Dengue fever is a mosquito-borne tropical illness that is now endemic to most tropical countries and (unlike malaria) has no chemical prophylaxis and (unlike ye

Dengue fever is a mosquito-borne tropical illness that is now endemic to most tropical countries and (unlike malaria) has no chemical prophylaxis and (unlike yellow fever) has had no effective vaccine. This article describes how there is now a dengue vaccine, which seems like great news, but there's a catch. Dengue has a very unusual feature where getting it twice is not only possible but is frequently much worse than getting it the first time, in the specific case where the second strain is different from the first strain. The second (or subsequent) infection with a different strain can cause a much more severe illness which may be fatal. https://en.wikipedia.org/wiki/Dengue_fever#Predisposition https://en.wikipedia.org/wiki/Dengue_fever#Predisposition https://en.wikipedia.org/wiki/Dengue_fever#Severe_disease https://en.wikipedia.org/wiki/Dengue_fever#Severe_disease "While each serotype can cause the full spectrum of disease, virus strain is a risk factor." According to the article, the new vaccine is effective at reducing the risk of dengue infections, but is also comparable to a prior infection in terms of increasing the harmfulness of subsequent infections! So if you get the vaccine without ever having had dengue before, you're less likely to get dengue but it's also more likely to be worse if you do, which is probably a bad tradeoff for most people. The article goes on to say that the FDA has now approved the new vaccine for use (only) in areas where dengue is present and (only) when the patient has a confirmed history of prior infection -- in which case the vaccine is believed to reduce risk of subsequent infections without also increasing their severity. But a prior trial period of the vaccine in the Philippines did not have this restriction, which means that it's believed to have decreased infection risk while increasing infection severity in some children, some of whom died when they later contracted dengue from mosquito bites. Parents' awareness of this history also reduced their willingness to have their children receive other vaccines that were known to be safe: "As result, vaccine coverage for childhood diseases in the Philippines, such as the measles, has dropped, WHO says. And the Philippines is now facing a large measles outbreak, with more than 26,000 cases and more than 355 deaths during 2019." :-(