Determining Actual Malaria Risk
According to the World Health Organization, in 2015 there an estimated 214 million malaria infections reported and 438,000 deaths worldwide. Although these numbers have been decreasing, infections in travelers have been increasing, and these statistics do not include the numerous cases that go unreported or undiagnosed.
Quantifying the actual risk to the traveler is impossible. Many have tried to calculate risk, but they vastly underestimate true risk. The most common measurement used here is the number of diagnosed malaria cases in a returned traveler divided by the number of travelers to that country. The attached figure (taken from the Journal of Travel Medicine, Sept 2017 issue) points out the flaw in this formula.
Some countries are not holoendemic with malaria. You may travel to a country with malaria risk, but never enter a malaria risk zone within that country, so these people shouldn't be included in the calculation. Some malaria is diagnosed after travel while at home (it can develop into clinical disease several months later in some cases), some is diagnosed while traveling, some is never diagnosed and/or self treated, and some die while abroad. Often only the diagnosed at home cases are counted, and all travelers to that country are counted. This massively leads to underestimation of real risk.
The best method to determine risk is treat each traveler independently and conglomerate all risk factors of both the individual and the itinerary. Staying in air conditioned hotels with windows results in a lower risk than open air accommodations. Sometimes urban only itineraries lead to a lower risk than rural destinations. For instance, a traveler going to Philippines and only staying in Manila will essentially have zero risk. But if traveling around the same island of Luzon, they will have some, but low, risk. Heading slightly south to the beautiful island of Palawan (Puerto Princesa, Coron, or El Nido are popular destinations) results in a much higher risk with prophylaxis recommended for all travelers.