Vaccination for Travellers

I am sure many of the residents of Taman Tun being among some of the privileged citizens of the country by virtue of their position in society are fairly frequent travellers. Many of our people often travel to places off the beaten track. This article and hopefully more to follow will try to offer information to help our residents better prepare for their trips. I have tried to avoid medical jargon as far as possible.
Vaccination is the administration of a vaccine to stimulate a protective immune response that will prevent disease in the vaccinated person if contact with the corresponding infectious agent occurs subsequently. Thus vaccination, if successful, results in immunization: the vaccinated person has been rendered immune to disease caused by the infectious pathogen. In practice, the terms “vaccination” and “immunization” are often used interchangeably.

For travellers, vaccination offers the possibility of avoiding a number of dangerous diseases that may be encountered abroad. Immunized travellers will also be less likely to contaminate other travellers or the local population with a number of potentially serious diseases. Despite their success in preventing disease, vaccines rarely protect 100% of the recipients. The vaccinated traveller should not assume that there is no risk of contracting the disease(s) against which he/she has been vaccinated. It is also important to remember that immunization is not a substitute for avoiding potentially contaminated food and water. The risk to a traveller of acquiring a disease depends on the local prevalence of that disease and on several other factors such as: age, sex, immunization status and current state of health, travel itinerary, duration and style of travel (e.g. first class, adventure, hiking, relief work).

Based on the traveller’s individual risk assessment, a health care professional can determine the need for immunizations and/or preventive medication (prophylaxis) and provide advice on precautions to avoid disease. There is no single schedule for the administration of immunizing agents to all travellers. Each schedule must be personalized and tailored to the individual traveller’s immunization history, the countries to be visited, the type and duration of travel, and the amount of time available before departure.

Following vaccination, the immune response of the vaccinated individual will become fully effective within a period of time that varies with the vaccine, the number of doses required and whether the individual has previously been vaccinated against the same disease. For this reason, travellers are advised to consult a travel medicine practitioner or physician 4–8 weeks before departure in order to allow sufficient time for optimal immunization schedules to be completed. However, an imminent departure still provides the opportunity to provide both advice and possibly some immunizations.

Choice of vaccines for travel

Vaccines for travellers include: (1) those that are used routinely, particularly but not only in children; (2) others that may be advised before travel to disease-endemic countries; (3) those that, in some situations, are mandatory.

In deciding which vaccines would be appropriate,
the following factors are to be considered for each vaccine:
– risk of exposure to the disease;
– age, health status, vaccination history;
– reactions to previous vaccine doses, allergies;
– risk of infecting others;
– cost.
Travellers should be provided with a written record of all vaccines administered (patient-retained record), preferably using the international vaccination certificate (which is required in the case of yellow fever vaccination). The certificate can be ordered from WHO at www.who.int/ith/en/.

SEASONAL INFLUENZA
The influenza viruses are classified into types A, B and C on the basis of their core proteins. Only types A and B cause human disease of any concern. All of the currently identified 16 HA and 9 NA subtypes of influenza A viruses are maintained in wild, aquatic bird populations. Humans are generally infected by viruses of the subtypes H1, H2 or H3, and N1 or N2.

Respiratory transmission occurs mainly by droplets disseminated by unprotected coughs and sneezes. Short-distance airborne transmission of influenza viruses may occur, particularly in crowded enclosed spaces. Hand contamination and direct inoculation of virus is another possible source of transmission.

Influenza occurs all over the world, with an annual global attack rate estimated at 5–10% in adults and 20–30% in children. In temperate regions, influenza is a seasonal disease occurring typically in winter months: it affects the northern hemisphere from November to April and the southern hemisphere from April to September. In tropical areas there is no clear seasonal pattern, and influenza circulation is year around typically with several peaks during rainy seasons.

Travellers, like local residents, are at risk in any country during the influenza season. In addition, groups of travellers that include persons from areas affected by seasonal influenza (e.g. cruise ships) may experience out-of season outbreaks. Travellers visiting countries in the opposite hemisphere during the influenza season are at special risk, particular if they do not have some degree of immunity through recent infection or regular vaccination. The elderly, people with pre-existing chronic diseases and young children are most susceptible to complications.

Influenza viruses constantly evolve, with rapid changes in their characteristics. To be effective, influenza vaccines need to stimulate immunity that protects against the principal strains of virus circulating at the time. Every year, the composition of influenza vaccines is modified separately for the northern and southern hemispheres. Since the antigenic changes in circulating influenza viruses can occur abruptly and at different times of the year, there may be significant differences between prevailing influenza strains in the northern and southern hemispheres. The internationally available vaccines contain three inactivated viral strains, the composition of which is modified every 6 months to ensure protection against the strains prevailing in each influenza season. The composition of vaccines is therefore adjusted for the hemisphere in which the vaccine will be used. Thus, a vaccine obtainable in one hemisphere may offer only partial protection against influenza infection in the other hemisphere, although in some years the viruses in the vaccine may be antigenically identical. Available influenza vaccines do not protect against avian influenza.

Travellers with conditions that place them at high risk for complications of influenza should be vaccinated every year. In years in which the northern and southern hemisphere influenza vaccine strains differ, high-risk individuals travelling from one hemisphere to the other shortly before or during the other hemisphere’s influenza season should obtain vaccination for the opposite hemisphere in a travel clinic. Where this is not possible, the traveller should arrange vaccination as soon as possible after arriving at the travel destination. Otherwise, receiving a vaccination at least 2 weeks before travel is advisable.

Influenza A(H1N1) virus is a new re-assortment that has never before circulated among humans. This virus is not related to previous or current human seasonal influenza viruses. Vaccines against H1N1 are available.

Dr Nirmal Singh

Ref. International Travel And Health 2010(WHO)