BA.2の病原性がBA.1より高いということはない。

WHO says Omicron subvariant is not more virulent

Real-world data on severity of COVID-19 cases from countries where BA.2 has surged are reassuring

BY MEREDITH WADMAN / 23 FEB 2022 / SCIENCEINSIDER HEALTH

Van Kerkhove of the World Health Organization, shown in Geneva in 2020, spoke reassuringly yesterday about the severity of an Omicron subvariant.

The World Health Organization (WHO) yesterday declared that a more contagious version of Omicron, the SARS-CoV-2 variant that has already swept the globe in recent months, makes people no sicker than the original Omicron. The subvariant does not merit a separate designation with its own Greek letter, WHO said.

The statement from the international organization that designates variants of concern came on the heels of (～のすぐ後に続いて) new population studies of COVID-19 from South Africa and Denmark, whose data were made available in non–peer-reviewed preprints. WHO also cited still-confidential data from the United Kingdom, which is expected to release a new briefing report on Omicron on Friday. Yesterday’s statement, from WHO advisers who track SARS-CoV-2 evolution, said the combination of real-world findings from all three countries showed no evidence that the subvariant, dubbed BA.2, causes more severe disease than BA.1, the original Omicron.

“We don’t see any difference in terms of severity between BA.2 compared to BA.1. … That’s important,” Maria Van Kerkhove, a co-leader of WHO’s COVID-19 response, told an audience on social media.

The subvariant should continue to be closely and separately tracked by countries, WHO’s Technical Advisory Group on SARS-CoV-2 Virus Evolution wrote, but “should remain classified as Omicron.” Some scientists have argued that the differences between the two variants are significant enough that BA.2, which has about 50 mutations distinguishing it from the earliest pandemic strain of SARS-CoV-2, roughly 30 of them shared with BA.1, should be labeled an independent variant of concern. (BA.1 also has additional mutations it does not share with BA.2; the net effect is a difference of about 40 mutations between the two strains.)

Some of the worry about BA.2’s ability to cause severe disease stems from a preprint posted last week, which suggested the virus made hamsters sicker (先日紹介した東大データ) than BA.1. “We should more carefully consider BA.2 as a unique variant different than Omicron,” says systems virologist Kei Sato of the University of Tokyo, lead author on the hamster work. “Because this variant might be more pathogenic in humans and more transmitted in the human population.”

Even as COVID-19 cases are declining around the globe, BA.2 continues to grow as a proportion of the cases for which viral sequences are available: It represented 21% of sequenced cases worldwide earlier this month. The subvariant has come to dominate in Denmark, India, and other countries. In South Africa, it grew from 27% to 86% of sequenced cases between 4 February and 11 February. In the United States, its prevalence tripled, to 3.6%, between late January and 5 February. By the week that ended on 19 February, it was 3.8%. Epidemiological data published last month, and confirmed since by Sato’s group, suggest BA.2 is up to 40% more contagious than BA.1.

Although it acknowledged the hamster study, the WHO advisory group appeared to give greater credence (信頼/kríːdns) to the recent epidemiological data on BA.2 virulence. The working group scrutinized the clinical severity of COVID-19 cases from South Africa, the United Kingdom, and Denmark. “In this data, there was no reported difference in severity between BA.2 and BA.1,” the WHO statement said, echoing Van Kerkhove’s remarks. The group also found, using initial population-level data, “that infection with BA.1 provides strong protection against reinfection with BA.2, at least for the limited period for which data are available.”

In South Africa, investigators at the National Institute for Communicable Diseases (NICD) recently analyzed data from more than 95,000 people who became infected with SARS-CoV-2 in South Africa between 1 December 2021 and late January. The researchers reported in a 19 February preprint they found no increased risk of hospitalization between those whose diagnostic tests indicated they had BA.1 and those whose tests pointed to BA.2. Nor did they find a higher risk of severe disease from BA.2 than from BA.1 in more than 3000 of these people who landed in the hospital.

この調査における、年齢層および、BA.2感染がBA.1感染後の再感染かは不明。

“We feel quite reassured,” says the preprint’s first author Nicole Wolter, principal medical scientist at NICD’s Centre for Respiratory Diseases and Meningitis. But because a large proportion of the South African population acquired COVID-19 early and has natural immunity, she and her co-authors call for additional data from other countries where SARS-CoV-2 has not infected such a broad swath of the population.

COVID-19の発症が、BA.2 virusによる初回感染による場合において、その病原性がどうなのかは、上記の疫学データでは証明されていないように思う。

Such data were made public on 22 February in a preprint from Denmark’s public health agency, the Statens Serum Institute, that analyzed reinfections in the country from late November 2021 through 11 February, a time period during which BA.2 became dominant there. They found people were rarely reinfected with BA.2 soon after a BA.1 infection, and that when such reinfections did occur they were mild, resulting in no hospitalizations or severe disease. Eighty-nine percent of the reinfected people were unvaccinated, and none was older than 38. The differences between symptoms in the initial and the second infection were also “negligible,” the agency noted in a public statement.

若年層に限定されたデータで、しかも、BA.1感染後のBA.2による再感染を解析しているので、初回感染がBA.2の場合に病原性がBA.1と比較してどうかということは言えない。

The UK Health Security Agency is expected to publish additional real-world data on BA.2 on 25 February. In England, where new daily cases are falling precipitously and hospitalizations are also declining, BA.2 represented 13% of sequenced cases between 1 February and 7 February.

Virologist Thomas Peacock of Imperial College London, responding to the Japanese preprint, noted that its findings of increased disease severity in hamsters were not consistent with the recent, “real-world” epidemiological findings on BA.2. “Model systems are only useful if they reflect reality,” he tweeted.

ハムスターを用いた感染実験で、BA.2の方がBA.1より病原性が高いというデータを発表した東大のDr. Satoだが、疫学データよりきっぱりと否定されて、ちょっと気の毒。