India could bear biggest impact from chikungunya, new maps suggest

The most comprehensive mapping to date of the global risk of chikungunya suggests India could experience the greatest long-term impact from the mosquito-borne virus.

Based on existing evidence of chikungunya transmission, the infectious disease model predicts 14.4 million people could be at risk of infections globally each year, with 5.1 million people at risk in India. It’s likely that chikungunya cases could also spread to regions not currently recording infections or considered at-risk, the analysis found, potentially increasing the number of people at risk each year to 34.9 million globally, with 12.1 million in India.

India, Brazil and Indonesia also ranked as the top three countries most-likely to experience substantial long-term impacts from chikungunya, with India and Brazil accounting for 48% of the global impact of the disease on healthcare systems and individuals. The analysis suggests chronic health impacts will be the biggest concern, with existing evidence suggesting around 50% of people infected with the virus are left with long-term disability.

The authors say their analysis provides fresh information for public health professionals in the region, who are already working hard to contain chikungunya outbreaks.

The study, conducted by researchers from the London School of Hygiene & Tropical Medicine (LSHTM), Nagasaki University and the International Vaccine Institute in Seoul, mapped all possible regions where chikungunya infections could occur in future, to enable countries to prepare more effectively and estimated age-groups and areas most at risk of long-term effects, to target for possible vaccine programmes.

Chikungunya outbreaks are a growing public health concern and have been reported in over 114 countries since its re-emergence in 2004. The virus is spread by Aedes aegypti and Aedes albopictus mosquitoes, more commonly known as yellow fever and tiger mosquitoes, respectively, and causes severe joint pain and high fever (over 39℃) in those who become infected.

While most patients fully recover from the initial phase after a few weeks, over 50% of chikungunya patients who survive the infection suffer from long-term joint pain and disability, and cases can occasionally be fatal. There are currently no specific treatments for chikungunya but two preventative vaccines have been approved for use in some countries.

The study is the first of its kind to predict the burden of chikungunya using machine learning to combine existing data on chikungunya infections with other factors that may influence the likelihood of infections. This included: the likely occurrence of Aedes aegypti or Aedes albopicus mosquitoes; temperatures suitable for transmission of the virus by mosquitoes, modelled on the spread of dengue; annual precipitation in each region; how suitable different environments might be for the chikungunya virus to spread among populations; and national level Gross Domestic Product (GDP).

Studies have estimated chikungunya burden previously but have often relied on only surveillance and outbreak reports, which are known to be an underestimate of the true number of cases.

The model was first used to predict how many people at risk of chikungunya infections may actually become infected per year, based on the force of infection. For infectious diseases, a higher force of infection generally means a disease can spread more quickly, making explosive outbreaks more likely.

All continents had regions where high numbers of chikungunya infections were predicted annually. On average, between 1.2-1.3% of people at risk of chikungunya could expect to be infected per year, which is lower than the risk of dengue (6%). However, the data suggested certain countries, such as Gabon in Africa, could experience infections in up to 11% of people at risk.

The team then estimated the most likely long-term consequences of chikungunya infection for individuals and countries worldwide, known as its burden. India, Brazil and Indonesia were found to be the most at-risk of long-term impacts, mostly due to chronic disability experienced by many after infection, rather than death by acute infection. Chronic illness accounted for 54% of the chikungunya burden, mostly affecting those aged between 40 and 60 years, while children under 10 and adults over 80 were found to be the most at risk of acute illness.

Hyolim Kang, who led the study as part of her PhD at the London School of Hygiene  & Tropical Medicine (LSHTM), and who is also a Research Fellow at Nagasaki University, said: “It’s been widely thought that mosquitoes carrying chikungunya would be confined to subtropical or tropical continents, but our analysis has found that the risk extends way beyond these regions.

“Prevention of the spread of this disease is important for everyone. There are no specific antiviral therapies for chikungunya and treatment relies solely on supportive care. Not only are infections extremely painful, even the healthiest of people can be infected and left with life-long disability.”

Sushant Sahastrabuddhe, Associate Director General of Innovation, Initiatives, and Enterprise Development at the International Vaccine Institute , and co-author of the study, said: “The potential spread of vectors carrying viruses like chikungunya won’t wait for us to carry out years of research, so it’s been extremely important to us that the modelling we’re working on is shared and used in real time to help public health professionals manage current cases and prepare for the future.”

Kaja Abbas, joint Associate Professor at the London School of Hygiene & Tropical Medicine (LSHTM) and Nagasaki University, and senior author of the study, said: “Our model-based estimates are useful to inform outbreak response immunization strategies for different age groups using the two licensed vaccines (Ixchiq® and Vimkunya®) in Brazil and broadly in any setting globally at risk of chikungunya outbreaks. We hope other countries will follow suit and look to prioritise regions conventional surveillance systems may have missed.”

The team say their maps are a snapshot representing the long-term annual average, and do not account for unexpected changes likely to be recorded going forward, such as extreme weather events and possible changes due to climate change.

The researchers hope their findings can inform prevention strategies across the globe, for example by the WHO SAGE Working Group on Chikungunya Vaccines, to identify countries or regions at risk of long-term burden from chikungunya and to support geographical prioritisation.