推奨される渡航ワクチンを見送り、その後ワクチンで予防可能な疾患に罹患した渡航者の約40％が後悔を報告している。渡航中に獲得したA型肝炎で入院した未接種の渡航者のうち、59％が入院を要した（GeoSentinel 2008-2020）。入院した未接種の感染症患者を対象とした症例集積による臨床後悔文献では64.7％に達する（Ioannou et al. 2022を代理指標として）
CDC/WHOの渡航先ガイダンスに従って推奨される渡航前ワクチンの1つ以上を拒否した米国/EUの成人国際渡航者。最も強力なデータはGeoSentinel帰国渡航者サーベイランス（Balogun et al. 2022 N=254のA型肝炎症例、98％未接種）から、Ioannou et al. 2022の入院した未接種者の後悔を擁護可能な代理指標として補完
No published study directly measures regret among travelers who skipped a recommended pre-travel vaccine and later contracted a vaccine-preventable disease — the 40% action-side estimate is therefore triangulated rather than measured, which is why the entry carries the proxy_only flag. The closest direct evidence comes from GeoSentinel surveillance of returned international travelers: Balogun et al. 2022 in the Journal of Travel Medicine analyzed 254 hepatitis A cases reported to GeoSentinel sites between 2008 and 2020 and found that 98% of cases with vaccination data were in unvaccinated travelers, and 59% of those with hospitalization data were admitted to hospital. The structural parallel to Ioannou et al. 2022 — where 64.7% of hospitalized unvaccinated COVID-19 patients said they would vaccinate “if they could turn time back” — supplies the regret rate that the GeoSentinel severity data anchors. Most travel-vaccine skippers return home asymptomatic; the population-weighted regret figure sits well below the hospitalized-cohort 65%, but well above ambient pre-decision regret because the skip-then-get-sick pathway produces the same vivid retrospective wish for vaccination across infectious diseases.
The inaction side runs at roughly 8% because most travel vaccines are highly effective with very low serious-adverse-event rates: hep A inactivated vaccine has near-100% seroconversion after a single dose with adverse events limited to mild local reactions; typhoid Vi polysaccharide and oral Ty21a are similarly well-tolerated; Japanese encephalitis Ixiaro has an established benign safety profile. The modal inaction-side regret is therefore financial and logistical rather than medical — a traveler who pays $300-700 out of pocket for the two-dose JE primary series before a one-week trip to urban Bangkok, only to discover later that CDC ACIP guidance explicitly does not recommend JE vaccine for that itinerary, expresses retrospective regret about the spend, not the safety. The one exception sits at the catastrophic end: yellow fever vaccine in first-time recipients aged 60 and older carries a YEL-AVD reporting rate of 1.0-3.2 per 100,000 doses per CDC MMWR, with case fatality near 50%. For a 70-year-old first-time vaccinee travelling to a destination that does not actually require proof of yellow fever vaccination on entry, this is the segment where inaction-side regret can crystallize as full-magnitude medical regret.
The travel-vaccine decision is sharply stratified by destination and itinerary in a way the broader adult-vaccine decision is not. CDC Yellow Book states directly: “ACIP does not recommend JE vaccine for travelers with very low-risk itineraries (e.g., shorter-term travel limited to urban areas, travel that occurs outside a well-defined JE virus transmission season),” with overall JE incidence among travelers from non-endemic countries estimated at less than 1 case per million travelers. By contrast, hepatitis A vaccine is rationally indicated for virtually every adult traveler to South-Central Asia, sub-Saharan Africa, Latin America, or any non-Western destination — the Lammert/LaRocque 2016 Global TravEpiNet data show 25% of US travelers seeking pre-travel care still decline at least one recommended vaccine, most commonly because they “are not concerned about the illness,” and that lack of concern is the precise attitudinal precursor that flips into regret after a hospital admission for hepatitis A. The action-dominates Gilovich pattern holds because the action-side regret population can crystallize into full medical-severity regret (hospitalization for typhoid or hep A) while the inaction-side regret population mostly experiences mild financial or inconvenience regret, with the 60+ first-time YF subpopulation as the narrow exception. For the broader CDC adult immunization schedule, see Skip adult vaccines vs. take them; for the underlying destination-risk math, see Typhoid fever and Japanese encephalitis (travel).
Among 254 international travelers with hepatitis A (185 confirmed, 69 probable) reported to the GeoSentinel Surveillance Network from 2008-2020, 98% (53 of 54 with vaccination data available) were unvaccinated; 59% (52 of 88 with hospitalization data) required hospitalization. The most common reasons for travel were tourism (47%) and visiting friends and relatives (28%); hepatitis A was acquired most often in South-Central Asia (25%) and sub-Saharan Africa (24%)
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“"Among 254 travellers with hepatitis A (185 confirmed and 69 probable), the median age was 28 years, 150 (59%) were male, and among 54 travellers with information available, 53 (98%) were unvaccinated. Among 88 travellers with information available, 59% were hospitalized. Travelers were most frequently tourists (n = 120; 47%), followed by those visiting friends and relatives (VFRs; n = 72; 28%). Hepatitis A was acquired most frequently in South-Central Asia (n = 63; 25%) and sub-Saharan Africa (n = 61; 24%). Despite availability of highly effective vaccines, travellers still acquire hepatitis A, even when traveling to low-endemicity destinations."
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Balogun, Brown, Angelo, Hochberg, Barnett et al. 2022, J Travel Med 29(2):taac013. GeoSentinel surveillance peer-reviewed study — does NOT directly measure regret, but establishes the consequence-severity base rate that drives action-side regret when an outcome materializes: 98% of travel-acquired hep A is in the unvaccinated, and 59% of those cases are hospitalized. The 40% population-weighted regret estimate is bounded above by hospitalized-cohort post-illness regret (~65%, see Ioannou proxy) and below by the much larger denominator of vaccine-skippers who travel without consequence. Used as the severity-of-consequence anchor for the action-side rate; the regret-rate itself is triangulated via the Ioannou proxy below.
[2]Infectious Disease Reports — COVID-19 Disease and Vaccination: Knowledge, Fears, Perceptions and Feelings of Regret for Not Having Been Vaccinated among Hospitalized Greek Patients Suffering SARS-CoV-2 Infection
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Among 162 hospitalized COVID-19 patients in two Greek tertiary care hospitals (56.2% unvaccinated, 97% with severe COVID-19), 64.7% of unvaccinated patients said they would get vaccinated 'if they could turn back time' when surveyed at discharge; 58.4% expressed this regret upon admission. Reused here as a defensible proxy for hospitalized-skipper regret because no travel-vaccine-specific direct regret survey exists in the published literature
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“"In total, 228 patients were asked to participate in the study, and finally, 162 (71.1%) agreed and participated. When asked whether they had been vaccinated against COVID-19, 56.2% replied that they had not. Severe COVID-19 was diagnosed in 97% of enrolled patients. When unvaccinated patients were asked whether they would get vaccinated if they could turn time back, 58.4% replied positively when asked upon admission, while 64.7% replied positively when asked on discharge. 53.9% replied positively when asked on discharge [about willingness to vaccinate]. When unvaccinated patients were asked whether they would suggest vaccination against COVID-19 to their relatives, 68.2% replied positively."
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Ioannou et al. 2022 Infectious Disease Reports 14(4):587-596. Reused as a defensible cross-disease proxy: the published literature contains no equivalent direct regret survey of travelers hospitalized for vaccine-preventable diseases (hep A, typhoid, yellow fever, JE). The structural decision is identical — adult declined a recommended vaccine, contracted a vaccine-preventable disease severe enough for hospital admission, was asked retrospectively about regret. The 64.7% hospitalized- cohort regret rate bounds the upper end of the population-weighted 40% action-side estimate; most travel-vaccine skippers never get hospitalized (GeoSentinel hep A denominator is the millions of unvaccinated travelers to endemic areas annually, not the ~250 sentinel cases captured), so the population-weighted regret rate runs well below the hospitalized-cohort figure. The proxy_only flag exists precisely to permit this triangulation; the entry is transparent about the absence of a direct travel-vaccine regret survey.
[3]Journal of Travel Medicine — Refusal of recommended travel-related vaccines among U.S. international travellers in Global TravEpiNet
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Of 23,768 US international travelers eligible for at least one recommended travel vaccine seen at Global TravEpiNet pre-travel consultation sites from July 2012 through June 2014, 25% (6,573) refused one or more recommended vaccines. Refusal rates by vaccine: meningococcal 44%, rabies 44%, Japanese encephalitis 41%, influenza 33%. The most common reason for declining was lack of concern about the illness
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“"Of 23 768 eligible travellers, 6573 (25%) refused one or more recommended vaccine(s). Travellers were most frequently eligible for typhoid, hepatitis A, and influenza vaccines. Refusal rates were highest for meningococcal (44%), rabies (44%), Japanese encephalitis (41%), and influenza (33%) vaccines. The most common reason for declining vaccines was that the traveller was not concerned about the illness. Lack of concern about disease, cost, and safety concerns were the three primary refusal categories across all vaccine types."
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Lammert, Rao, Jentes, Fairley, Erskine, Walker, Hagmann, Sotir, Ryan, LaRocque 2016 J Travel Med 24(1):taw075. Establishes the denominator and decision structure of the action-side population: ~25% of US travelers who actually sought pre-travel medical consultation declined at least one recommended vaccine; the true refusal rate among the broader population of international travelers who never sought pre-travel care is substantially higher. The primary reason — "not concerned about the illness" — is the exact attitudinal precursor that flips into regret after a severe outcome, per the Ioannou proxy. Does not itself measure regret; included to characterize the decision population.
ACIP explicitly does NOT recommend JE vaccine for travelers with very low-risk itineraries (shorter-term travel limited to urban areas, travel outside the JE transmission season). The overall incidence of JE among travelers from non-endemic countries to Asia is estimated at <1 case per 1 million travelers. Short-term tourists restricted to major urban areas are at minimal risk — yet JE vaccine is frequently administered to this population, generating financial/inconvenience regret on the inaction side
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“"ACIP does not recommend JE vaccine for travelers with very low-risk itineraries (e.g., shorter-term travel limited to urban areas, travel that occurs outside a well-defined JE virus transmission season). ACIP recommends JE vaccine for people moving to a JE-endemic country, longer-term (e.g., ≥1 month) travelers to JE-endemic areas, and frequent travelers to JE-endemic areas. The overall incidence of JE among people from non-endemic countries traveling to Asia is estimated to be <1 case per 1 million travelers. Shorter-term (e.g., <1 month) travelers whose visits are restricted to major urban areas are at minimal risk for JE."
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CDC Yellow Book — authoritative ACIP guidance defining the boundary between rationally-recommended JE vaccination (long-stay rural Asia) and over-prescription (short-stay urban tourists). The two-dose JE primary series runs ~$300-700 out of pocket in the US; a traveler who pays this for a one-week Bangkok itinerary that ACIP explicitly says does not require the vaccine generates the modal inaction-side regret event for this entry. Used to anchor the ~5-8% baseline financial/inconvenience regret component of the inaction-side estimate. See also [[japanese-encephalitis-travel]] for the absolute risk math.
[2]CDC MMWR Recommendations and Reports — Yellow Fever Vaccine: Recommendations of the Advisory Committee on Immunization Practices (ACIP)
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政府報告書
Reported rates of serious YF vaccine adverse events: YEL-AND 0.4-0.8 per 100,000 doses, YEL-AVD 0.3-0.4 per 100,000 doses overall. For first-time vaccinees aged 60-69: YEL-AND 1.6 per 100,000, YEL-AVD 1.0-1.1 per 100,000. For first-time vaccinees aged 70+: YEL-AND 1.1-2.3 per 100,000, YEL-AVD 2.3-3.2 per 100,000. Serious adverse events occur essentially exclusively in first-time vaccine recipients — no YEL-AVD cases have been reported following booster doses
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“"The reporting rate for YEL-AND is 0.4-0.8 cases per 100,000 doses distributed. The reporting rate of YEL-AVD is 0.3-0.4 cases per 100,000 doses distributed. For persons aged 60-69 years, the YEL-AND rate is 1.6 cases per 100,000 doses distributed and the YEL-AVD rate is 1.0-1.1 cases per 100,000 doses distributed. For persons aged 70 years and older, the YEL-AND rate is 1.1-2.3 cases per 100,000 doses distributed and the YEL-AVD rate is 2.3-3.2 cases per 100,000 doses distributed. All YEL-AND cases reviewed occurred in first-time vaccine recipients; YEL-AVD has occurred only following a recipient's first YF vaccination, with no cases reported in persons receiving booster doses."
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CDC MMWR — authoritative federal-government surveillance data on yellow fever vaccine serious adverse events. The 60+ first-time-vaccinee YEL-AVD rate of 1-3 per 100,000 is the one place on the inaction side where catastrophic safety regret can be elevated (case fatality ~50% for YEL-AVD). This drives the ~3% safety-regret component of the inaction-side estimate among the small subpopulation of 60+ first-time vaccinees travelling to YF zones where proof-of-vaccination may not even be mandatory. Combined with the ~5-8% financial/inconvenience regret from JE over-prescription (per the CDC JE Yellow Book source above), supports the 8% weighted inaction-side estimate. The proxy_only flag is required because neither source directly measures regret; both are authoritative inputs to a structured estimate.
注意事項
No direct regret survey exists for the travel-vaccine-skipping decision specifically — the entry uses proxy_only because both sides are triangulated rather than measured. The action-side 40% estimate combines (a) GeoSentinel surveillance establishing that 98% of travel-acquired hepatitis A in returned travelers is in the unvaccinated and 59% of cases are hospitalized (Balogun et al. 2022), with (b) the Ioannou et al. 2022 hospitalized- unvaccinated COVID-19 cohort regret rate of 64.7% reused as a defensible proxy for what travel-vaccine-skipper regret looks like after the dread outcome materializes. Most travel-vaccine skippers return home asymptomatic, so the population-weighted regret rate sits well below the hospitalized-cohort 65%; the 40% estimate is bounded above by that ceiling and below by the very low ambient regret of skippers whose trips passed without incident. The recommendation set is also heterogeneous: hepatitis A vaccine is high-value across nearly all non-Western destinations and the recommendation is rational for almost every adult traveler (see [[hepatitis-a-travel]] for the absolute per-trip risk math, and [[typhoid-endemic]] for the parallel typhoid case in South Asia); Japanese encephalitis vaccine, by contrast, is explicitly NOT recommended by CDC ACIP for short-stay urban tourists in Asia (see [[japanese-encephalitis-travel]]), and the modal inaction-side regret is paying $300-700 out of pocket for a JE primary series that ACIP guidance says was not indicated. The inaction-side 8% estimate also incorporates the elevated yellow fever vaccine adverse-event rate among first-time vaccinees aged 60+ (YEL-AVD ~1-3 per 100,000 doses, case-fatality ~50%) per CDC MMWR — for older first-time travelers headed to a destination that does not actually mandate proof of vaccination, this is the one segment where catastrophic safety regret can crystallize on the inaction side. This entry covers the travel-specific subset of the broader adult-vaccine decision; for the parent decision about CDC adult immunization schedule (flu, COVID, shingles, HPV) see [[skip-adult-vaccines-vs-vaccinate]]. Travelers with specific medical contraindications (severe egg allergy precluding YF vaccine, immunocompromise precluding live vaccines, pregnancy) are outside the population for which this regret asymmetry applies — those are clinically guided decisions to defer specific vaccines and rely on alternative prevention, not the traveler-skepticism decision the entry addresses.
