Omicron "BA.2" derivative strain: hospitalization risk is almost the same as the original strain - new research

Recently CNN.co.jp reported the following:

オミクロン「ＢＡ．２」派生株、入院リスクは従来株とほぼ変わらず　新研究

2022.02.24 Thu posted at 21:00 JST

 （ＣＮＮ） 新型コロナウイルスの変異株オミクロンの一種で「ＢＡ．２」と呼ばれる新たな派生株について、現実世界での入院リスクは従来株の「ＢＡ．１」とほぼ変わらないとする新たな研究が発表された。これより前には、研究室内や動物を用いた実験からＢＡ．２がＢＡ．１より重い症状を引き起こす可能性があるとの研究結果が示されていた。

英国とデンマークで先に行われていた研究によれば、ＢＡ．２の感染力はＢＡ．１よりも約３割強い。世界保健機関（ＷＨＯ）は、現在世界で検出される新型コロナ感染のおよそ５分の１がＢＡ．２だとしている。

感染件数は世界で減少傾向にあるが、ＢＡ．２が全体に占める割合は相対的に増えている。少なくとも４３カ国ではＢＡ．１以上に蔓延（まんえん）しており、新たな感染拡大の波に対する懸念が浮上している。

ただ今回新たに発表された研究から、ＢＡ．２で入院患者や死者の数が大幅に増える公算は小さいとの見通しが明らかになった。

新たな研究の１本目は南アフリカでのもの。同国では今月の１週間で、ＢＡ．２の新規感染に占める割合が２７％から８６％に急増した。ただ研究者らが９万５０００人以上の陽性者を調べたところ、ＢＡ．２に感染したとみられる人の入院率は約３．６％と、ＢＡ．１感染の兆候が表れた人の入院率３．４％とほぼ同率だった。

高齢など重症化リスクに影響を及ぼし得る要因を考慮すると、研究者らはＢＡ．１とＢＡ．２の感染者の間に入院リスクの差は認められないとした。どちらに感染していても、入院する人に占めるワクチン接種済みの人の割合は４人に１人程度だったという。

筆頭研究者によると、第４波の後のＢＡ．２出現で新たなピークは発生せず、尾が長引いている状況だという。第４波は「以前の波の間の時期より高いレベル」で長引いているものの、「年末の休日期間後の学校再開や、制限緩和が影響している可能性もある」としている。

南アでの感染者数は高止まりしているが、大半はＢＡ．２による感染であり、入院患者は減少を続けている。

２本目の研究はデンマークの国家血清研究所が発表した。それによると、ＢＡ．１やデルタ株に感染して回復した人であってもＢＡ．２に再感染する可能性はあるが、そうしたケースは見たところまれであり、大半はワクチン未接種者の間で起こる。症状も軽症の場合がほとんどだという。

現在デンマークで最も蔓延しているＢＡ．２は、先月の第２週にＢＡ．１の感染例を上回った。

再感染のリスクを調べるため、研究者らはオミクロン株が主流となった昨年１１月下旬から今年２月中旬までのウイルスゲノムを解析。２０～６０日の期間内に２度の感染を経験した２６３人中、１９０人がＢＡ．２に感染していたことを突き止めた。このうち１４０人はデルタ株に次いでＢＡ．２に感染し、４７人はＢＡ．１に次いでＢＡ．２に感染していたという。

後者をより詳しく分析すると、４７人中３０人は２０歳以下の若年層。４０歳以上は１人もおらず、４７人中４２人はワクチン未接種だった。

再感染時の症状は、２８人が無症状か軽症だった。５人は中程度と分類される症状を経験したが、これはインフルエンザと同様のもの。入院患者や死者の報告はなかった。

カナダ・サスカチュワン大学のワクチン及び感染症機構に所属するウィルス学者、アンジェラ・ラスムセン氏は上記の研究から分かることとして「再感染は最近ＢＡ．１から回復した人の間で起こり得るが、かなり珍しい。またすべての事例で重症化はなかった」と述べた。同氏は当該の研究を検証したものの、研究自体には関与していない。

ラスムセン氏によればＢＡ．２への再感染は一つのリスクではあるものの、人口全体で考えれば規模は小さく、ワクチンのブースター（追加）接種や最近のオミクロン株への感染が多くの場合で防御効果を発揮するという。

今回の２つの研究は医学誌に未発表であることを意味する「プレプリント」の状態でインターネット上に投稿されたもので、外部の専門家による査読もまだ済んでいない。

Translation

(CNN) A new study showed that for the new derivative strain called "BA.2", which was a variant strain of the new coronavirus Omicron, the risk of hospitalization in the real world was almost the same as that of the original "BA.1". Prior to this, from experiments using laboratories and animals, a study had shown that BA. 2 could cause more severe symptoms than BA.  1.

According to previous studies in the United Kingdom and Denmark, the infectivity of BA.2 was about 30% stronger than BA.1. According to the World Health Organization (WHO), about one-fifth of the new corona infections currently detected in the world were said to be BA.  2.

While the number of infections was declining worldwide, the ratio of but BA. 2 to the whole was relatively increasing.  BA.2 more than BA.1 was spreading in at least 43 countries, raising concerns about a new wave of infestation.

However, from the newly published research, it became clear that it was unlikely the number of inpatients and deaths could increase significantly due to BA. 2.

The first of the new studies was in South Africa. In this country, in one week in this month, the proportion of BA.2 new infections surged from 27% to 86%. However, when the researchers examined more than 95,000 positive cases, the hospitalization rate of those who were thought to be infected with BA.2 was about 3.6%, the hospitalization rate for those who showed signs of BA. 1 infection was about the same at 3.4%.

Considering factors that could affect the risk of aggravation, such as old age, the researchers found that between BA. 1 and BA.2 infected persons, there was no difference in hospitalization risk in these two. Regardless of which one was infected, about 1 in 4 people hospitalized were vaccinated.

According to the lead researcher, with the appearance of BA. 2 after the fourth wave, it was said that no new peak had occurred, just had a prolonged tail. Although the fourth wave had been prolonged at "a higher level than the in-between period with the previous waves", "it may be affected by the resumption of school after the holiday period at the end of the year and the relaxation of restrictions."

The number of infected people in South Africa remained high, but most of them were BA.2 infections, and the number of hospitalized patients continued to decrease.

The second study was published by the Danish National Institute of Serum. According to it, even those who had recovered from infection with BA. 1 or Delta strains, it was possible to be re-infected by BA.2, yet such cases were apparently rare and mostly occurred among unvaccinated individuals. Most of the symptoms were mild.

Currently the most prevalent in Denmark was BA.2. In the second week of last month. The number of infections exceeded BA. 1.

To investigate the risk of reinfection, researchers analyzed the viral genome from late November last year to mid-February this year when the Omicron strain became mainstream. Of the 263 people who experienced two infections within a period of 20-60 days, it was confirmed that 190 had BA. 2. It was found out that of these, 140 were the second time BA. 2 infections, while 47 people first had  BA.1 then BA.2 infections.

A more detailed analysis of the latter revealed that 30 out of 47 were young people under the age of 20. No one was over 40 years old, and 42 out of the 47 were unvaccinated.

At the time of reinfection, in 28 patients they were asymptomatic or with mild symptoms. Five experienced symptoms classified as moderate, similar to influenza. There were no reports of hospitalization or deaths.

Angela Rasmussen, a scholar affiliated with the Vaccine and Infectious Diseases Organization at the University of Saskatchewan, Canada said "Reinfection can occur among people who have recently recovered from BA. 1, but it is quite rare. In all cases, there was no serious illness. " She validated the study, but she was not involved in the study herself.

According to Rasmussen, although there was a risk in reinfection with BA. 2, it was in small scale for the entire population, and booster (additional) vaccination together with recent protection measures against infection with Omicron strains were said to be effective in many cases.

These two studies were posted on the Internet in a "preprint" state, which meant they had not been published in a medical journal, and had not yet been peer-reviewed by outside experts.

              So, according to two recent unpublished studies, it is showed that for the so called "BA.2"derivative strain, which is a variant strain of coronavirus Omicron, its risk of hospitalization could be almost the same as that of the original strain the "BA.1".