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H1N1新型流感問答集

作者:台大醫院小兒部李秉穎醫師

建立:2009.05.14

修改:2010.01.29

 

秋冬會有更嚴重的第二波疫情?

因為流感比較不容易在夏天流行,所以在大流感發生大突變以後,其流行曲線會呈現二至三波的形狀,其中低下去的谷底就是天氣變熱的時候。1918年的西班牙流感,第一波比較輕微,但在六個月後造成嚴重傷亡的疫情。1957年的大突變也類似,一開始比較輕微,後來出現嚴重的第二波疫情。1968年的大突變則第一波與第二波疫情都還算輕微。有人推測病毒突變導致第二波疫情比較嚴重,這種看法的正確性值得商榷。通常第一波疫情大多侵犯青壯年等有工作能力的人,第二波以後形成大規模社區感染,更多孩童、老人、慢性疾病患者罹病,而且社區感染會使親密接觸傳染機會增加,傳染到的病毒量可能因而大增,致死率自然上升,例如1918年西班牙流感的第一波很多病患是第一次世界大戰剛結束時要返家的戰士。1918年西班牙流感後來的確有突變,但那是經過數年的慢慢突變,通常流感病毒不會在短短一年中發生兩次以上重大突變。 新型流感疫苗上市後,因為對於疫苗不當的質疑,導致疫苗接種率不如理想,發生第二波、第三波疫情的可能性極高。

 

 

2009.5.6 法新社相關報導

新型流感第二波?南半球疫情發展是關鍵

H1N1新型流感在全球擴散的情況雖然趨緩,但專家今天警告說,世界各國必須提防第二波疫情。根據之前全球疫情的經驗,第二波毒性可能增強許多。

專家們指出,20世紀三次全球流感大流行,包括造成4000萬人死亡的1918年西班牙 流感,起初都是在北半球春末發生相對緩和的疫情。

倫敦 聖巴索羅繆醫院(Saint Bartholomew's)和皇家倫敦醫院(Royal London Hospital)著名病毒專家奧斯佛(John Oxford)說,全球疫情通常一開始會有一個「前導波」,預示之後會有另一波疫情。

他在電話中告訴法新社記者說:「讓人不安的是,1918年夏季曾有並不嚴重的疫情,那應該讓人警覺到秋冬將發生嚴重疫情,但是並沒有。」

威契塔州立大學(Wichita State University)歷史學家和豬流感專家甘納(George Gehner)告訴法新社記者說:「1889、1918、1957和1968年,都是在春季和夏初,也就是北半球流感季節結束的時候,出現一種到處蔓延的新型流感。」

奧斯佛跟甘納都說,出現「第二波」的可能性很高,但未必比第一波致命。即使第二波確實毒性更強,世界各國應付致命流感的能力也已經提升許多。

奧斯佛說:「如果重演1918的情況,那麼我們的準備工作就完全白費了。我們有抗病毒藥物和傳染的知識庫,而且我猜想能很快製造出疫苗。」

一個世紀之前如果有抗生素,可以拯救數百萬人的性命,因為在1918年,約有60%的患者是因二度細菌感染引發肺炎和其他呼吸疾病而死亡。

目前的新型流感疫情在南半球即將來臨的冬季如何發展,還有墨西哥疫情中有是否有許多老人死亡,將對未來幾個月會發生什麼事提供重要的線索。

衛生專家最擔心的情況是病毒持續變化,跟鳥類、豬隻和人類流感的病毒基因混合。

奧斯佛說,許多科學家相信,1918年的流感病毒經過突變,能「誘騙」人類的免疫系統,使它對肺部組織進行攻擊。

1918年流感跟當前流感一項讓人不安的相似之處,在於死者屬於同一個年齡層。雖然為數不多,但墨西哥許多死於流感的患者是年輕人,這點跟1918年相同。

季節流感每年造成25萬到50萬人死亡,其中大多數是幼童和老年人。

墨西哥衛生官員仍在整理感染流感與死亡患者的資料。根據官方統計,墨西哥這次有近800人感染新型流感,26人死亡。

奧斯佛說:「如果死者當中都沒有老年人,我會非常意外。那會是好消息,因為這個年齡層的人一般比較容易生病。」

他說,如果60歲以上的人大致都沒有嚴重病情,也可能表示當中有些人經過從前的大流行之後,還有若干剩餘的「跨流感免疫力」,特別是1957年那一次。

 

 

2009.5.12 中央社相關報導

新型流感病毒可能突變 引發高達3波大流行

世界衛生組織(WHO)今天表示,H1N1新型流感病毒仍可能突變成毒性更強病毒,並引發全球大流行,預料可能會重複蔓延全球高達三波。

世衛指出,任何大流行的衝擊將會迴異,因為在擁有強大保健體系國家境內,病毒可能只會引發溫和疫情,但它卻能「重創」保健體系脆弱國度,後者往往短缺藥物且醫院設備不良。

世衛昨晚公布標題為「評估流感大流行嚴重性」(Assessing the severity of an influenza pandemic)文件指出,通常被稱為豬流感的H1N1新型流感病毒,「傳染力似乎較季節性流感更強」,且幾乎全球人口都對這種新疾病沒有免疫力。

世衛表示,「大流行的整體嚴重性,由於大流行往往傾向於重覆蔓延全球至少兩波,有時甚至三波,而進一步受影響。」

相較於季節性流感,H1N1新型流感病毒的傳染力似乎更強。定期出現的季節性流感每年約造成25萬到50萬人喪生,致死率(fatality rate)不到0.1%。

世衛指出,季節性流感的二次侵襲率(secondaryattack rate),即接觸者遭到已感染者傳染的百分比,介於5%到15%之間。「依目前估計,H1N1新型流感的二次侵襲率介於22%到33%之間」。

 

2009.5.11 世界衛生組織相關文章

Assessing the severity of an influenza pandemic

(http://www.who.int/csr/disease/swineflu/assess/disease_swineflu_assess_20090511/en/index.html, access 14 May 2009)

The major determinant of the severity of an influenza pandemic, as measured by the number of cases of severe illness and deaths it causes, is the inherent virulence of the virus. However, many other factors influence the overall severity of a pandemic’s impact.

Even a pandemic virus that initially causes mild symptoms in otherwise healthy people can be disruptive, especially under the conditions of today’s highly mobile and closely interdependent societies. Moreover, the same virus that causes mild illness in one country can result in much higher morbidity and mortality in another. In addition, the inherent virulence of the virus can change over time as the pandemic goes through subsequent waves of national and international spread.

Properties of the virus


An influenza pandemic is caused by a virus that is either entirely new or has not circulated recently and widely in the human population. This creates an almost universal vulnerability to infection. While not all people ever become infected during a pandemic, nearly all people are susceptible to infection.

The occurrence of large numbers of people falling ill at or around the same time is one reason why pandemics are socially and economically disruptive, with a potential to temporarily overburden health services.

The contagiousness of the virus also influences the severity of a pandemic’s impact, as it can increase the number of people falling ill and needing care within a short timeframe in a given geographical area. On the positive side, not all parts of the world, or all parts of a country, are affected at the same time.

The contagiousness of the virus will influence the speed of spread, both within countries and internationally. This, too, can influence severity, as very rapid spread can undermine the capacity of governments and health services to cope.

Pandemics usually have a concentrated adverse impact in specific age groups. Concentrated illnesses and deaths in a young, economically productive age group will be more disruptive to societies and economies than when the very young or very old are most severely affected, as seen during epidemics of seasonal influenza.

Population vulnerability


The overall vulnerability of the population can play a major role. For example, people with underlying chronic conditions, such as cardiovascular disease, hypertension, asthma, diabetes, rheumatoid arthritis, and several others, are more likely to experience severe or lethal infections. The prevalence of these conditions, combined with other factors such as nutritional status, can influence the severity of a pandemic in a significant way.

Subsequent waves of spread


The overall severity of a pandemic is further influenced by the tendency of pandemics to encircle the globe in at least two, sometimes three, waves. For many reasons, the severity of subsequent waves can differ dramatically in some or even most countries.

A distinctive feature of influenza viruses is that mutations occur frequently and unpredictably in the eight gene segments, and especially in the haemagglutinin gene. The emergence of an inherently more virulent virus during the course of a pandemic can never be ruled out.

Different patterns of spread can also influence the severity of subsequent waves. For example, if schoolchildren are mainly affected in the first wave, the elderly can bear the brunt of illness during the second wave, with higher mortality seen because of the greater vulnerability of elderly people.

During the previous century, the 1918 pandemic began mild and returned, within six months, in a much more lethal form. The pandemic that began in 1957 started mild, and returned in a somewhat more severe form, though significantly less devastating than seen in 1918. The 1968 pandemic began relatively mild, with sporadic cases prior to the first wave, and remained mild in its second wave in most, but not all, countries.

Capacity to respond


Finally, the quality of health services influences the impact of any pandemic. The same virus that causes only mild symptoms in countries with strong health systems can be devastating in other countries where health systems are weak, supplies of medicines, including antibiotics, are limited or frequently interrupted, and hospitals are crowded, poorly equipped, and under-staffed.

Assessment of the current situation


To date, the following observations can be made, specifically about the H1N1 virus, and more generally about the vulnerability of the world population. Observations specific to H1N1 are preliminary, based on limited data in only a few countries.

The H1N1 virus strain causing the current outbreaks is a new virus that has not been seen previously in either humans or animals. Although firm conclusions cannot be reached at present, scientists anticipate that pre-existing immunity to the virus will be low or non-existent, or largely confined to older population groups.

H1N1 appears to be more contagious than seasonal influenza. The secondary attack rate of seasonal influenza ranges from 5% to 15%. Current estimates of the secondary attack rate of H1N1 range from 22% to 33%.

With the exception of the outbreak in Mexico, which is still not fully understood, the H1N1 virus tends to cause very mild illness in otherwise healthy people. Outside Mexico, nearly all cases of illness, and all deaths, have been detected in people with underlying chronic conditions.

In the two largest and best documented outbreaks to date, in Mexico and the United States of America, a younger age group has been affected than seen during seasonal epidemics of influenza. Though cases have been confirmed in all age groups, from infants to the elderly, the youth of patients with severe or lethal infections is a striking feature of these early outbreaks.

In terms of population vulnerability, the tendency of the H1N1 virus to cause more severe and lethal infections in people with underlying conditions is of particular concern.

For several reasons, the prevalence of chronic diseases has risen dramatically since 1968, when the last pandemic of the previous century occurred. The geographical distribution of these diseases, once considered the close companions of affluent societies, has likewise shifted dramatically. Today, WHO estimates that 85% of the burden of chronic diseases is now concentrated in low- and middle-income countries. In these countries, chronic diseases show an earlier average age of onset than seen in more affluent parts of the world.

In these early days of the outbreaks, some scientists speculate that the full clinical spectrum of disease caused by H1N1 will not become apparent until the virus is more widespread. This, too, could alter the current disease picture, which is overwhelmingly mild outside Mexico.

Apart from the intrinsic mutability of influenza viruses, other factors could alter the severity of current disease patterns, though in completely unknowable ways, if the virus continues to spread.

Scientists are concerned about possible changes that could take place as the virus spreads to the southern hemisphere and encounters currently circulating human viruses as the normal influenza season in that hemisphere begins.

The fact that the H5N1 avian influenza virus is firmly established in poultry in some parts of the world is another cause for concern. No one can predict how the H5N1 virus will behave under the pressure of a pandemic. At present, H5N1 is an animal virus that does not spread easily to humans and only very rarely transmits directly from one person to another.

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