Malaria parasites change their antigenic ‘coats’ when challenged by the immune system. They just turn on a gene, pull on a new Pfemp protein (60+ varieties) and they’re disguised. That makes it really hard to generate and maintain a decent immune response.
The new vaccine gives recipients 50% protection and requires 3 doses. Protection might fade over time but it’s looking good for a year or so. GSK is evasive on the cost, but it will likely be more than $10 per course even though they only plan to take a 5% markup over expenses.