誰在傳播猴痘重要嗎？

資料來源：www.thelancet.com/infection Vol 22 September 2022 / 財團法人台灣紅絲帶基金會編譯

Flickr – NIAID 於 2022 年 6 月 28 日線上發布 https://doi.org/10.1016/S1473-3099(22)00431-5

–

    最近，全球報告了許多猴痘病例。儘管這些病例中的大多數似乎與男男性行為者 (MSM) 社區有關，但並非所有病例都如此。西非和中非猴痘病毒進化枝的病例在過去 20 年中一直在上升。由於西非分支的傳播性和毒性較低，目前的傳播是出乎意料的，因為對早期主要是在非洲村莊爆發的觀察，顯示猴痘爆發是自限性的。從技術上講，基本再生數（R0）小於1。R0是在沒有免疫力和沒有介入措施的社區中，在整個傳染期內，單個病例產生的繼發病例的平均數。顯然，在當前出現的病例中，低傳播率肯定發生了變化。這種變化是由於病毒的突變還是由於不同類型和頻率的接觸所引起的，這很有趣，但在考慮如何阻止疫情爆發時並不一定相關。主要問題只是如何將每個感染者的平均繼發病例數減少到 1 以下。在這種情況下，感染主要是在 MSM 社區內傳播還是進入其他人群也無關緊要。我們只能推測當前猴痘的R0 值。

    猴痘過去的傳染性遠低於天花，其 R0 介於 3.5 和 6.4 之間。過去並未對西非猴痘進化枝的 R0 進行估計，但剛果盆地中傳染性更強的進化枝的有效再生數（Rt值，由易感和非易感宿主組成的人群中每個感染病例的平均繼發病例數）估計約為 0.3，而其 R0 則估計在 1.46 和 2.67 之間。我們迫切需要新的估計，特別是因為這些估計可以追溯到天花疫苗接種覆蓋率很高的時期，這可能導致對 R0 的低估。對猴痘使用 R0=3 可能仍然被認為是一個非常大（非常悲觀）的值。與天花一樣，被感染的個體出現症狀的時間相當長，並且與流感或 SARS-CoV-2 相比，他們的感染期相當長。因此，猴痘的平均生成時間相當長（約 20 天）這使得猴痘極易受到干預：即使 R0=3，也需要幾個月的時間才能出現幾千例病例。隨著意識的提高，現在發現病例的速度將比暴發流行之初快得多。即使從出現症狀到發現和隔離病例需要 1 週的時間，傳染期（在出疹後 15-27 天）也減少了 50% 以上。如果感染主要不是在前驅期或早期感染期間中傳播，則假定的再生數 3 將下降到小於 1·5。大多數公共衛生系統的標準做法是對所有確診病例進行接觸者追踪。由於猴痘的潛伏期較長（7-17天），已知接觸者中病例可在傳播感染前進行接觸。他們可以被隔離，或者至少，一旦他們出現第一個症狀，他們就會意識到感染。重要的是，與 SARS-CoV-2 不同，似乎沒有證據顯示人類會感染無症狀的正痘病毒，或者至少在天花上這被認為是正確的；針對猴痘的研究仍在進行中。由於口咽部病變是空氣傳播所必需的，而皮疹是透過身體接觸傳播的，因此在感染傳播之前至少必須存在一些症狀是合理的。因此，對接觸者的認識或隔離應會進一步顯著地減少感染的傳播，將使得剩餘的再生數接近或低於 1。1980 年代的數據顯示，天花疫苗接種對猴痘提供了 85% 的保護。在 1970 年代，天花疫苗接種計畫開始停止接種，到 1984 年，所有國家都停止為公眾接種天花疫苗。 研究顯示，接種過天花疫苗的人群中正痘病毒抗體的血清陽性率很高。這進一步降低了猴痘的有效再生數。使用新推出的猴痘疫苗，已知接觸者於接觸後的疫苗接種——或者更廣泛地病例的社會環境（這種被稱為疫苗環狀接種方式）——如果在感染後立即應用，甚至可以保護已經感染的人。此外，提高疾病認識可能導致更早發現新病例，也可以考慮對風險群體和免疫功能低下的個人進行預防性疫苗接種。

*Kristan A Schneider, Martin Eichner；我們聲明沒有競爭利益。

kristan.schneider@hs-mittweida.應用計算機科學和生物科學系，德國米特魏達Hochschule大學，米特魏達 09648, 德國(KAS)； 

德國杜賓根大學臨床流行病學和應用生物統計學研究所（ME）； Epimos, Bischofsheim in der Rhön, 德國 (ME)

Does it matter who is spreading monkeypox?

www.thelancet.com/infection Vol 22 September 2022

Flickr – NIAID Published Online June 28, 2022 https://doi.org/10.1016/ S1473-3099(22)00431-5 

    Recently, many cases of monkeypox were reported worldwide. Although most of these cases seem to be associated with the community of men who have sex with men (MSM), not all of them are. Cases with west and central African monkeypox virus clades have been rising in the past 20 years. The current spread, due to the less transmissible and less virulent west African clade, was unexpected because observations on earlier outbreaks, mostly in African villages, indicated that monkeypox outbreaks are self-limiting. In technical terms, the basic reproduction number (R0) was less than 1. R0 is the average number of secondary cases produced by a single case during the whole infectious period in a community without immunity and without interventions. The low transmissibility, obviously, must have changed during this current emergence of cases. Whether this change was due to mutations in the virus or due to a different type and frequency of contacts is interesting, but not necessarily relevant when contemplating how to stop the outbreak. The main question simply is how to reduce the average number of secondary cases per infected person to below 1. In this context, it also does not matter whether the infection mainly spreads within an MSM community or finds its way into other groups of the population. We can only speculate about the current value of R0 for monkeypox. 

    Monkeypox used to be far less transmissible than smallpox, the R0 of which lay between 3·5 and 6.4 The R0 has not been estimated for the west African monkeypox clade, but the effective reproduction number (the average number of secondary cases per infectious case in a population made up of both susceptible and nonsusceptible hosts) of a more transmissible clade in the Congo Basin was estimated to be about 0·3, while its R0 was estimated to be between 1·46 and 2·67. New estimates are urgently needed, particularly because these estimates date back to a time when smallpox vaccination coverage was high, which might have led to an underestimation of the R0. Using R0=3 for monkeypox might still be considered a very large (highly pessimistic) value. As with smallpox, individuals who are infected take rather long to develop symptoms and they have— compared with influenza or SARS-CoV-2—a rather long infectious period. Therefore, the average generation time of monkeypox is rather long (about 20 days). This renders monkeypox highly vulnerable to interventions: even with R0=3, it takes months until a few thousand cases occur. With the raised awareness, cases will be detected much quicker now than in the beginning of the outbreak. Even if it takes 1 week from onset of symptoms to detect and isolate cases, the contagious period (15–27 days after the onset of rash) is reduced by over 50%. If the infection is not predominantly passed on during the prodrome or early enanthem periods, the assumed reproduction number of 3 drops to less than 1·5. The standard practice of most public health systems is to initiate contact tracing for all confirmed cases. As the latent period of monkeypox is long (7–17 days), cases among known contacts can be contacted before they spread the infection. They can either be quarantined or, at least, they will be aware of the infection as soon as they develop first symptoms. Importantly, unlike for SARS-CoV-2, there seems to be no evidence of asymptomatic Orthopoxvirus infections in humans, or at least this is believed to be true for smallpox; studies for monkeypox are still ongoing. Because oropharyngeal lesions are necessary for airborne transmission and skin rash is required for transmission by physical contact, it is plausible that at least some symptoms must be present before the infection can be passed on. Therefore, awareness or quarantine of contacts should further reduce the spread of infection considerably, bringing the remaining reproduction number close to or below 1. Data from the 1980s suggest that smallpox vaccination provided 85% protection against monkeypox. In the 1970s, smallpox vaccination programmes were starting to be discontinued and, by 1984, all countries had ceased vaccinating the general public against smallpox. Studies suggest high seropositivity of Orthopoxvirus antibodies in the smallpox-vaccinated population. This further reduces the effective reproduction number of monkeypox. Using the newly available monkeypox vaccines, post-exposure vaccination of known contacts—or more generally the social environment of cases (this was termed ring vaccination )—can even protect people who have already been infected if applied soon after infection. Additionally, increased awareness might lead to earlier detection of new cases, and the prophylactic vaccination of risk groups and of individuals who are immunocompromised can also be considered.

We declare no competing interests. 

*Kristan A Schneider, Martin Eichner

kristan. schneider@hs-mittweida. de Department of Applied Computer Sciences and Biosciences, Hochschule Mittweida, Mittweida 09648, Germany (KAS);

Institute for Clinical Epidemiology and Applied Biometrics, University of Tübingen, Tübingen, Germany (ME); Epimos, Bischofsheim in der Rhön, Germany (ME)