第四劑疫苗開打！打三劑保護力剩多少？看懂各廠牌突破性感染比例

打完三劑疫苗，還有可能確診嗎？答案是有可能，且機率可能比想像中高。指揮中心20日宣布65歲以上民眾、免疫不全或低下者等特定族群，可以施打第四劑疫苗，補足消退的保護力。

所謂突破性感染（breakthrough infection），指的是明明已接種疫苗，卻仍確診。突破性感染廣泛地來說，不管施打幾劑、隔多久接種下一劑疫苗，只要接種後感染，都屬突破性感染。因此，在討論突破性感染時，通常還需要考慮到其他細部因素：施打劑量、劑次、各劑之間施打的間隔、施打完畢多久等。

突破性感染哪個廠牌多？國內疫苗數據一次看

政府力拚第三劑、第四劑疫苗覆蓋率，民眾此時最好奇，哪劑疫苗的保護力最好？該選哪個廠牌？4月9日，指揮中心公開國內的突破性感染數據，不過需要注意，官方並未細分接種的疫苗組合，僅以「最後1劑疫苗」的廠牌相互比較，僅供參考。

國內第三劑疫苗的接種人數，各廠牌以莫德納疫苗（769萬人）最多，其次為BNT疫苗（307萬人），接續則是高端（49萬人）。而據指揮中心統計，發生「突破性感染」的比例如下（依最後一劑計）：

BNT疫苗：每10萬人中7.5人

莫德納疫苗：每10萬人中6.5人

高端疫苗：每10萬人中2.6人

AZ疫苗：每10萬人中10.2人（第三劑接種人數不到3萬，統計基數過小，參考價值低）

三劑疫苗保護力多少？國外實打數據曝光

為什麼會發生突破性感染？疫苗效力會隨時間下滑，已廣泛獲研究證實：接種第三劑疫苗後，保護力約從第10週開始大降，即便接種第二針追加劑，防感染效果仍有限。美國疾病管制局（CDC）就發現，不論施打莫德納，第三劑注射後2～4個月，感染、住院保護力都下降。

保護力消退主因，除了抗體水平本身衝不上去，還包括Omicron病毒不斷出現新變種，包括主導高雄港傳播鏈的亞型BA.2，就具備超越原株33%的傳播力，且具免疫逃脫能力。

澳洲生醫科學與藥學專家巴特列（Nathan Bartlett）援引數據指出，平均而言，接種莫德納疫苗作為追加劑（第三針）2～4週後，有症狀感染保護力約回升到60～70%；15週後降回30～40%。在日本，Omicron確診者就有7成是打過疫苗的人。即使接種第三劑，還是會感染。好消息是住院、重症保護力仍在，疫苗從90%降到75%，莫德納疫苗能衝到90～95%左右。

誰會發生突破性感染？慢性病不是高風險群

美國醫學網站Web Med分析，容易造成突破性感染的要素包括：

先天免疫不全、免疫低下，或因接受後天治療（如：化療）等，使免疫力下降

感染到較新的變種病毒，原本設計好的疫苗效果打折。

疫苗施打人員失誤、不當操作接種流程，影響接種效果。

許多人好奇，有慢性病的患者是不是比較容易突破性感染？值得注意的是，慢性病並不在高風險因子之列。疾管署副署長羅一鈞日前也指出，根據國內目前觀察，突破性感染跟慢性病並無相關性，「比較跟疫苗接種兩劑或三劑，或已經接種的時間長短有關係。」

還需釐清的是，免疫力跟個人的體質有關，每個人在不同狀態下接種疫苗、誘發免疫反應的效果也不一樣。即使是同一個人，也會因某段時間較操勞、或營養不均等，讓免疫力下降。最好的方式，是平時在飲食、作息上養好自己的免疫系統，提高疫苗效果、也幫助身體迎戰感染。

我突破性感染，還會傳染給別人嗎？

即便施打疫苗，確診後仍有傳染給他人的風險。Delta、Omicron等新冠變異株傳染力相當高，與初期病毒株不可同日而語。根據CDC初期研究等資料，即使是完整接種兩劑，甚至追加到三劑、四劑的人，發生突破性感染（確診）的大有人在，而且病毒量跟沒打疫苗的人比，並不會比較少，也就是說傳染力不見得比較低。

What’s the difference in protection against Omicron between 2 doses and 3 doses of vaccine?

Australian Health Minister Greg Hunt said last week he expects the Australian Technical Advisory Group on Immunisation (ATAGI) to change the definition of “fully vaccinated” to three doses instead of two.

It comes as evidence emerges suggesting the highly infectious Omicron variant has the ability to escape the protection two vaccine doses offer.

So, how effective are two doses compared to three against Omicron?

Let’s break it down.

Two doses don’t protect much against Omicron

Vaccine protection against Omicron is reduced for two reasons.

First, antibodies generated by vaccination gradually wane over time. There are now many countries that are more than a year into their COVID vaccine rollout, so many people have received their second COVID jab over six months ago.

Without boosting, their antibody levels will have dropped significantly. Australia was a little slower off the mark – but now finds itself in a similar situation.

The second reason is Omicron can escape vaccine-induced immunity because of its constellation of mutations. Its spike protein (the bit that helps the virus access our cells) is significantly different to Delta’s, and to the original virus from which our vaccines are based.

The critical part of the spike protein is the “receptor binding domain”. It latches onto a protein on our cells called ACE-2 so the virus can gain entry. Delta had two mutations in the receptor binding domain, and Beta had three. Omicron has 15 mutations in its receptor binding domain. As a result, only some of the antibodies the vaccine induces will still bind to Omicron’s spike and inhibit it getting into your cells.

For these reasons, emerging evidence suggests two doses of a COVID vaccine provide just 0-10% protection against infection with Omicron five to six months after the second jab.

So, you really cannot claim you are “fully vaccinated” with just two doses now, particularly if it’s been months since your second dose.

Some protection against severe disease and hospitalisation remains. UK data suggests two doses of AstraZeneca or Pfizer offer around 35% protection against hospitalisation by six months after the second dose.

What about three doses?

Having a booster dose bumps up your antibodies – which is particularly important for Omicron because only some of those antibodies are protective. Emerging evidence suggests protection from symptomatic Omicron infection is restored to 60-75% two to four weeks after a Pfizer or Moderna booster dose.

However, third-dose protection also wanes, down to 30-40% against Omicron infection after 15 weeks.

So, unfortunately breakthrough infections will still be common. Fortunately, protection against hospitalisation remains much higher, up around 90% after a Pfizer booster dose and only dropping to 75% after 10-14 weeks, and 90-95% up to nine weeks after a Moderna booster.

Pfizer and Moderna are currently developing vaccines matched to Omicron, which if approved, should induce better immunity against this variant.

How much more protection is gained from having a third COVID vaccination dose?

Protection against COVID-19 severity; second vs third dose of Pfizer, Moderna or AZ vaccines against COVID-19.

Note: Insufficient data currently available for 6+ month interval of 3 doses.