Younger children will need two doses of the new swine flu vaccine, but clinical trials suggest the drug is working well, health officials said Monday.

"Immunologically this is acting like seasonal flu and we are very pleased with that," said Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases. "The response in younger children is less robust, but that is not unexpected."

A single 15 microgram dose of swine flu vaccine is enough for kids 10 and older, while children under 10 will need two doses, 21 days apart.

Younger kids need the double dose because their immune systems are not fully mature, Fauci said.

Officials called the trial results "encouraging" given that three quarters of healthy kids over 10 developed an immunity to the virus.

"Overall, this is very good news for the vaccination program," Fauci said. "76 percent is not bad at all - it's quite good I think."

One dose was about half as effective in kids nine and younger, Fauci said, but the second dose will boost their immunity substantially.

Kids in the clinical trial reported only minor side effects that are typical of the seasonal flu vaccine, such as soreness in the arm after a shot, Fauci said

Some 47 children have died of swine flu since it first emerged in the spring. Unlike seasonal flu, which tends to be worst among the elderly, swine flu hits kids hardest.

The officials recommend children and adults get the seasonal flu vaccine ahead of the swine flu vaccine to prevent logistical problems with separating the doses.

The first of swine flu vaccine will be nasal sprays, not shots, meaning many at-risk groups will have to wait for the injectable vaccine to be released in late October.

The city Department of Health, which will offer free vaccines to all school aged children this fall, says the spray and shot are equally effective among healthy people ages 2 to 49.

健康官员们在周一说，年幼的孩子们将需要两个剂量新的猪流感疫苗，而临床试验显示这种药物正在很好地起作用。

"从免疫学上看，这样的行为象季节性流感，并且我们对此很满意，"美国国家过敏症和传染病协会的主任安东尼 福奇博士说。"在年幼的孩子中的反应不太稳健，但是那并不意外。"

对于 10 岁和 10 岁以上的孩子们，单个 15 微克剂量的猪流感疫苗就足够了，而 10 岁以下的孩子们将需要二个剂量，间隔 21 天。

年幼的孩子们需要双倍剂量，因为他们的免疫系统没有完全成熟，福奇说。

官员们称试验的结果"令人鼓舞",已知四分之三 10 岁以上的健康孩子对这种病毒发生了免疫力。

"总的来说，这对疫苗接种计划来说是非常好的消息，"福奇说。"76% 一点也不差--我觉得它很不错。"

一个剂量大约是九岁和九岁以下孩子们的有效剂量的一半，福奇说，但是第二剂量会显着地增强他们的免疫力。

临床试验的孩子们只报告有轻微的副作用，是季节性流感疫苗的典型特征，例如注射后的手臂酸痛，福奇说

自从猪流感在春天首次出现以来，大约 47 名儿童死于猪流感。不像季节性流感，往往是稍年长者情况最差，猪流感打击孩子最严重。

官员们推荐的儿童和成人在猪流感疫苗之前接种季节性流感疫苗，采取分开的剂量防止后勤供应问题。

第一剂猪流感疫苗将是鼻腔喷雾剂而不注射，意味着许多处于风险的群组将不得不等待 10 月下旬发放的注射疫苗。

城市卫生部门--他们今年秋天将对所有学校年龄的孩子们提供免费疫苗--说，在年龄 2 至 49 岁的健康人中，喷雾和注射是同样有效的。