Dear malariacontrol.net user,
In our last posting, we described several updates and improvements we`ve made to our model. This round, we`d like to report on a research question we`ve addressed with the model: what is the optimal strategy for distributing a malaria vaccine?
The question is particularly relevant since one promising vaccine—RTS,S—is currently undergoing phase III clinical trials (the last stage of testing before licensing) in infants and children in seven African countries.
As you know, malaria transmission can be reduced by spraying insecticide to reduce the number of mosquitoes, and by reducing the number of bites by sleeping under insecticide-treated bed nets. Treating patients with anti-malarial drugs also reduces transmission because it reduces the chance that a mosquito will become infected with an active malaria parasite when it bites the patient. For the same reason, a malaria vaccine could reduce the overall malaria burden, both by protecting an individual from disease, but also because overall transmission will be reduced: that is, mosquitoes who bite the vacinated individual won`t become infected with the malaria parasite through that bite.
Assuming the RTS,S vaccine fulfills its promise and is licensed, the next question will be how to best distribute the vaccine. One possibility would be to include the malaria vaccine in the suite of immunizations planned through the World Health Organization's Expanded Programme on Immunization (EPI), which aims to provide universal access to immunization against several infectious diseases during the first three months of life. Another possibility would through a mass malaria vaccination campaign, or some combination of both ideas.
The malaria model developed and calibrated though your efforts is the ideal tool to address this problem. Researchers here at the Swiss Tropical and Public Health Institute submitted an ensemble of models of P. falciparum dynamics to assess the likely public health impact of the RTS,S malaria vaccine to malariacontrol.net; your machines did the rest.
These models included different assumptions about the decay of acquired immunity against malaria, different transmission efficiencies, and, critically, patient access to treatment. The passage of thousands of hypothetical individuals through different stages of malaria infection was simulated; movement between stages occurs stochastically (by chance) at a probability based on field data. Each model was used to predict the health benefits during 14 years of RTS,S deployment through four different immunization strategies:
(1) EPI without catch-up vaccination for infants who were not immunized during their first three months of life
(2) EPI with catch-up vaccination for infants who were not immunized during their first three months of life
(3) EPI and supplementary vaccination of school children
(4) Mass vaccination campaigns every five years
The predicted benefits of EPI vaccination programs over the 14-year period were modest and similar over a wide range of settings. However, there were differences depending on the malaria transmission levels, or entomological inoculation rate [EIR].
Strategy number 2 (EPI with an initial catch-up phase) averted the most deaths per vaccine dose at higher EIRs (11 and 20 infectious bites per person per year).
At the lowest EIR (2 infectious bites per year), strategy 4, (mass vaccination) substantially reduced transmission, leading to much greater health improvement per dose than other strategies, even at modest coverage.
You can see this difference in the lower right figure on the graphic below:
The conclusion: targeted mass vaccination with RTS,S in low transmission settings may have greater health benefits than vaccination through national EPI programs.
Most importantly, the computer-intensive approach provides more secure predictions than can be obtained using any single model. – this computer intensive approach was only possible through our combined efforts. You can read more about the work at
Ensemble Modeling of the Likely Public Health Impact of a Pre-Erythrocytic Malaria Vaccine
As always, thank you for your support and collaboration!
Still to come…
Of course, one question leads to another. The next step will be to combine the outputs of multiple models with economic analyses to provide a rational basis for the design of vaccine-containing malaria control and elimination programs. Stay tuned for more work units!
Swiss Tropical and Public Health Institute