Age-specific incidence/fatality of novel influenza during the 2009 epidemic in Japan

Age-specific incidence/fatality of novel influenza
during the 2009 epidemic in Japan

Etsuji Okamoto

Abstract：The author analyzed the daily, age-specific incidence data from the mandatory reporting of thenovel (H1N1) influenza which plagued the country in 2009 as well as the subsequent hospitalization
reporting data to illustrate age-related epidemiological pattern of the disease. The young population, particularly teen agers, showed higher susceptibility and vulnerability in terms of incidence,
hospital admission rate and relative fatality than the elderly population. The observed discrepancy
between the young and the old was larger than the traditional seasonal influenza although theseverity of the disease did not differ much. The age-related epidemiological pattern suggests
the“novel”nature of the disease and provides valuable evidence for future tactics against therecurrence of the epidemic.
Key words : novel influenza, surveillance, epidemic, fatality, H1N1Introduction(bay-side tactics) was escaped by the disease andOn 28th April 2009, WHO announced the out-

the entire country was plagued with the epidemic.

break of novel (swine) influenza (A/H1N1) in Mexi-

The outbreak started from a high school andco and raised the alert level to phase 4 (it later fur-quickly spread to the entire country despite large-

ther raised the alert level to phase 6 on 12 June).

scale school closures suggesting high susceptibility

Many countries including Japan strengthened theand transmissibility among the young, school-age

surveillance and quarantine to keep the disease atpopulation.
In the article, the author analyzes the age-relat-bay.
Unfortunately Japan’s strengthened quarantine

ed epidemiologic pattern of the influenza outbreakin Japan in 2009 based on the mandatory reporting
surveillance data and illustrates the“reverse【correspondence】
Etsuji Okamoto
Department of Health & Welfare Service Research
National Institute of Public Health
2-3-6, Minami, Wako-shi, Saitama 351-0197 JAPANTEL&FAX：048-458-6208

ageism”of its incidence, admission rate and relative fatality.
The then bureau chief of public health, Dr. Ueda
later described the initial response by the Ministryof Health, Labor & Welfare (MHLW)1）．MHLW̶ 78 ̶

ヘルスサイエンス・ヘルスケア

Volume 11，No.2（2011）

learned the first information on the outbreak fromfollow-up was conducted and it was unquestion-

the latest issue of MMWR published by CDC on

ably presumed that no infected persons hadrdth23 April. On 24 April, the world media started

entered the country yet. Consequently the so-to report the outbreak from Mexico. MHLW wascalled“bay-side”tactics, keeping the infected per-alerted by the high fatality in Mexico.

sons at bay through quarantine were pursued.

There were two direct flights/week between

On 9th May, the first infected persons wereMexico and Japan and a flight arrived from Mexi-

detected in in-flight inspection at Narita interna-thco on 25 April. MHLW conducted in-flight inspec-

tional airport. They were high school students andtion on passengers and no patients were found.

teachers flying back from Canada. They wereHowever, no retrospective survey was conducted

taken into a nearby hospital and a total of 49

for passengers entering the country before that

exposed persons were detained in a nearby hoteldate. Such retrospective survey would have been

for a week5）．It appeared that the strengthened

difficult without health questionnaire including thequarantine had successfully stopped the entrance

passenger’s name, address and phone number.

of the disease. Actually the disease was alreadyHowever it would not have been entirely impossi-spreading in somewhere else.

ble by requesting airliners to disclose passenger’sOn 16th May, minister Masuzoe announced that

information. Such disclose of personal information

the first confirmed case was found in Kobe city.

was permissible for public interest like disease

The case was a high school boy who developedcontrol. However, the then minister of MHLW,

fever on 11th May6）．The examining doctor sent

Masuzoe described that airliners tended to be

the specimen to the Kobe City Public Health Insti-

uncooperative ．

tute next day for PCR testing but the exam was2）

thApril, next day after the WHO alert,

delayed because the boy had no history of travel-

MHLW strengthened the airport quarantine anding abroad and hence did not fulfill the case defini-issued a mandate for reporting of diagnosed casestion. Later active surveillance by the city foundto doctors together with a formal definition ofthat influenza had already been prevalent amongcases3） (Japan maintains a“sentinel”surveillance

students of the high school. The first student

for seasonal influenza, in which selected hospitals

developed the symptom as early as 5th May (Japanand clinics [sentinels] report the number of new

had a five consecutive holidays 2-6 May and manycases weekly to the National Institute of Infectious

clinics were closed, which might have delayed vis-Diseases. The mandatory reporting of novel

iting doctors).

On 29

influenza was a separate measure to the existing

Despite the strengthened quarantine and rigor-sentinel surveillance). The strengthened airport

ous detention of exposed persons at the airport,

quarantine included in-flight inspection and distrib-

the disease already entered into the country andution of health questionnaire. The questionnaire

the epidemic was under way. The fact that a highincluded the passengers’
name, address and tele-school student with no history of travelling abroad

phone number for follow-up.

presented symptom on as early as 5th May sug-Actually, a molecular analysis suggested thatgests that the case was a secondary infection.

infected persons had possibly entered the country

Given the incubation time of the novel influenza, it

as early as 22th April eventually spreading the dis-was highly likely that the very first case infectingease in Kobe-Osaka region ．However, no suchthe students entered into the country before 28th4）

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Age-specific incidence/fatality of novel influenza during the 2009 epidemic in Japan

April or might have gone undetected by thebasis (the report was not released on weekends

strengthened quarantine.

except in the early phase) but no age-sex break-The number of reported cases increased dramatically after the discovery of the first case.

down was available, hence the analysis was limited to age-specific incidence.

th

Within three days, by 19 May, a total of 172 conResults and Discussion

firmed cases were reported from Kobe and Osaka
areas. The local governments of Kobe city and1）Results

Osaka prefecture implemented extensive schoolAge-specific incidence was calculated by divid-closures, deciding to close not only schools withing the number of reported cases with age-specificinfected students but all schools in both districts,population. The result is shown in [Table.1].

thfor one to two weeks from 16 May. As a result,

A total of 4,986 cases were reported between

over 4,200 schools with around 650,000 children/

16th May and 24th July 2009 or 39.1 cases per mil-students were closed7）． Recognizing that thelion population. By far the highest incidence wasdisease was already prevalent in the country,observed in teenagers (10-19 years old)： 195/mil-MHLW discontinued the strengthened quaran-lion population and the lowest among the elderlytine. Mandatory reporting continued until 24 July,>=60yo: 1.3/million.

by which time the cumulative number of patients

After the mandatory reporting was discontin-reached 4,986. After the mandatory reporting wasued after 24th July, reporting was required onlydiscontinued, only hospitalization cases werefor hospitalized cases but with more detailedreported.

patient information such as the presence of underlying conditions. The hospitalization surveillanceMaterials and Methodswas continued to 23rd March 2010 whose results

The daily data of mandatory reporting of con-are summarized in [Table.2].
Admission rate was highest among 0-9 yo:

firmed novel (swine) influenza cases between 19thth

May and 24 July 2009 in Japan were analyzed.

1.03/1000 population followed by teenagers: 0.26.

A cumulative total of 4986 confirmed cases wereRemarkably, the admission rate of the elderly wasreported by 24th July and a total of 17640 hospital-lower than high teenagers (0.05 for >=80yo vs. 0.26

ized cases were reported between 16th July 2009

for 10-19 yo). As for fatality of admitted patients,

and 23rd March 2010. Ten-year age-specific num-after excluding those with underlying diseases, theber of reported cases was available on a daily

middle age (40-49 yo) showed the highest case-Table.1

Age-specific incidence of novel influenza（16 May-24July 2009）̶ 80 ̶

ヘルスサイエンス・ヘルスケア

Volume 11，No.2（2011）

fatality (8 deaths/406 admissions or 19.7 per thou-ent pattern emerged: children aged 0-9 years

sand admissions) followed by 30-39 and 50-59 yoshowed the highest relative fatality during hospi-groups (12.3, 12.4 per thousand admissions respec-talization (5.694) followed by people aged 20-49.

tively). However no fatalities were observed with

Teen agers showed high hospitalization rate but

60-69 and >=80 yo groups and only 4 per thousand

their relative fatality was low.

admissions for 70-79 yo group (2 deaths/505 admis-

worthy was that the elderly over 60 yo showedsions).

lower relative fatality than younger generation.

What was note-

Although it appears that middle age hospitalizedpatients had the highest fatality, one should com-2）Discussion

pare inter-age fatality considering age-specificInfluenza is a serious health threats to thosemortality. Since older people have higher mortali-with underlying conditions and the elderly. This isty than young people, one should compare by rela-a prime reason why influenza vaccination is rec-tive fatality (the ratio between crude fatality

ommended for the elderly. However, when mea-

[deaths/admissions] and age-specific mortality).

sured by age-specific incidence, the elderly showsTen-year age-specific mortality (both men andlower incidence than the young. Kawado estimat-women) was obtained from Japan’s 2009 life table.

ed age-specific incidence of influenza based on

When compared with relative fatality, a differ-

Japan’s sentinel surveillance data as shown in

Table.2

Age-specific outcomes of hospitalization due to influenza（16th July 2009-23rd March 2010）

Table.3

Estimated age-sex-specific incidence of influenza（per thousand population）

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Age-specific incidence/fatality of novel influenza during the 2009 epidemic in Japan[Table.3] 8）．When compared between the age

Acknowledgments

group [15-59yo] and [>=60], the elderly consistently

This study was supported by Ministry of Sci-

showed lower incidence than the young in bothence & Education Grants-in-aid(B)[19390181]‘IT

men and women. The ratio between the elderly

infrastructure for active disease surveillance’

and the young appears to be declining over timefrom the Japan Society for the Promotion of Sci-

but the incidence of the elderly is nearly half ofence. This paper was presented at the 139th

the younger generation.

Annual Meeting & Exposition of the American

The epidemic of novel influenza in 2009 showed

Public Health Association held in Washington DC,

a far larger age difference in every measure ofUSA on 31st October 2011 for the session“Inter-severity of the epidemic, incidence, admission ratenational Health for Older Adults”.

and relative fatality, in which the young people
around teen agers showed more than ten-fold

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These findings are consistent with those basedon the surveillance of the U.S., in which
researchers found that contacts 18 years of age or
younger were twice as susceptible as those 19 to
50 years of age (relative susceptibility 1.96), andcontacts older than 50 years of age were less susceptible than those who were 19 to 50 years ofage (relative susceptibility 0.17) 9）．The high transmissibility among the young is also supported byapplying three mathematical models to Japan’s
surveillance data10）．
These age-related epidemiologic patterns suggest that the influenza A (H1N1)
was literally“novel”strain different from theexisting “seasonal” influenza although the severity of the illness did not differ much.
Conclusions
The elderly showed stronger resistance against
novel influenza than young population in terms ofincidence, hospital admission rate and relative
fatality, an age-related epidemiological pattern different from the traditional seasonal influenza.
Preventive and precautionary measures targetedto the young and vulnerable population will benecessary to combat the recurrence of the pandemic in the future.

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