コロナウィルスの謎
その答えは精査しなければならないが、
答えはこれではないか。
コロナウィルスの遺伝子を分類した
英独研究チーム
世界で検出された新型コロナウイルスを遺伝子型で分類すると、中国など東アジアに多い型や、欧米で多く見つかっている型など3タイプに分けられることがわかった。ウイルスはヒトなどの細胞内で増殖しながら絶えず変異しており、タイプを追跡することで、感染経路や流行の分析、ワクチンの開発につながる可能性がある。
独英の研究チームが昨年12月から先月までに検出された約160人分のウイルスの遺伝子を分析した。
チームはまず、コウモリから見つかったウイルスに最も似た群をAタイプとした。中国南部の広東省の住民だけでなく、日本人や武漢市滞在歴のある米国人から見つかるなど、米国や豪州など東アジア以外でも多く見つかっているという。
一方、Aタイプから分かれたBタイプは、武漢市を含む中国やその周辺国で多く見つかった。チームは、東アジアに多いタイプと分類。東アジアの人に広がりやすいタイプだった可能性があるという。
さらに、Bタイプに由来するCタイプはフランスやイタリア、スウェーデン、米国、ブラジルなど欧米が中心だった。
シンガポールや香港、台湾、韓国でも見つかっているが、中国本土では見つかっていないという。論文は、米科学アカデミー紀要(https://doi.org/10.1073/pnas.2004999117)に発表された。
気になる点は1月末にイタリアで見つかったコロナウィルスはシンガポールのクラスターと関係しているという報告だ。
これが源流なら、なぜ、シンガポールなのか解明を待たなければならない。欧米からの感染者はこれまで以上の監視体制が必要になる。サイレントが一人でも入れば、状況は一変する。
この理論は4月29日発表の米国国立研究機関の査読前の論文で裏打ちされた。米国のものは最下段に掲載している。この推論は発表時、世界初だったが、今では有力になっている。
原文
Researchers from Cambridge, UK, and Germany have reconstructed the early "evolutionary paths" of COVID-19 in humans - as infection spread from Wuhan out to Europe and North America - using genetic network techniques.
By analysing the first 160 complete virus genomes to be sequenced from human patients, the scientists have mapped some of the original spread of the new coronavirus through its mutations, which creates different viral lineages.
"There are too many rapid mutations to neatly trace a COVID-19 family tree. We used a mathematical network algorithm to visualise all the plausible trees simultaneously," said geneticist Dr Peter Forster, lead author from the University of Cambridge.
"These techniques are mostly known for mapping the movements of prehistoric human populations through DNA. We think this is the first time they have been used to trace the infection routes of a coronavirus like COVID-19."
The team used data from virus genomes sampled from across the world between 24 December 2019 and 4 March 2020. The research revealed three distinct "variants" of COVID-19, consisting of clusters of closely related lineages, which they label 'A', 'B' and 'C'.
Forster and colleagues found that the closest type of COVID-19 to the one discovered in bats - type 'A', the "original human virus genome" - was present in Wuhan, but surprisingly was not the city's predominant virus type.
Mutated versions of 'A' were seen in Americans reported to have lived in Wuhan, and a large number of A-type viruses were found in patients from the US and Australia.
Wuhan's major virus type, 'B', was prevalent in patients from across East Asia. However, the variant didn't travel much beyond the region without further mutations - implying a "founder event" in Wuhan, or "resistance" against this type of COVID-19 outside East Asia, say researchers.
The 'C' variant is the major European type, found in early patients from France, Italy, Sweden and England. It is absent from the study's Chinese mainland sample, but seen in Singapore, Hong Kong and South Korea.
The new analysis also suggests that one of the earliest introductions of the virus into Italy came via the first documented German infection on January 27, and that another early Italian infection route was related to a "Singapore cluster".
Importantly, the researchers say that their genetic networking techniques accurately traced established infection routes: the mutations and viral lineages joined the dots between known cases.
As such, the scientists argue that these "phylogenetic" methods could be applied to the very latest coronavirus genome sequencing to help predict future global hot spots of disease transmission and surge.
"Phylogenetic network analysis has the potential to help identify undocumented COVID-19 infection sources, which can then be quarantined to contain further spread of the disease worldwide," said Forster, a fellow of the McDonald Institute of Archaeological Research at Cambridge, as well as the University's Institute of Continuing Education.
The findings are published today in the journal Proceedings of the National Academy of Sciences (PNAS). The software used in the study, as well as classifications for over 1,000 coronavirus genomes and counting, is available free at http://www.
Variant 'A', most closely related to the virus found in both bats and pangolins, is described as "the root of the outbreak" by researchers. Type 'B' is derived from 'A', separated by two mutations, then 'C' is in turn a "daughter" of 'B'.
Researchers say the localisation of the 'B' variant to East Asia could result from a "founder effect": a genetic bottleneck that occurs when, in the case of a virus, a new type is established from a small, isolated group of infections.
Forster argues that there is another explanation worth considering. "The Wuhan B-type virus could be immunologically or environmentally adapted to a large section of the East Asian population. It may need to mutate to overcome resistance outside East Asia. We seem to see a slower mutation rate in East Asia than elsewhere, in this initial phase."
He added: "The viral network we have detailed is a snapshot of the early stages of an epidemic, before the evolutionary paths of COVID-19 become obscured by vast numbers of mutations. It's like catching an incipient supernova in the act."
Since today's PNAS study was conducted, the research team has extended its analysis to 1,001 viral genomes. While yet to be peer-reviewed, Forster says the latest work suggests that the first infection and spread among humans of COVID-19 occurred between mid-September and early December.
The phylogenetic network methods used by researchers - allowing the visualisation of hundreds of evolutionary trees simultaneously in one simple graph - were pioneered in New Zealand in 1979, then developed by German mathematicians in the 1990s.
These techniques came to the attention of archaeologist Professor Colin Renfrew, a co-author of the new PNAS study, in 1998. Renfrew went on to establish one of the first archaeogenetics research groups in the world at the University of Cambridge.
2020年4月21日追記
北京大学の研究チームが発表した研究報告があがってきた。それによると、中国国内で発見された新型コロナウィルスをL型とS型と呼んでいる。
これは中国国内にはサンプル採取時点でAとB型しか存在しないため、感染力に大きな差を認めていた。
新型コロナ、2タイプか=感染力違う可能性―北京大
新型コロナ欧州株 感染力が強い変異を獲得 米研究所
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