Travel vaccine preventable diseases—updated logarithmic scale with monthly incidence rates

There is a steady increase in the number of travellers, but travel related epidemiology is a moving target. Since 1987 I have occasionally published a logarithmic scale illustrating estimates on the monthly incidence rate of vaccine preventable diseases in non-immune travellers visiting lower income countries, last in a more detailed review three years ago.1 To prevent older and obsolete figures to be shown in meetings and to result in inadequate conclusions, let me show here the latest assessment and briefly comment on the relevant changes. References relating to the latest epidemiological studies on individual infectious diseases are mentioned in the figure (as the number of references is limited the author will indicate details upon request). Additionally few notes from daily observations will be included (Figure 1). Vaccine preventable disease travel health risks: Estimated incidence per month in lower income countries among non-immunes Influenza remains the most frequent vaccine preventable illness. In a recent review it has been stressed how this vaccine should particularly be recommended to those travelling in groups or on cruise ships.2 Obviously senior travellers are at greatest risk of complications by this infection. Over the past four decades, the risk of hepatitis A has continuously decreased from 1 in 300 to 1 in 3000 and now 1 in 10 000.3 Some years ago two major outbreaks were recorded in travellers returning from Egypt.4 Currently there is an ongoing intercontinental problem among men who have sex with men (MSM), which mainly is related to the EuroPride parade, which took place in Amsterdam in summer 2016.5 While reviewing vaccination certificates one occasionally notes that those consulting for pre-travel recommendations have received hepatitis A booster injections every 10 years. This is unnecessary since for long it has been documented that this vaccine offers long-term protection (at least 30 years); while hepatitis A serology is not indicated in those who have received two doses of vaccine with the correct interval, it may be beneficial in those unable to provide a clear history of previous infection or vaccination.6 For the first time, it has been possible to include measles in the figure. Among Swedish residents <75 years old who had been travelling abroad 2009–13 overall 31 cases (from Europe 15, Asia 14, Africa 2) were diagnosed in 576 million days of travel.3 Since 95% of the Swedish population have received two doses of measles vaccine by official country estimates, it can be very roughly estimated that among the non-immune travellers the monthly incidence rate is in the order of magnitude of 3 per 100 000. Basing on 36 cases of pertussis detected in the same Swedish study a similar rate can be extrapolated also for this infection.3 Also tick-borne encephalitis (TBE) has recently been included in the figure. This based on a review of data according to which 39 cases of TBE were associated with international travel in a single year.7 Usually zero to two cases of yellow fever were recorded in international travellers per year until 2002; subsequently, no cases associated with international travel were reported until 2017, when associated with outbreaks in Angola and later in Brazil several dozen cases were recorded in 2017 and 2018. Particularly worrisome was the exportation to China,8 but in the past months also several Europeans and South Americans from outside Brazil were infected with a substantial case fatality rate.9 Since there are no data on the proportion of exposed travellers who are protected by immunization, it is impossible to even grossly estimate the risk of yellow fever in non-immune travellers, this is why this infection is mentioned at the bottom with a question mark. Besides protecting travellers of infection yellow fever immunization also is relevant for simple procedural reasons: Tanzania and also its territory of Zanzibar, a country without a single case of yellow fever in more than 50 years, haphazardly check vaccination certificates in travellers arriving from Europe.10 Rabies continues to be a widespread problem mainly in Asia and Africa.11 In contrast, there is far less risk in Latin America where only a few dozen cases of rabies were recorded annually among the estimated 59 000 deaths worldwide.11 A recent case in a French long-term traveller should not only be a ‘wake-up call for improved access to timely and adequate post-exposure prophylaxis’ (PEP), but it also serves to highlight the importance of pre-exposure prophylaxis (PrEP) particularly for those with prolonged cumulative exposure and other risk groups.12 We have no data on the number of travellers who obtained PEP, or on how many—including entire families—had to change their travel plans to obtain rabies vaccine and rabies immune globulin (RIG). It is impossible to estimate how many have been saved by PEP. After extensive review of published and unpublished data, the World Health Organization (WHO) has recently issued revised recommendations, which make PrEP now limited to IM or ID doses given only on Days 0 and 7 easier and more affordable.11 The third PrEP vaccine dose on Day 21 or 28 is no longer indicated. In view of successful trials completed with single visit rabies PrEP in the sense of priming,13 there are hopes that in future a single visit PrEP strategy may be adopted, but so far that has not been endorsed by the WHO or any national expert group. Similarly to TBE also, Japanese encephalitis (JE) is a rare, but potentially devastating infection in travellers. The latest estimate on attack rates in those with destinations in endemic countries is 1 in 400 000.14 While we know about risk groups, such as those visiting friends and relatives (VFR) who are at higher risk of bites by mosquitoes transmitting the virus, the experience teaches us that a substantial proportion of JE victims are short-term beach vacationers or other who at least in theory belong to the so-called lower risk groups.14 Finally poliomyelitis, in the year 2017 only 22 wild poliovirus infections have been recorded in Afghanistan and Pakistan. Additionally 91 cases of circulating vaccine-derived poliovirus were noted in various Asian and African countries.15 The last case of a traveller was recorded in a student returning from Pakistan to Australia in 2007. Thus, with the exception of travellers visiting the at-risk countries and those joining the hajj, booster doses of poliomyelitis vaccine soon will become unnecessary. Conflict of interest: The author declares that in the past three years he has obtained support to attend meetings, honoraria for lectures, advisory boards or research from GlaxoSmithKline, Fondation Mérieux, Pfizer, Sanofi, Valneva.