I REFER to the letter “Dengue vaccine strategy” (The Star, April 21), where the writer said “What we may be able to expect are reductions in severe disease, reduced hospitalisation due to dengue and possibly a reduction in deaths due this dreaded mosquito-borne disease.”
While it is indeed true about what this Dengvaxia vaccine can do for us, it must be remembered that all cases of the disease must be hospitalised to isolate (for the period of communicability, which is usually five days) such cases in special dengue wards to prevent the spread of the disease.
While the vaccine will protect those receiving it (65% overall, and about 90% of severe cases which cause death and severe debilitating illness), the country cannot expect to complete immunisation of all at risk, certainly not within a decade. Hence, there will still be many in the next decade without protection.
Vaccination, at the outset, would be expected to cover only those who are able to afford it. Also, the manufacturer and WHO do not recommend using the vaccine on children below nine years old.
Besides the strategy of early diagnosis/prompt notification/ prompt isolation of cases/prompt fogging of areas where cases arise, vaccination, diligent use of effective mosquito-repellents (such as DEET), and efforts at reducing the Aedes population will help a long way in controlling the disease (even though the WHO in its fact sheet website says reducing the Aedes population is the ONLY way of controlling dengue, and now has ventured to recommend the Dengvaxia vaccine).
It is a public health principle that all cases of an infectious disease with high morbidity, high mortality and high transmission rate must be isolated to prevent it from spreading. Isolation (with barrier nursing, which is not required in dengue) was the mainstay in the control of SARS, H1N1 and Ebola. The US Centers for Disease Control and Prevention (CDC) directs all cases of yellow fever (also an Aedes-borne disease) to be isolated.
The US CDC also says that only 50% of dengue infections are asymptomatic, meaning that theoretically we can reduce 50% of dengue cases through early diagnosis/prompt notification/prompt isolation/prompt fogging of affected areas.
In practice, this percentage may be less because of the transovarial-transmission of the dengue virus directly from the parent mosquito to its offspring. Transovarial (vertical) transmission also occurs in yellow fever. Yet, the US CDC recommends the isolation of all cases of yellow fever.
Vaccination, use of mosquito repellents and reduction of the Aedes population would be the mainstay in preventing the spread of dengue from asymptomatic, infected humans.
The need to continue efforts at reducing the Aedes population is vital. However, surveys done in Malaysia indicate that only 55% of mosquito breeding grounds are indoors and in the immediate vicinity, such as gardens and roof gutters.
The Aedes is a day-biting mosquito, meaning it bites throughout the day although its peak biting hours are the cooler early mornings and evenings. The classical understanding is that the primary vector, the Aedes aegypti, breeds indoors while the secondary vector, the Aedes albopictus, breeds outdoors. Hence, from the results of the survey above, it would mean that A. albopictus would be more than a secondary vector in this country. Plenty of biting, it appears, occurs outdoors.
Some point out that we may be wrong in just fogging (and larviciding) the area where cases arise. They claim that in “hot spots” in this country, transmission occurs outside homes while Malaysians are on their way to work in the morning, either as pedestrians or commuters. Hence, it may be imperative to interview cases to find their route from home to work and back, and then search out possible breeding places along the way. This would require more effort than is being done now.
Concerning the release of sterile male Aedes (genetically-modified, irradiated or Wolbachia-infected), there are financial and logistic constraints. For one, the cost is exorbitant. Secondly, the A. aegypti is classically understood to have a flight range of only 400m to 800m while that of A. albopictus is only 200m. That means sterile males of both species, theoretically, must be released in sufficient numbers at circles of human habitats of 800m to 1,600m in diameter in the case of A. aegypti, and 400m in the case of A. albopictus.
According to one entomologist, they need not and do not fly more than 50m to find a mate. Also, the farmed sterile males do not really have any advantage over the wild males in finding a female; the wild males actually wait centimetres from the pupae in water to mate with the emerging females.
For the eradication of the Aedes indoors, the entomologist recommends the use of commonly available knock-down aerosol insecticides. These need to be sprayed for about five seconds in all rooms once a week. The occupants should leave the house for about two hours after spraying.
Let us hope the Dengvaxia vaccine will be reasonably priced and adequately available so that all those at risk will quickly become protected. Let us hope too that the authorities concerned, such as the WHO, Health Ministry, state health departments and local councils will make greater effort and be more innovative in controlling and preventing the scourge.
The Health Ministry must also sell effective mosquito-repellents such as DEET at subsidised prices in its clinics. The ministry must repeatedly and adequately educate the public on the availability of such repellents, besides educating on the diligent use of such repellents.
Malaysians need not die unnecessarily, nor suffer from severe debilitating illness because of dengue.
RETIRED PUBLIC HEALTH CONSULTANT