengue transmission using data collected during the trials.
In general, the researchers found that all 10 of the vaccination programs that were assessed translated into significant reductions in the number of Dengue cases at the population level over the first 10 years following program initiation. Their research suggests that routine vaccination at age 9 with CYD-TDV/Dengvaxia can reduce the number of Dengue cases by as much as 25% within the first 5 years of the vaccine program’s initiation and by as much as 25% within 10 years.
Although the most efficient age for vaccination varied according to transmission intensity, the authors believe that 9 years of age “is the most efficient age across all settings” for administration of the vaccine. Still, many participating countries instituted “catch-up” programs, in which large populations were vaccinated at later ages, with a significant effect. Indeed, vaccine impact increases in proportion to vaccine coverage. At 90% coverage via catch-up vaccination, for example, Dengue cases could be reduced by as much as 50%.
“For all countries considered, broader catch-up campaigns were always associated with larger reductions in Dengue cases and hospitalizations,” the authors of the study wrote. “For a program including 20 catch-up cohorts, the reductions in hospitalizations ranged from 45% (Thailand) to 68% (Honduras). The number of Dengue cases prevented by vaccine dose administered indicates that catch-up campaigns of adequate size are likely to improve the efficiency of the program.”
Overall, the authors believe that the combination of “routine vaccination and catch-up campaigns provide an opportunity for a more rapid reduction in the Dengue burden compared with routine vaccination alone.” In their concluding remarks, they added, “The implementation of a vaccination program was not found to stop variation of Dengue incidence over time but rather limited the frequency and intensity of these variations. However, the combination of routine vaccination and large catch-up campaigns significantly limited the risk of a high dengue incidence during the first few years following vaccine introduction.”
Although the news regarding CYD-TDV/Dengvaxia has been largely positive, there are some concerns over the safety of the vaccine. In the aforementioned Phase III trials, conducted in Southeast Asia and Latin America, the vaccine was initially able to reduce the incidence of infection by 60% and the number of hospitalizations due to infection by 80%. However, researchers observed that many of those who had been vaccinated later developed severe Dengue.