Better together
Combining vaccines may help eliminate malaria.
In 2016, malaria caused an estimated 216 million illnesses and 445,000 confirmed deaths globally. The disease is caused by a parasite, and mosquitos infected with the parasite transmit them to humans when they bite. In humans, the parasites enter the body and head to the liver before spending part of their life cycle in red blood cells, which cause the symptoms of the disease.
Prevention efforts have reduced the burden of malaria but eliminating the disease will require new tools. One option is to use vaccines. The world’s first malaria vaccine — a so-called pre-erythrocytic vaccine (PEV) — targets the stages preceding the parasite reaching the liver. This vaccine prevents malaria parasites from infecting people, but it is only partially effective. Scientists are also developing transmission-blocking vaccines (TBVs). These TBVs block the development of malaria parasites in mosquitos that bite vaccinated humans. So far, the most promising TBV candidates are also only partly effective.
It is possible that using PEV and TBV vaccines together could boost their effectiveness, since the TBV vaccines reduce the number of parasites that infect each mosquito. This means that fewer parasites are injected into the next person. Currently, the PEVs work better when there are fewer parasites infecting a person.
Now, Sherrard-Smith et al. show that combining TBVs with PEVs enhances their antimalarial effects. In the experiments, Sherrard-Smith et al. treated mice with either TBV or PEV vaccines, or both. Then, the mice were exposed to mosquitos infected with the malaria parasite. As expected, the TBV and PEV treatments were only partially effective when used alone. But exposing the mice to both TBVs and PEVs eliminated the parasites from the mosquitos and the mice.
The combined benefit of TBVs and PEVs were greater than would be expected if either vaccine was acting alone and the effects were simply multiplied, suggesting they enhance each other’s effects. More studies of TBVs in humans are needed to prove they are safe and effective in the real world. More studies also are needed to confirm what Sherrard-Smith et al. found in mice would happen in humans treated with a combination of TBV and PEV vaccines. But if such future studies prove this combination approach is effective, it could be a powerful tool in the fight against malaria.
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