Last week we mentioned that there is a vaccine for dengue fever but that it’s not recommended for travellers and expats. This probably seems quite strange, so this week we’d like to explain why.
Dengue is a common and at times severe virus infection transmitted by mosquitoes. There were 98,000 cases of dengue in Thailand last year, along with 100 deaths, and yet 3 out of every 4 infections are thought to give no symptoms at all. Like most viral illnesses there is no cure and hospital treatment for severe cases consists of supportive care and hope.
The symptoms of dengue begin a few days after a mosquito bite. Typically, there’s a sudden fever, pain behind the eyes, headache and severe body aches (so bad that in earlier times dengue was known as ‘break bone fever’). In extreme cases there may be bleeding from the eyes, nose and mouth, explaining dengue’s Thai name of ‘khai leuat ork’ (fever blood out).
The Aedes mosquito that transmits dengue has white stripes and is easy to recognise but not so easy to avoid. She bites in the daytime as well as at night and can transmit other similar viruses like Zika, chikungunya and yellow fever virus.
There are 4 distinct strains of dengue virus and the interaction between them and the human immune defence system is key to why vaccination can be dangerous.
Infection with a dengue virus results in long-term immunity to that strain. For a year or two there is cross-over immunity to the other strains also, but this protection is lost as the antibody levels fall, making it possible to catch dengue again. The strains can be caught in any order, with the second infection usually much worse than the first, third or fourth (whichever strain causes it).
This happens because antibody levels from the first infection pass through a ‘danger zone’ as they fall from high to low levels over time. If the second virus comes along when antibodies are in the danger zone then the immune response goes into overdrive, generating an excessive, harmful and sometimes fatal reaction. Levels above or below the danger zone are safe.
This is why we don’t recommend the dengue vaccine – if you’ve never had a dengue infection and you take the vaccine then you will make a high level of antibodies, the same as happens with a first dengue infection. As those antibody levels fall you will for a while be at a much higher risk for severe and possibly fatal dengue than if you’d never been vaccinated at all.
Checking for antibodies before giving the vaccine would avoid this problem but unfortunately the antibody test for dengue is unreliable because there are false positive results from antibodies to similar viruses (chikungunya, yellow fever and Zika). Interestingly, several Covid-19 patients have had false-positive dengue tests also, perhaps indicating a similarity between the antibodies for dengue and Covid-19. The significance of this observation is unknown.
In the absence of a safe vaccination program, the best way to protect yourself against dengue is to
eliminate mosquito breeding sites around the home, ie remove or drain anything that may collect small pools of water during the rainy season. We also recommended wearing long sleeves, long trousers and to use insect repellant at this time of year, even during the daytime.
The Be Well Team Hua Hin, 26 April 2020