Dengue risk beyond the tropics

Mosquito avoidance measures are important, even if you think the destination is low risk for exotic disease transmitted by mosquitoes.

The recent report of dengue haemorrhagic fever in a young Italian traveller to Sharm-El-Sheik in Egypt earlier this month (May 2024) is a good reminder.

We think of dengue as a serious health issue in tropical regions of the world – Southeast Asia, East and West Africa and tropical South America. North Africa would be seen as an unusual place for dengue, yet we know its geographical range will increase with climate change.

Where do we stand with Dengue Fever Vaccines currently?

In May 2024, the World Health Organisation (WHO) published a position paper on dengue vaccines. Vaccine position papers are more concerned with the use of vaccines in state-based programs for large populations who have significant risk of the disease. Individual states must look at the burden of the disease and the cost of a vaccine program.

According to WHO, almost 4 billion people are at serious risk of dengue fever currently. The population at risk is expected to grow through increasing urbanization and climate change. An estimated 100 to 400 million cases occur each year with more than 36,000 deaths. The public health cost is enormous.

A WHO position paper also provides travel health practitioners with an insight into the use of a dengue vaccine and guidelines on the benefits of vaccination versus any risks associated with its use. Cost is also a consideration.

The Dengue Virus and the Role of Vaccines

Dengue fever can arise from 4 different serotypes, Dengue 1,2,3, or 4. Dengvaxia vaccine covers each of the 4 serotypes as a live attenuated recombinant vaccine. The vaccine is provided as 3 doses, at 6 monthly intervals. It can be used in those aged 9 to 45 years or 9 to 60 years depending on the guidelines of the approving authority. In Australia it is recommended for the age group 9 to 45 years. It is more targeted for those who have experienced a bout of dengue fever in the past. These individuals might be more likely to experience a more serious disease if infected again. Individuals who have lived previously in a dengue risk area might also be given the vaccine on the basis they might have experienced asymptomatic disease in the past. Pre – screening is suggested prior to vaccination, and this limits its use in national immunization programs.

The position paper discussed 2 dengue vaccines, Dengvaxia (Sanofi) and QDenga (Takeda). Dengvaxia is approved for use in Australia but not widely available. It can be sourced under the Special Access Scheme on a case-by-case basis. The vaccine is quite expensive.

Qdenga Vaccine - Efficacy, Access and Use

Qdenga is not currently available in Australia or New Zealand (May 2024), however, we are increasingly seeing European travellers heading to Southeast Asia having been vaccinated prior to departure. The vaccine is currently being used in the private market in some European countries, Thailand, Indonesia and in government approved programs in Brazil and Argentina.

Qdenga received prequalification from WHO on May 10th, 2024, and is likely to be assessed for use in Australia in the not-too-distant future. Prequalification is important to ensure vaccines are safe and effective.

Qdenga is also a tetravalent dengue vaccine. It is live attenuated for Dengue-2 but live recombinant for each of the other 3 strains. This makes it different to Dengvaxia. It is highly immunogenic and appears to provide good protection for up to 51 months of follow up with greater than 98% protection for Dengue-2 and over 80% for the other 3 strains in those who were dengue negative at the start of the program. The vaccine is given as 2 doses, spaced 3 months apart and recommended for those between 6 and 60 years. The course should be completed 14 days prior to entering the area of risk.

Guidelines for Travellers

The position paper also provides some guidelines for travellers. It concluded that the vaccine might be considered in persons from non-endemic countries if they have previously experienced dengue fever and returning to an area of risk. It might also be considered in frequent travellers, long term travellers, migrants from endemic areas and expatriates who might have experienced asymptomatic disease in the past. Care still needs to be taken in those who have never been to an area of risk and likely to have no immunity to any strain of the disease. These individuals might not be fully protected against Deng-3 or Deng-4 and might experience serious disease. The highest benefit with least risk applies to those travelling to areas experiencing outbreaks of either Dengue virus 2 or 1.

The vaccine might also be considered in people with comorbidities which might lead to more serious disease.