より感染力の強いオミクロンの変異株BA.2の出現には要警戒

Sudden rise of more transmissible form of Omicron catches scientists by surprise

“Sister” of original may extend Omicron surges, but no evidence yet it causes more severe disease

BY MEREDITH WADMAN / 31 JAN 2022 / SCIENCEINSIDER / HEALTH

わたしのnoteにおいては、最新の科学・経済・社会等の問題に関して、英語の記事を引用し、その英文が読み易いように加工し、「英語の勉強ツール」と「最新情報収集ツール」としてご利用頂くことをmain missionとさせて頂きます。勿論、私論を書かせて頂くこともしばしです。

On 7 December 2021, as the Omicron variant of the pandemic coronavirus began to pummel (拳で殴る/pʌ́ml) the world, scientists officially identified a related strain. BA.2 differed by about 40 mutations from the original Omicron lineage, BA.1, but it was causing so few cases of ­COVID-19 that it seemed a sideshow (副次的なこと、付け足し/sáidʃòu) to its rampaging (暴れ回る/ræmpéidʒ) counterpart (BA.1).

BA.1より、明らかに感染力の強いBA.2が世界中で広がり始めている。これまで接種したワクチンの効果は明確でなく、また、BA.1に感染した人はBA.2には感染しないという事も不明。ただ、今、感染爆発を起こしているBA.1の感染力より強い感染力を持っていること自体が脅威であり、BA.1で学習したことを、BA.2が感染爆発した場合には、フル稼働すべき。

“I was thinking: ‘BA.1 has the upper hand (優位、優勢). We’ll never hear again from BA.2,’” recalls Mark Zeller, a genomic epidemiologist at the Scripps Research Institute. Eight weeks later, he says, “Clearly that’s not the case. … I’m pretty sure [BA.2] is going to be everywhere in the world, that it’s going to sweep (一気に広がる) and will be the dominant variant soon in most countries if not all.”

現在、日本では、BA.1の感染爆発が発生しているが、BA.1がピークアウトしたと思った途端、BA.2による感染爆発が続く可能性があると思われる。

Zeller and other scientists are now trying to make sense of why BA.2 is exploding and what its emergence means for the Omicron surge and the pandemic overall. Already a U.K. report issued last week and a large household (家族、家庭、一家、世帯、所帯/háushòuld) study from Denmark posted this week as a preprint make it clear BA.2 is inherently (本質的に、もともと/) more transmissible than BA.1, leaving scientists to wonder which of its distinct mutations confer an advantage.

But so far, BA.2 does not appear to be making people sicker than BA.1, which itself poses less risk of severe disease than variants such as Delta and Beta. In Denmark, where by 21 January BA.2 accounted for 65% of new COVID-19 cases, “We see a continuous, steep decline in the number of intensive care unit patients and … now a decrease in the number of hospital admissions related to SARS-CoV-2,” says Tyra Grove Krause, an infectious disease epidemiologist at the country’s public health agency. In fact, the Danish government is so confident the variant won’t cause major upheaval that it lifted almost all pandemic restrictions on 1 February.

日本においても、オミクロン株感染は軽症例が圧倒的に多いので、感染症分類における2類から5類への変更が検討されている。ただ、BA.1の後、すぐにBA.2の感染爆発が発生すると、現在の医療の状況の様な事が繰り返される可能性がある。

Still, some scientists predict BA.2 will extend Omicron’s impact. “I would guess we’ll see [BA.2] create a substantially longer tail of circulation of Omicron than would have existed with just [BA.1], but that it won’t drive the scale of epidemics we’ve experienced with Omicron in January,” computational biologist Trevor Bedford of the Fred Hutchinson Cancer Research Center tweeted on 28 January. In South Africa, BA.2 already may be stalling (失速する/stɔl) the rapid decline in new infections seen after the country’s Omicron wave peaked in December 2021.

Although BA.2 represented less than 4% of all Omicron sequences in the leading global virus database as of 30 January, it has been identified in 57 countries, with the earliest documented case dating to 17 November in South Africa. It likely now dominates in India, according to Bijaya Dhakal, a molecular biologist at the Sonic Reference Laboratory in Austin, Texas, who examined sequence data uploaded from eight large Indian states. In the United Kingdom, the proportion of likely BA.2 cases doubled from 2.2% to 4.4% in the 7 days that ended on 24 January.

In the United States, the Centers for Disease Control and Prevention is not yet tracking BA.2 separately. But Bedford estimates it accounted for 7% of new U.S. cases as of 30 January, up from 0.7% on 19 January. “In each country and across time, we see that the epidemic growth rate of Omicron BA.2 is greater than Omicron BA.1,” he says.

The report last week from the UK Health Security Agency (UKHSA) backs up that assessment in England, finding BA.2 was spreading faster than BA.1 in all regions where enough data were available to make an assessment. UKHSA data also show that in late December 2021 and early January, transmission was higher among household contacts of BA.2 cases, at 13.4%, than in contacts of other Omicron cases (10.3%).

The study from Denmark, which sequences the virus from virtually every person who gets COVID-19, paints a more dramatic picture. In households where the first case was BA.1, on average 29% of other people in the household became infected. When the first case was BA.2, 39% of household members were infected.

BA.2の方が、BA.1より感染力が強い。

Omicron was already known to have mutations that help it evade antibodies, but the Danish researchers also found that BA.2 may be even better at dodging (巧妙に避ける/dɑdʒ) vaccine-induced immunity: Vaccinated and boosted people were three times as susceptible to being infected with BA.2 as with BA.1. Vaccinated but unboosted people were about 2.5 times as susceptible, and unvaccinated people 2.2 times as susceptible. Early U.K. data, however, showed vaccinated people, if boosted, had about the same level of protection against symptomatic infections with BA.1 or BA.2—63% and 70%, respectively.

In one hopeful and unexpected finding from Denmark, those who were vaccinated or vaccinated and boosted passed on BA.2 to household members less often, relative to BA.1. The same didn’t hold for unvaccinated people, who passed BA.2 to their household contacts at 2.6 times the rate they passed BA.1.

Much as scientists a few weeks ago wondered whether a previous infection with Delta or another variant would protect people from Omicron overall, some are now looking for data on whether Omicron’s first surge created a shield against BA.2. “To what extent does a BA.1 infection protect you against reinfection with BA.2?” Zeller asks. “From what I have seen in Denmark, it’s not going to be 100%.”

Scientists are also probing the variant’s ability to dodge vaccine-induced antibodies in lab dish studies. And drugmaker GlaxoSmithKline is testing its monoclonal antibody, sotrovimab, made with Vir Biotechnology, against BA.2 in lab studies. It’s the only widely authorized antibody that still thwarts (阻害する/θwɔ́rt) BA.1.

Scientists note BA.1 and BA.2 are about as far apart on the evolutionary tree as earlier variants of concern—Alpha, Beta, and Gamma—are from each other. Some even think BA.2 shouldn’t even be considered Omicron. “I hope in the near future that BA.2 gets its own variant of concern because people assume it’s very similar which it’s not,” Zeller says.

Not so similar as this depiction of the SARS-CoV-2 evolutionary tree suggests, the BA.1 and BA.2 strains of the Omicron variant are about as genetically distinct as earlier variants Alpha, Beta, and Gamma are from each other.

遺伝子配列からevolutionary treeが作られるが、BA.1株とBA.2株は、Alpha株, Beta株, Gamma株の3つの株の間で塩基配列が異なるのと同程度に配列が異なっており、BA.1株とBA.2株は類似の変異株と考えるのは誤りである。

BA.2 doesn’t have all of the mutations that help BA.1 avoid immune detection, but it has some its sibling (兄弟姉妹/síbliŋ) doesn’t (some its sibling has all of the mutations mentioned above). Thomas Peacock, a virologist at Imperial College London, notes that most of the differences are in an area of the spike protein, called the N-terminal domain (NTD), that houses antibody targets. “What we don’t know is: Just because there are changes, are they changes that actually do something?” says Emma Hodcroft, a molecular epidemiologist at the University of Bern.

But one NTD difference—a deletion at amino acids 69 and 70 that is present in BA.1 and not in BA.2—could give researchers a tool for monitoring the spread of the up-and-coming (注目の、活動的な) Omicron strain. Certain SARS-CoV-2 polymerase chain reaction tests detect three genetic sequences of the virus, but the mutation in BA.1’s NTD gene eliminates one of those targets. Polymerase chain reaction tests pick up all three targets in BA.2, providing a proxy (代理、代理権、代理人/prɑ́ksi) for distinguishing the Omicron strains if there is no full virus sequence.

How the sibling strains were born is also preoccupying scientists. Viral evolution in a single immunocompromised patient is one theory, says Andrew ­Rambaut, an evolutionary biologist at the University of Edinburgh. “It’s possible that long-term infection could produce quite a lot of diversity within a single individual. It could be compartmentalized. So different variants living in different parts of the body.” Both Omicron strains could have also evolved in animals infected with human-adapted SARS-CoV-2, then spread back into people.

Why BA.2 is emerging only now is one more mystery, Hodcroft says. She speculates that a factor as simple as which Omicron caught an earlier flight out of South Africa, where both strains were first identified, may be the explanation. “BA.2 may have just been trapped for a little bit longer. But when it did finally get out and start spreading it started to show that it can edge out (徐々に追い出す、押しのける) its big sister (BA.1).”