你有沒有想過，為什么疫苗的接種部位通常是胳膊而不是屁股或大腿？盡管擼起袖子確實要方便得多，但是疫苗接種部位的選擇沒有你想的那么簡單。

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?

數以億計的人已經接種了新冠疫苗，為什么打疫苗都是擼起袖子而不是卷起褲腿？為什么多數疫苗都是打在胳膊上？這背后有什么科學原理呢？

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，多數人（但不是所有人）接種疫苗都是打在胳膊上，號稱“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，比如輪狀病毒疫苗。其他一些疫苗是皮下注射的，比如麻疹、腮腺炎和風疹疫苗。但是，許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，為什么肌肉注射占比這么大，注射的肌肉部位是否有講究？

肌肉含有免疫細胞

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.

肌肉是絕佳的疫苗注射部位，因為肌肉組織含有重要的免疫細胞。這些免疫細胞能識別疫苗中引發(fā)免疫反應的抗原。

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.

以新冠疫苗為例，肌肉組織中的免疫細胞會識別這些抗原，然后讓抗原進入淋巴結。

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.

將疫苗注射進肌肉組織可以讓疫苗先停留在局部，肌肉部位的免疫細胞會向其他免疫細胞發(fā)出警報，通知它們開始工作。

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.

一旦疫苗被肌肉內的免疫細胞識別，這些細胞會攜帶抗原進入淋巴管，然后淋巴管會將這些帶有抗原的免疫細胞輸送給淋巴結。

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.

作為人體免疫系統(tǒng)的關鍵組成部分，淋巴結所含的免疫細胞更多，這些免疫細胞能夠識別疫苗中的抗原，從而啟動制造抗體的免疫程序。

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.

成簇狀分布的淋巴結就在疫苗注射部位附近。許多疫苗都打在三角肌處，因為三角肌和腋下的淋巴結距離很近。如果疫苗是打在大腿上，距離腹股溝淋巴結聚集處也不遠。

肌肉注射可以讓疫苗反應停留在局部

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，肌肉注射往往會讓疫苗反應停留在局部。將疫苗注射進三角肌可能會引發(fā)注射處的局部紅腫或酸痛。

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組織上，那么出現刺激和紅腫反應的幾率會增大，因為脂肪組織的血液供應不足，從而導致對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.

含有佐劑的疫苗必須打在肌肉上，以避免大范圍的刺激和紅腫反應。佐劑的使用可以提高對抗原的免疫反應。

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

決定疫苗注射部位的另一個因素是肌肉的大小。

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.

成年人和三歲以上兒童接種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種疫苗，因為他們的胳膊肌肉組織較小、尚未發(fā)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個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種診室內脫下褲子的畫面嗎？擼起袖子要方便得多，也更易被人們接受。

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.

綜上所述，大多數成年人和孩子在接種新冠疫苗和流感疫苗時，胳膊是最佳的疫苗注射部位。

英文來源：The Conversation

翻譯&編輯：丹妮