A substantial portion of older Americans may have some immunity to the swine-origin H1N1 influenza virus, a finding that may prove useful when and if a vaccine to the new flu strain becomes available.

    The questions of whom to target with a swine flu vaccine and how to stretch the supply if it is limited are among the most important issues facing public health officials over the next four months.

    Scientists at the Centers for Disease Control and Prevention announced yesterday that a study using stored blood samples found that one-third of people older than 60 have antibodies that might protect them from infection with the new virus. If further research is able to better define who has partial immunity, those people might need only one dose of vaccine, not two.

    "Our working hypothesis is that everyone who gets this vaccine is likely to need two doses," Anne Schuchat, CDC's deputy director for science and public health, said yesterday. She added, however, that the new study suggests "perhaps there will be some people where preexisting immunity will be there, and one dose would lead to a 'primed' response. That is definitely . . . something we're interested in."

    If a swine flu vaccine is produced, about 2 billion doses would be ready by next fall, the World Health Organization estimates. Public health authorities presumably would recommend it for people at greatest risk for severe illness and death.

    As of yesterday, the United States had 5,764 confirmed cases and nine deaths, in 47 states and the District of Columbia. Epidemiologists believe, however, that more than 100,000 people have been infected since the new virus came to public attention a month ago.

    Worldwide, 41 countries have reported 11,034 cases and 85 deaths -- numbers that are almost certainly also an undercount.

    The blood study, published yesterday in CDC's Morbidity and Mortality Weekly Report, gives an immunological explanation for a surprising observation in the swine flu outbreak: that very few old people are getting sick.

    Nearly two-thirds of the U.S. cases are in people between 5 and 24 years old. Less than 1 percent are in people older than 65, those most susceptible to typical seasonal outbreaks of influenza. Of the people ill enough to be hospitalized, 40 percent have been 19 to 49.

    In the study, researchers tested blood collected since 2005 for research on the effectiveness of seasonal flu vaccine. They exposed the blood to samples of the swine flu to see whether it contained antibodies that attacked the virus.

    Samples from children 6 months to 9 years old contained virtually no antibodies against the swine flu strain. However, 6 percent of people 18 to 40, 9 percent of people 18 to 64 and 33 percent of people older than 60 had the antibodies.

    When blood samples taken after the same people had received seasonal flu vaccine were tested, the percentage with active antibodies against the swine flu strain increased in the two older groups. Specifically, for the 18-to-64-year-olds, it increased from 9 to 25 percent; and for the older-than-60 group, from 33 to 43 percent.

    Overall, the findings suggest that many older people may have been exposed to a flu virus decades ago that bore a similarity to the new strain and triggered an immune response. Seasonal flu shots appear to boost that "memory" response a little.

    A vaccine made from the new strain would be expected to both increase and sharpen the response -- perhaps enough that a single shot would suffice. However, people whose immune systems have never encountered a flu strain even remotely like the new one would almost certainly need two shots to gain protection.

    Schuchat, the CDC epidemiologist, said no firm conclusions can be drawn yet.

    It is not known whether the "cross-reactive" antibodies found in the study will prove to be protective against illness, or whether the 79 children and 280 adults whose blood samples were tested are representative of the population at large. Studies to answer those questions are underway.

    據《華盛頓郵報》5月22日報道，研究發現美國一些老年人可能對甲型H1N1流感病毒有些免疫力，如果新型流感疫苗研制獲得成功，或許能證明這一發現非常有用。四個月后公共健康官員要解決的最重要的問題是：確定流感疫苗的對象、在疫苗數量有限的情況下如何擴大其供給。

    5月21日美國疾病預防控制中心的科學家宣布，在美國三分之一的60歲以上老人血樣中發現了抗體，該抗體能保護他們免受新型病毒的傳染。如果進一步研究能更好地確定哪些人具有部分免疫力的話，那么這些人可能僅需要一劑流感疫苗，不需要兩劑疫苗。“我們假設每個人可能需要兩劑流感疫苗，”美國疾病防控中心國家防疫與呼吸道疾病研究中心主任安妮醫生21日說。但她補充道，最新研究顯示“可能有些人體內預先存在流感免疫力，他們服用一劑就會奏效。這無疑會引起我們的興趣。”

    世界衛生組織估計，如果要生產甲型H1N1流感疫苗，到明年秋季能生產出20億劑疫苗。5月21日，經確認美國47個州和哥倫比亞特區共有甲型H1N1流感病例5,764個，9人死亡。然而流行病學家相信，自從一個月前新型流感病毒引起公眾注意以來，感染病毒的人數超過了10萬。全世界41個國家有11,034個甲型H1N1流感病例，85人死亡，這些數字肯定少于實際數目。

    5月21日美國疾病預防控制中心發病與死亡周報公布了血液檢驗報告，該報告對觀察甲型H1N1流感爆發所獲得的資料做了免疫學方面的解釋，研究人員吃驚地發現，很少有老年人感染甲型H1N1流感病毒。感染甲型H1N1流感病毒的人中，近三分之二的人年齡在5歲至24歲之間，年齡超過65歲的不足1%，而這些人最容易受到典型的季節性流感爆發的影響。病情嚴重需要就醫者占40%，其年齡在19歲至49歲之間。

    在這次研究中，研究人員檢驗了自從2005年以來為研究季節性流感疫苗的作用而抽取的血樣。他們讓血液感染甲型H1N1流感病毒，看看血液中是否包含攻擊病毒的抗體。事實上，6個月到9歲孩子的血液樣品中不包含甲型H1N1流感病毒抗體。但是18歲至40歲中的6%、18歲至64歲中的9%和60歲以上33% 的人具有抗體。同樣是這些人，研究人員在其注射了季節性流感疫苗后抽取了他們的血液，并進行了檢驗，結果發現兩組年齡偏大的人的血液中所含的甲型H1N1流感病毒抗體比例增加了，具體來說，18至64歲人的甲型H1N1流感病毒抗體從9%增加到25%，60歲以上人的甲型H1N1流感病毒抗體從33%增加到43%。

    總而言之，這些發現表明，許多老年人可能在幾十年前感染過流感病毒，這些病毒與新型流感病毒有些相似之處，它們激活了體內的一些免疫反應。季節性流感疫苗的注射似乎刺激了這種“免疫反應”。注射新型流感病毒疫苗有望使這種反應增大并且變得尖銳，注射一劑就能奏效，但是，如果免疫系統從來沒有遭遇過流感病毒的破壞，更不用說是新型流感病毒了，那么這些人肯定需要注射兩劑疫苗才能幸免于難。美國疾病預防控制中心國家免疫與呼吸道疾病中心主任安妮說，現在還沒有得出最終結論。

    研究中發現的可交叉反應的抗體是否具有抵抗病毒的能力？接受血液檢驗的79個孩子和280個成年人的情況是否代表大多數人？對這些問題的研究正在進行中。