A vaccine against dengue?

  

Of late, there has been a sharp spike in the Aedes aegypti mosquito population, with the warmer-than-usual weather shortening the breeding and maturation cycles of the mosquitoes, as well as the incubation periods for the dengue virus.

From Jan 3-28 this year, the Health Ministry’s iDengue remote-sensing tool has recorded 12,897 cases nationwide, with 23 fatalities. Almost half the cases come from the hotspot state of Selangor, which tops the list with 6,267 cases.

The availability of a dengue vaccine, though not perfect, is vital in offering protection against the disease.

“Because it reduces the severity of the disease, it greatly reduces hospitalisation,” notes Prof Duane Gubler of the emerging infectious diseases programme at the Duke University-National University of Singapore Graduate Medical School in Singapore.

The vaccine, which hit the market last December, has been approved for use in Mexico, the Philippines and Brazil. The vaccine works better for individuals aged nine to 45 years, and on those who have been previously infected.

Our Health Ministry has yet to approve it for use.

Even Singapore, which has one of the best vector control programmes in the world, is being weighed down with the dengue problem, with 2,223 cases and one fatality recorded from Jan 3-28.

“The vaccine should reduce transmission and prevent a major epidemic. Overall, the vaccine has good public health benefits beyond efficacy. This will result in significant economic savings for the government of Malaysia.

“But, the vaccine alone will not prevent dengue. You have to do mosquito control, with the goal of reducing mosquito population, and at the same time, raise herd immunity against different serotypes of the virus. You probably will never be able to prevent transmission completely, but I do think you can prevent an epidemic,” says the world-renowned dengue expert who was in Kuala Lumpur recently.

The vaccine may not be ideal, but if the majority of people in endemic areas have already had at least one infection, there is protection against two or more strains, and against dengue haemorrhagic fever.

Porf Gubler says, “The fact that it’s not as efficient against the Den-2 serotype is unimportant. If you look at all severe diseases, it’s associated with the first or second infection. If you look at the people who have been infected for the third or fourth time, the illness was either mild or asymptomatic.”

“That means if you can be protected against Den-1 and 3, or Den-1 and 4, or Den-3 and 4, you are still protected against severe disease. You can introduce the vaccine in a controlled fashion, and combine it with good surveillance and good risk management so that the risk and adverse effects can be controlled.”

While it is widely believed that weather phenomenon El Nino has had effect on dengue, Prof Gubler, who has been on the Intergovernmental Panel on Climate Change, says otherwise.

“Global climate change has not played any role in the global emergence on epidemics, or what I call the 20th-century pandemic, which is dengue. It has not been a principal driving force. Instead, urbanisation, globalisation and lack of effective mosquito control have been the major drivers of dengue.

“Environmental and climate factors are very important in the biology of diseases like dengue. So yes, things like temperature, humidity, evaporation … all of these influence transmission, but as far as El Nino goes, we have had epidemics without the El Nino years. What that tells you is that it’s a complex set of factors that influence epidemic transmissions.”

However, there are some exciting new mosquito control methods in the pipeline and Prof Gubler is fairly optimistic that these tools will combat dengue.

They include at least three vaccines, anti-viral drugs, therapeutic antibodies, vector control tools and residual insecticides that can be used as sprays.

Prof. Gubler is a doyen of dengue research and has made significant contributions in the prevention and control of dengue in this region. Photo: The Star/Muhamad Shahril Rosli

One tool already in place is the sterile mosquito technique where the maleAedes aegypti mosquitoes are infected with the Wolbachia bacteria, which causes their mating with wild females to produce sterile eggs.

Again, Prof Gubler points out that while there is tremendous potential, like vaccines, they need to be used together.

“The good thing about Wolbachia and residual insecticide is that they will actually kill mosquitoes that are breeding where health inspectors cannot find them.

“Old cities that have been built on top of each other have these unused wells, unused cisterns, rooms that collect water, etc. These are ideal mosquito larvae habitats and they breed millions!

“So governments need to look at their countries and prioritise incidences in urban areas and introduce targeted programmes, looking at ecology, epidemiology and culture. When you introduce vaccine and vector control, the impact will be powerful. But it’s not going to be easy,” notes Prof Gubler.

In the history of the disease, there have been only three success stories – in post-World War II America, in Singapore in the early 1960s and Cuba in 1981.

The United States started a programme called Aedes aegypti eradication in 1946, and they eliminated these mosquitoes, effectively stopping the yellow fever epidemic for almost 50 years.

Prof Gubler says, “I say, success breeds failure, because by the early ‘90s, public health officials thought they were spending too much money on the programme when there were no more cases. So they stopped, the mosquitoes came back, and the result was another epidemic.”

Likewise in Singapore and Cuba, which introduced legislations to punish those breeding the pests.

Once the war on infectious diseases was won, people became complacent. And the mosquitoes returned with a vengeance, bringing along other viruses as well. The Aedes aegypti mosquito also carries the yellow fever, chikungunya and zika viruses.

The eminent professor, who has been in the field for almost 50 years, says we haven’t been able to emulate these success stories due to different priorities and the fact that dengue is not considered a public health problem.

“We live in a very different world now. Back in the ‘50s, ‘60s and ‘70s, cities were small. Now they’re big, so the mosquito population is taking advantage of the urban sprawl.

“Social factors also contribute to this. Programmes were successful because it was paramilitary type and highly disciplined. If you went into a house and wanted to inspect it, people let you in. Now, you’ll be thrown out!” he says, chuckling.

On the consumption of papaya leaves and crab soup to increase platelet levels, Prof Gubler says there are no scientific findings to support these testimonies.

“If people find it helps, then go ahead and take it. Part of medicine is how the patient feels. A lot of ability to recover is based on attitude, so if I give you a placebo and you feel better, that’s good.

“There are some drugs currently in human trials that could be useful in treating dengue, but none are available yet.”

Source: http://www.star2.com/