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.

    据《华盛顿邮报》5月22日报道，研究发现美国一些老年人可能对甲型H1N1流感病毒有些免疫力，如果新型流感疫苗研制获得成功，或许能证明这一发现非常有用。四个月后公共健康官员要解决的最重要的问题是：确定流感疫苗的对象、在疫苗数量有限的情况下如何扩大其供给。

    5月21日美国疾病预防控制中心的科学家宣布，在美国三分之一的60岁以上老人血样中发现了抗体，该抗体能保护他们免受新型病毒的传染。如果进一步研究能更好地确定哪些人具有部分免疫力的话，那么这些人可能仅需要一剂流感疫苗，不需要两剂疫苗。“我们假设每个人可能需要两剂流感疫苗，”美国疾病防控中心国家防疫与呼吸道疾病研究中心主任安妮医生21日说。但她补充道，最新研究显示“可能有些人体内预先存在流感免疫力，他们服用一剂就会奏效。这无疑会引起我们的兴趣。”

    世界卫生组织估计，如果要生产甲型H1N1流感疫苗，到明年秋季能生产出20亿剂疫苗。5月21日，经确认美国47个州和哥伦比亚特区共有甲型H1N1流感病例5,764个，9人死亡。然而流行病学家相信，自从一个月前新型流感病毒引起公众注意以来，感染病毒的人数超过了10万。全世界41个国家有11,034个甲型H1N1流感病例，85人死亡，这些数字肯定少于实际数目。

    5月21日美国疾病预防控制中心发病与死亡周报公布了血液检验报告，该报告对观察甲型H1N1流感爆发所获得的资料做了免疫学方面的解释，研究人员吃惊地发现，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，近三分之二的人年龄在5岁至24岁之间，年龄超过65岁的不足1%，而这些人最容易受到典型的季节性流感爆发的影响。病情严重需要就医者占40%，其年龄在19岁至49岁之间。

    在这次研究中，研究人员检验了自从2005年以来为研究季节性流感疫苗的作用而抽取的血样。他们让血液感染甲型H1N1流感病毒，看看血液中是否包含攻击病毒的抗体。事实上，6个月到9岁孩子的血液样品中不包含甲型H1N1流感病毒抗体。但是18岁至40岁中的6%、18岁至64岁中的9%和60岁以上33% 的人具有抗体。同样是这些人，研究人员在其注射了季节性流感疫苗后抽取了他们的血液，并进行了检验，结果发现两组年龄偏大的人的血液中所含的甲型H1N1流感病毒抗体比例增加了，具体来说，18至64岁人的甲型H1N1流感病毒抗体从9%增加到25%，60岁以上人的甲型H1N1流感病毒抗体从33%增加到43%。

    总而言之，这些发现表明，许多老年人可能在几十年前感染过流感病毒，这些病毒与新型流感病毒有些相似之处，它们激活了体内的一些免疫反应。季节性流感疫苗的注射似乎刺激了这种“免疫反应”。注射新型流感病毒疫苗有望使这种反应增大并且变得尖锐，注射一剂就能奏效，但是，如果免疫系统从来没有遭遇过流感病毒的破坏，更不用说是新型流感病毒了，那么这些人肯定需要注射两剂疫苗才能幸免于难。美国疾病预防控制中心国家免疫与呼吸道疾病中心主任安妮说，现在还没有得出最终结论。

    研究中发现的可交叉反应的抗体是否具有抵抗病毒的能力？接受血液检验的79个孩子和280个成年人的情况是否代表大多数人？对这些问题的研究正在进行中。