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Editors CornerWhy Vaccinate Travelers?

Why Vaccinate Travelers?

The primary goal of vaccination is to immunize susceptible individuals against a pathogen, preventing outcomes that range from asymptomatic infection (with the potential for onward transmission) to clinical disease, hospitalization, long-term complications, and death.

At the population level, vaccination aims for a broader impact by immunizing all susceptible individuals. This approach leads to a significant reduction in disease burden, transmission, hospitalizations, mortality, and associated healthcare costs.

The same rationale applies to individual travelers, who may be susceptible to various pathogens due to exposure to new environments at their destinations.

An estimated 1.1 billion international tourist arrivals (overnight visitors) were recorded between January and September 2024, marking an 11% increase compared to the same period in 2023. These 1.1 billion travelers represent a substantial population potentially susceptible to numerous vaccine-preventable diseases, posing risks that range from individual infections to the potential for outbreaks.

Accordingly, immunoprophylaxis through vaccination for travelers should consider the following factors related to their destination.

Risk of acquiring infection/disease to the traveler:

  1. Vectors, mosquitoes: Mosquitoes are the primary vectors responsible for transmitting a significant proportion of diseases worldwide. Malaria remains the leading mosquito-borne disease associated with high mortality, particularly in Africa (Plasmodium falciparum). However, other diseases such as Dengue, Yellow Fever, Chikungunya, Japanese Encephalitis, West Nile Virus, Venezuelan Equine Encephalitis, Ross River Fever, Barmah Forest fever, La Crosse Encephalitis, Zika Fever, Oropuche Fever, as well as newly detected Keystone Virus and Rift Valley Fever and several others also contribute substantially to global disease burden. In addition to Plasmodium species and viruses, interestingly, in January 2024, an Australian research group proved that Mycobacterium ulcerans, the causative pathogen of Buruli Ulcer is transmitted by mosquitoes. These infections can result in severe illness, hospitalizations, long-term complications, and, in some cases, mortality. Currently, vaccines are available for travelers against Chikungunya, Japanese Encephalitis, and Yellow Fever. Additionally, a recently developed Dengue vaccine has demonstrated rapid immunogenicity without the need for prior serostatus testing and may soon be approved for use by travelers hopefully in all countries.
  2. Vectors, ticks: Ticks are significant vectors for numerous diseases, transmitting pathogens that include viruses (e.g., Tick-Borne Encephalitis, Powassan, Colorado Tick Fever, and Crimean-Congo Hemorrhagic Fever), parasites (e.g., Babesiosis), and bacteria (e.g., Lyme Disease and other Borrelioses, Typhus, Rocky Mountain Spotted Fever, Rickettsioses, Bartonellosis, Ehrlichiosis, and Tularemia). Currently, four licensed vaccines are available for Tick-Borne Encephalitis. In regions where the disease is highly endemic, the World Health Organization (WHO) recommends vaccination for all age groups, including children, as well as for travelers to these areas who engage in activities that increase the risk of tick bites. Additionally, a Lyme Disease vaccine is under development, showing promising early results.
  3. An endemic human-to-human transmitted disease in which there is no vaccine in the country of origin. A prime example of this condition is Meningococcal Disease. Many countries with intermediate to high endemicity rates (1–10 cases per 100,000) pose a transmission risk to susceptible individuals. Additionally, mass gatherings in endemic areas, such as the Hajj pilgrimage in Mecca, have been linked to significant outbreaks. As a result, vaccination prior to travel is mandatory for individuals attending such events. Another important scenario involves the issue of so-called “sex tourism,” a broad and often criminal activity. A notable example is how Mpox spread globally, primarily through sexual transmission. Other sexually transmitted infections (STIs) associated with this context include Hepatitis B, Hepatitis C, HIV, Syphilis, Gonorrhea, and more. It is crucial to note that vaccines are available for some of these diseases, including Mpox, Hepatitis B, and, to a certain extent, Gonorrhea (via the Meningococcal B vaccine, which offers partial protection against Neisseria gonorrhoeae).
  4. An outbreak: In the context of an ongoing outbreak, the primary recommendation is to avoid traveling to the affected region. However, if travel is essential, the traveler’s susceptibility must be carefully assessed. For example, outbreaks of diseases covered by vaccines included in National Immunization Programs (NIPs), such as measles, diphtheria, or polio, may require vaccination prior to travel if the individual’s immunization status is unknown or incomplete. Currently, there are also outbreaks of diseases for which vaccines are not universally part of NIPs. Examples include Ebola and Marburg in Africa, as well as Crimean-Congo Hemorrhagic Fever, Chikungunya, Yellow Fever, Japanese Encephalitis, Hepatitis A, and others. Notably, there are now two licensed vaccines available for Ebola, providing a measure of protection for travelers to at-risk regions.
  5. Enteric diseases: In most developed countries and many developing nations, water purification has significantly reduced the prevalence of bacterial enteric infections. However, numerous developing countries continue to face challenges in ensuring access to clean drinking water. This has resulted in recurring outbreaks of diseases such as Cholera, Typhoid Fever, other Salmonellosis, Shigellosis, diarrheagenic E. coli, and other bacterial infections. While these diseases pose risks to local populations, visitors are at an even higher risk due to their presumed lower immunity from minimal prior exposure. Currently, there are three WHO-prequalified Cholera vaccines available. For Typhoid Fever, two newer-generation vaccines have replaced the older, reactogenic inactivated whole-cell vaccine previously used. These include the live oral Ty21a vaccine and the injectable Vi polysaccharide vaccine, both of which have demonstrated safety and efficacy in large-scale clinical trials and are internationally licensed for individuals aged 2 years and older. Meanwhile, vaccines against Enterotoxigenic E. coli (ETEC), Shigella, and non-typhoidal Salmonella are under development, with many showing promising results in early clinical trials. In addition to these bacterial infections, Norovirus—a viral disease frequently linked to outbreaks in crowded settings such as cruise ships, schools, and other group environments—remains a significant concern. Encouragingly, a Norovirus vaccine is currently in clinical development, offering hope for future prevention.
  6. Others: Certain host-related conditions can increase susceptibility to infections, necessitating additional considerations. Individuals with diabetes, hypertension, or any form of immunosuppressive condition may require a more thorough evaluation to determine if additional vaccines are needed beyond those routinely recommended for travelers.

Risk of disseminating infection/disease from the traveler:

This is a critical issue. An unimmunized traveler, whether asymptomatic or in the prodromal phase of a disease, can directly or indirectly transmit infections to other susceptible individuals. This can escalate from small clusters of infections to outbreaks and even large-scale epidemics. Examples of such diseases include Measles, Influenza, Mpox, Dengue, Chikungunya, Zika, Polio, Diphtheria, COVID-19, Ebola, and many others.

Without appropriate measures—such as confinement, prophylaxis (e.g., vaccination) for contacts, and other preventive actions—the consequences can sometimes be catastrophic.

Organizations providing recommendations for traveler immunizations:

The WHO offers a comprehensive and regularly updated website with global recommendations for travelers. Additionally, numerous national organizations (e.g., the US CDC), regional societies (e.g., the Latin American Society for Travel Medicine), and private entities (e.g., Travel Health Xpert) provide supplementary information and guidelines. These resources complement the recommendations issued by the foreign ministries of most countries, ensuring travelers have access to the latest and most relevant advice.

Gaps and areas for improvement:

For several diseases, vaccination recommendations often face challenges due to limited data on the vaccine or an incomplete understanding of the benefit-risk balance regarding disease acquisition. This creates gaps in advising vaccination for certain pathogens. Notable examples include Chikungunya and Dengue, where periodic evaluation of recommendations is essential to ensure they remain aligned with emerging evidence and evolving epidemiological trends, as well as safety data of the immunogens.

In summary, vaccination for travelers represents a unique and dynamic field within vaccinology and public health. It requires a comprehensive approach that balances personalized assessments with community-based strategies. However, inequitable access to many vaccines remains a significant challenge, highlighting the urgent need for targeted actions to address this disparity.

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