Epidemiological characteristics and low case fatality rate of pandemic (H1N1) 2009 in Japan
Taro Kamigaki, Hitoshi Oshitani
Pandemic (H1N1) 2009 has been causing large outbreaks in Japan. Yet, the case fatality rate (CFR) remains low and only 85 deaths have been confirmed as of December 17, 2009. Surveillance data was analyzed to define epidemiological characteristics of pandemic (H1N1) 2009 in Japan. It was shown that most of the reported influenza-like illness cases and hospitalizations have occurred in those aged 5-9 years and 10-14 years, in whom CFR is extremely low. However, CFRs are higher in small children (<5 years) and adults. The transmission to these age groups may possibly have been minimized through aggressive suspension of classes in schools.
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The mortality rates per estimated cases and per reported hospitalizations were very low among age groups of 5-9 years, 10-14 years, and 15-19 years. These mortality rates were higher in children aged 0-4 years than in older children. Moreover, the mortality rates, both per estimated cases and per reported hospitalizations, increased significantly with age in adults. The mortality rate per estimated cases in the 50-59-year group was more than 100 times higher than that in the 15-19-year group. The mortality rates per hospitalization were also much higher in adults, particularly in those over 40 years. Our data indicate that CFR in small children (i.e., <5 years) and adults, especially the elderly, are higher than among those aged 5-19 years. However, most of the infections to date have occurred in these age groups (i.e., 5-19 years), while infections in other age groups are still very limited. This may be one possible reason why the CFR in Japan remains low.
In this study we have discussed another possible reason for the low CFR in Japan. Epidemiological characteristics of pandemic H1N1 in Japan are, to date, unique. Most cases and hospitalizations have occurred in those aged between 5 and 14 years. Children in these age groups are highly susceptible to infection of pandemic H1N1, but they are less likely to develop severe complications, and the CFR is extremely low. On the other hand, severe complications are more common in smaller children aged <5 years and adults aged over30 years, particularly in the elderly. However, the attack rate in these age groups is relatively low in Japan. It is still not clear what has produced this epidemiological pattern in Japan. The measure to aggressively suspend classes in schools may have contributed to a reduction in transmission into the community. However, cases in smaller children and adults are gradually increasing. More recent data indicated that the number of deaths particularly in adult patients was increasing.
By December 17, 2009, according to the MHLW, 122 deaths have been confirmed in Japan. Among these cases, 37 died in December, 29 (78%) of whom were older than 40 years. This data suggests an increasing trend toward fatal cases in the adult population. The overall mortality impact in Japan may depend on how infections are spreading in different age groups in coming months.
PLoS Currents: http://tinyurl.com/y8vtxmf
Taro Kamigaki, Hitoshi Oshitani
Pandemic (H1N1) 2009 has been causing large outbreaks in Japan. Yet, the case fatality rate (CFR) remains low and only 85 deaths have been confirmed as of December 17, 2009. Surveillance data was analyzed to define epidemiological characteristics of pandemic (H1N1) 2009 in Japan. It was shown that most of the reported influenza-like illness cases and hospitalizations have occurred in those aged 5-9 years and 10-14 years, in whom CFR is extremely low. However, CFRs are higher in small children (<5 years) and adults. The transmission to these age groups may possibly have been minimized through aggressive suspension of classes in schools.
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{snips}
The mortality rates per estimated cases and per reported hospitalizations were very low among age groups of 5-9 years, 10-14 years, and 15-19 years. These mortality rates were higher in children aged 0-4 years than in older children. Moreover, the mortality rates, both per estimated cases and per reported hospitalizations, increased significantly with age in adults. The mortality rate per estimated cases in the 50-59-year group was more than 100 times higher than that in the 15-19-year group. The mortality rates per hospitalization were also much higher in adults, particularly in those over 40 years. Our data indicate that CFR in small children (i.e., <5 years) and adults, especially the elderly, are higher than among those aged 5-19 years. However, most of the infections to date have occurred in these age groups (i.e., 5-19 years), while infections in other age groups are still very limited. This may be one possible reason why the CFR in Japan remains low.
In this study we have discussed another possible reason for the low CFR in Japan. Epidemiological characteristics of pandemic H1N1 in Japan are, to date, unique. Most cases and hospitalizations have occurred in those aged between 5 and 14 years. Children in these age groups are highly susceptible to infection of pandemic H1N1, but they are less likely to develop severe complications, and the CFR is extremely low. On the other hand, severe complications are more common in smaller children aged <5 years and adults aged over30 years, particularly in the elderly. However, the attack rate in these age groups is relatively low in Japan. It is still not clear what has produced this epidemiological pattern in Japan. The measure to aggressively suspend classes in schools may have contributed to a reduction in transmission into the community. However, cases in smaller children and adults are gradually increasing. More recent data indicated that the number of deaths particularly in adult patients was increasing.
By December 17, 2009, according to the MHLW, 122 deaths have been confirmed in Japan. Among these cases, 37 died in December, 29 (78%) of whom were older than 40 years. This data suggests an increasing trend toward fatal cases in the adult population. The overall mortality impact in Japan may depend on how infections are spreading in different age groups in coming months.
PLoS Currents: http://tinyurl.com/y8vtxmf