Climate change ranks among the WHO’s top 10 threats for 2019—a dubious honor indicating not only its potential effects on human health and vulnerable populations. How will vaccine-preventable diseases be affected by climate change, and what can we in the global immunization field do to mitigate those consequences?

3 Things We Know About Vaccines in a Climate Change Future

Chamily (35) stands in the flooded yard of her home in Sarishabari, Jamalpur, Bangladesh. © 2016 Probal Rashid, Courtesy of Photoshare

By Molly Sauer

Climate change ranks among the WHO’s top 10 threats for 2019—a dubious honor indicating not only its potential effects on human health and vulnerable populations, but also the urgent need for a dedicated global effort to understand and mitigate its impact. How will vaccine-preventable diseases be affected by climate change, and what can we in the global immunization field do to mitigate those consequences?

Since the Intergovernmental Panel on Climate Change was founded more than 30 years ago, our understanding of what that means has evolved significantly and we can now predict and model detailed scenarios for environmental, health, social, and economic outcomes under key global temperature increase thresholds.

We know that increasing air pollution will exacerbate respiratory health issues, and rising temperatures will increase heat-related deaths. Extreme weather events, droughts, and floods will inevitably cause injuries, food and health system breakdowns, and infrastructure failure.

When it comes to infectious diseases, especially those that can be prevented through immunization, the picture is evolving. Risk factors and vulnerabilities are often complex and compounding. A child living in a low-income setting, experiencing undernutrition, with limited access to healthcare is already at higher risk of infection and severe outcomes from a vaccine-preventable disease—exposing that child to extreme weather, or forcing them to relocate as a climate refugee makes them even more vulnerable.

As decisionmakers and experts convene in New York this month for the U.N. General Assembly and Climate Action Summithere are 3 things we know—and 2 things we need to know more about—to help understand the intersection of climate change and immunization.

An adolescent girl collects rainwater from drought-affected lower land in a rural area of West Bengal, India. © 2012 Prasanta Biswas, Courtesy of Photoshare

WHAT WE KNOW 

1.  It’s likely more people will be affected by mosquito-borne diseases like dengue, yellow fever, and malaria 

Climate change is affecting rainfall patterns and increasing temperatures in many parts of the world, and mosquito-borne diseases are directly linked to these climatic factors. The WHO projected that 2030 would see 60,000 additional malaria deaths due to climate change. But innovations in vector control and vaccines for these and other mosquito-borne diseases can help mitigate risk. The world’s first malaria vaccine(RTS,S; GSK) is now undergoing a pilot introduction in parts of Ghana, Malawi, and Kenya.

We also anticipate increased transmission of other mosquito-borne diseases like dengue, yellow fever, and Japanese Encephalitis in some areas, as vector habitat and changing land use patterns place new populations at risk. Continuing to develop vaccines, and ensuring adequate supply and improving access to existing vaccines, for these diseases will be a critical part of adaptation and risk mitigation efforts. And we need more data on the issue, given the complexity of interactions between climate, socioeconomic factors, and vector-borne diseases.

2.  Diarrheal diseases are likely to increase and some regions may see changes in seasonality

Diarrheal diseases may surge as water and sanitation systems are inundated and damaged by extreme weather events and flooding. The resulting exposure to contaminated drinking water and waste places communities worldwide at risk, particularly as displacement and crowding increase. The WHO estimates climate change will contribute to an additional 48,000 deaths due to diarrhea in 2030—already, diarrhea is a leading cause of child illness and death, responsible for more than 440,000 under-5 deaths in 2017 alone.

On top of WaSH system failures contaminating food and water sources and increasing risk of infection, climate change may also affect disease patterns for different diarrheal pathogens—and even affect them all differently. Cholera is particularly linked to climate change, with endemic cholera zones like coastal Bangladesh expected to see increasing transmission; other regions of the world may see emerging disease as waters warm. Oral cholera vaccines could play a key role in reducing outbreak risk.

Typhoid transmission is also expected to increase under climate change scenarios. Other diarrheal diseases may see changes in seasonality. Conditions like overcrowding, population displacement, and malnutrition will increase vulnerability and exacerbate the vicious cycle of diarrhea and malnutrition. Available vaccines can help improve resiliency, especially among already vulnerable young children—accelerating vaccine introductions and ensuring supply and access are critical. But we also need more pathogen-specific evidence on how vaccine-preventable diarrheal diseases like rotavirus—as well as those with vaccines in development, like enterotoxigenic Escherichia coli (ETEC), shigella, and norovirus—are affected by shifting climate factors.

3.  Robust immunization programs are critical to support climate refugees and mitigate health security concerns

From natural disasters to rising sea levels, to temperature changes making areas uninhabitable, climate change is expected to trigger significant population displacement. Communities around the world—particularly those already vulnerable due to poverty, conflict, or resource shortages—are already feeling these effects. The World Bank estimates that more than 140 million people will be displaced by climate change in Sub-Saharan Africa, South Asia, and Latin America alone by 2050. Ensuring high immunization coverage and robust systems can help reduce vulnerability among displaced populations and in refugee settings, and promote health security.

A haze obscures the city of Addis Ababa, Ethiopia. © 2013 Wallace Mawire/Freelance Photographer, Courtesy of Photoshare

WHAT WE DON’T KNOW—HOW CLIMATE CHANGE WILL IMPACT IMMUNIZATION SYSTEMS

1.  What is the link between air pollution and vaccine-preventable causes of pneumonia?

In 2012, air pollution was linked to a staggering 1 in every 8 global deaths, with children particularly affected. Risk stacking means that those children most likely to be exposed to air pollution are already especially vulnerable to its effects, due to underlying health issues and other risk factors like socioeconomic status and access to care. Pneumonia claims more child lives than almost any other cause, responsible for over 800,000 under-5 deaths in 2017. Not all causes of childhood pneumonia can be prevented with currently available vaccines—tackling pneumonia requires a comprehensive approach that protect against, prevent, and treat disease. But immunization can help improve children’s resiliency to other infections and the ill-effects of exposure to air pollution.

We need to gather more evidence on the links between vaccine-preventable, pneumonia-causing pathogens—pneumococcus, Hib, pertussis, influenza, and RSV—and air pollution. We need to continue to develop vaccines for other leading causes of pneumonia to broaden that protection. And we need to work to identify and address populations at greatest risk of air pollution exposure and low access to protection, prevention, and treatment for pneumonia.

2.  How is climate change influencing our susceptibility to vaccine-preventable diseases?

There is limited evidence demonstrating the links between shifting climate and changing range and burden of vaccine-preventable diseases, in part because this requires long-term data sets for both outcomes to effectively capture trends. As more longitudinal data is gathered in key countries, there is now an opportunity to more directly link the two and improve predictions of where, when, and how these effects will be felt. This has the potential to help countries and partners prepare adaption plans and ready systems for the inevitable—establishing immunization as an essential component of those plans and ensuring resiliency of the vaccine delivery system is critical here. In particular, data from the countries most vulnerable to climate change is crucial if we are to understand the potential risks and interactions, build resiliency, and promote health security.

Country governments and global partners need to work together to ensure that we gather these important insights to address emerging questions, document experiences in areas already experiencing the effects of climate change, and strengthen systems to cope with a shifting burden of infectious diseases under a climate change future.

Molly Sauer is a research associate and the Deputy Director for Policy, Advocacy, and Communications at IVAC. Her work focuses on supporting evidence-based immunization decision-making, introduction, and scale-up in low- and middle-income countries; facilitating global scientific consensus and policy initiatives for vaccines and vaccine-preventable diseases; and strengthening immunization-related technical, policy, and advocacy institutional capacity.