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Abstract

  1. Top of page
  2. Abstract
  3. Methods
  4. Results
  5. Discussion
  6. Acknowledgment
  7. Declaration of Interests
  8. References

Age distribution of 4,986 cases of influenza A (H1N1) 2009 in Japan was analyzed. Cases with a travel history within 10 days preceding the illness onset were significantly older than indigenous cases (p < 0.01) reflecting age-specific travel patterns. Border controls should account for the high frequency of infection among adults.

The importance of age specificity in influenza A (H1N1) 2009 virus infection has been increasingly recognized. The infection is most frequently seen among those aged <20 years,1,2 and severe cases accumulate in young adults, reflecting the second highest frequency of infection in this group.3 While these patterns evoke the concept of age-related disease control policies, including school closures, and treatment and prevention in relation to preexisting immunity,4,5 the impact of human travel and age, and implications for preventing widespread pandemics have yet to be clarified.6 This article reports the age specificity of imported and indigenous cases in Japan.

Methods

  1. Top of page
  2. Abstract
  3. Methods
  4. Results
  5. Discussion
  6. Acknowledgment
  7. Declaration of Interests
  8. References

All confirmed cases of H1N1 2009 virus infection were mandatorily reported to the Japanese Government by the end of July 2009. In total, 4,986 confirmed cases, experiencing the onset of illness from May 5 to July 24, were included in this analysis. A travel history to any country which reported confirmed cases was notified along with the age of the patient. Those with a travel history within 10 days preceding the onset were regarded as imported cases. Comparison of age between those with and without a travel history was performed using the Welch test.

Results

  1. Top of page
  2. Abstract
  3. Methods
  4. Results
  5. Discussion
  6. Acknowledgment
  7. Declaration of Interests
  8. References

Of 4,986 confirmed cases, 903 (18.1%) were imported. Figure 1 compares the age distribution between imported and indigenous cases. The mean (SD) and median ages of imported cases were 27.0 (15.6) and 26.0 years, and of indigenous cases were 17.6 (10.9) and 16.0 years. The age of imported cases appeared to be significantly older than indigenous cases (p < 0.01). While 83.4% of indigenous cases were aged <25 years, only 43.4% of imported cases were <25 years.

image

Figure 1. Age distribution of confirmed cases of influenza A (H1N1) 2009 virus infection in Japan stratified by travel history (n = 4,986). All the cases, experiencing illness onset from May 5 to July 24, 2009, were confirmed by means of reverse transcription-polymerase chain reaction. Imported cases were those with history of overseas travel within 10 days preceding the onset, whereas indigenous cases were those without a travel history. The ends of the box plot are the 25 and 75 percentiles, the middle line corresponds to the median estimate, and the whiskers extend to 1.5 times the interquartile range. Histograms show the frequency distribution of age separately for imported and indigenous cases (horizontal axis measures the frequency).

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Discussion

  1. Top of page
  2. Abstract
  3. Methods
  4. Results
  5. Discussion
  6. Acknowledgment
  7. Declaration of Interests
  8. References

The risk of infection among imported cases was not found to be accumulated in those aged <20 years, but rather those aged 25 years and older accounted for more than half of the imported infections. The differential age distribution most likely reflected age-specific travel patterns because adults >25 years were more likely to have experienced international travel. An important limitation of the present study is that imported case, which was defined as those with a travel history within 10 days before the illness onset, potentially includes those infected in Japan. Nevertheless, given the significantly different ages between crudely defined imported and indigenous cases, the age of actual imported cases may be even older than that reported in Figure 1.

As a future subject, in addition to the age-specific absolute number of cases, age-specific incidence of infection among travelers (ie, imported cases divided by the number of travelers) needs to be explored. Whereas the impact of imported cases on the transmission dynamics of importing country can be partly assessed by examining the age-specific number of imported cases,7,8 further clarification of the role of adults in accelerating global spread requires additional insight into the age-specific risk of infection among travelers.9

Epidemiological analysis of travel-associated cases of H1N1 2009 influenza is crucial for understanding the dynamics of international spread and elucidating the most effective strategies for disease control.10 Unlike the local spread of H1N1 2009 influenza, which is frequently driven by infection in schoolchildren, adults play an important role in accelerating international spread. Adults are also likely to be the source of interregional spread within a country. Two important implications are that prevention of international spread (eg, border controls) must not overlook the high frequency of infection even among older adults, and surveillance and monitoring of the spread of disease over long distances need to take into account the impact of age specificity of travel on geographic propagation.11

Acknowledgment

  1. Top of page
  2. Abstract
  3. Methods
  4. Results
  5. Discussion
  6. Acknowledgment
  7. Declaration of Interests
  8. References

The work of H. N. was supported by the JST PRESTO program.

Declaration of Interests

  1. Top of page
  2. Abstract
  3. Methods
  4. Results
  5. Discussion
  6. Acknowledgment
  7. Declaration of Interests
  8. References

The author states he has no conflicts of interest to declare.

References

  1. Top of page
  2. Abstract
  3. Methods
  4. Results
  5. Discussion
  6. Acknowledgment
  7. Declaration of Interests
  8. References