Hundreds of millions have rolled up their sleeves for the COVID-19 vaccine, but why haven't they rolled up their pants legs instead? Why do we get most shots in our arms? What's the science behind why we get most vaccines in our arm?

数以亿计的人已经接种了新冠疫苗，为什么打疫苗都是撸起袖子而不是卷起裤腿？为什么多数疫苗都是打在胳膊上？这背后有什么科学原理呢？

It's worth noting that most, but not all, vaccines are given in the muscle – this is known as an intramuscular injection.

值得注意的是，多数人（但不是所有人）接种疫苗都是打在胳膊上，号称“肌肉注射”。

Some vaccines, like the rotavirus vaccine, are given orally. Others are given just beneath the skin, or subcutaneously – think of the measles, mumps, and rubella vaccine. However, many others are given in the muscle.

有些疫苗是口服的，比如轮状病毒疫苗。其他一些疫苗是皮下注射的，比如麻疹、腮腺炎和风疹疫苗。但是，许多其他疫苗都是肌肉注射的。

But why is the muscle so important, and does location matter?

那么，为什么肌肉注射占比这么大，注射的肌肉部位是否有讲究？

肌肉含有免疫细胞

Muscles make an excellent vaccine administration site because muscle tissue contains important immune cells. These immune cells recognize the antigen, a tiny piece of a virus or bacteria introduced by the vaccine that stimulates an immune response.

肌肉是绝佳的疫苗注射部位，因为肌肉组织含有重要的免疫细胞。这些免疫细胞能识别疫苗中引发免疫反应的抗原。

In the case of the COVID-19 vaccine, the immune cells in the muscle tissue pick up these antigens and present them to the lymph nodes.

以新冠疫苗为例，肌肉组织中的免疫细胞会识别这些抗原，然后让抗原进入淋巴结。

Injecting the vaccine into muscle tissue keeps the vaccine localized, allowing immune cells to sound the alarm to other immune cells and get to work.

将疫苗注射进肌肉组织可以让疫苗先停留在局部，肌肉部位的免疫细胞会向其他免疫细胞发出警报，通知它们开始工作。

Once a vaccine is recognized by the immune cells in the muscle, these cells carry the antigen to lymph vessels, which transport the antigen-carrying immune cells into the lymph nodes.

一旦疫苗被肌肉内的免疫细胞识别，这些细胞会携带抗原进入淋巴管，然后淋巴管会将这些带有抗原的免疫细胞输送给淋巴结。

Lymph nodes, key components of our immune system, contain more immune cells that recognize the antigens in vaccines and start the immune process of creating antibodies.

作为人体免疫系统的关键组成部分，淋巴结所含的免疫细胞更多，这些免疫细胞能够识别疫苗中的抗原，从而启动制造抗体的免疫程序。

Clusters of lymph nodes are located in areas close to vaccine administration sites. For instance, many vaccines are injected in the deltoid because it is close to lymph nodes located just under the armpit. When vaccines are given in the thigh, the lymph vessels don't have far to travel to reach the cluster of lymph nodes in the groin.

成簇状分布的淋巴结就在疫苗注射部位附近。许多疫苗都打在三角肌处，因为三角肌和腋下的淋巴结距离很近。如果疫苗是打在大腿上，距离腹股沟淋巴结聚集处也不远。

肌肉注射可以让疫苗反应停留在局部

Muscle tissue also tends to keep vaccine reactions localized. Injecting a vaccine into the deltoid muscle may result in local inflammation or soreness at the injection site.

此外，肌肉注射往往会让疫苗反应停留在局部。将疫苗注射进三角肌可能会引发注射处的局部红肿或酸痛。

If certain vaccines are injected into fat tissue, the chance of irritation and inflammation reaction increases because fat tissue has poor blood supply, leading to poor absorption of some vaccine components.

如果疫苗被打在脂肪组织上，那么出现刺激和红肿反应的几率会增大，因为脂肪组织的血液供应不足，从而导致对某些疫苗成分的吸收不良。

Vaccines that include the use of adjuvants – or components that enhance the immune response to the antigen – must be given in a muscle to avoid widespread irritation and inflammation.

含有佐剂的疫苗必须打在肌肉上，以避免大范围的刺激和红肿反应。佐剂的使用可以提高对抗原的免疫反应。

Yet another deciding factor in vaccine administration location is the size of the muscle.

决定疫苗注射部位的另一个因素是肌肉的大小。

Adults and children ages three and older tend to receive vaccines in their upper arm in the deltoid. Younger children receive their vaccines mid-thigh because their arm muscles are smaller and less developed.

成年人和三岁以上儿童接种疫苗的部位通常是胳膊上方的三角肌。更年幼的小孩是在大腿中部接种疫苗，因为他们的胳膊肌肉组织较小、尚未发育完全。

Another consideration during vaccine administration is convenience and patient acceptability. Can you imagine taking down your pants at a mass vaccination clinic? Rolling up your sleeve is way easier and more preferred.

疫苗注射部位的另一个考量因素是便利度和患者的接受度。你能想象在人群聚集的疫苗接种诊室内脱下裤子的画面吗？撸起袖子要方便得多，也更易被人们接受。

All things considered, when it comes to the flu shot and the COVID-19 vaccine, for most adults and kids, the arm is the preferred vaccination route.

综上所述，大多数成年人和孩子在接种新冠疫苗和流感疫苗时，胳膊是最佳的疫苗注射部位。