コロナのパンデミック宣言から２年　出口は見えた？

【ＡＦＰ＝時事】２０２０年３月に世界保健機関（ＷＨＯ）が新型コロナウイルスのパンデミック（世界的な大流行）を宣言してから、１１日で２年が経過した。一部の国ではコロナ関連のさまざまな規制を緩和・解除し、「コロナとの共生」を目指す動きもみられる。ＷＨＯは世界人口の約７０％がワクチンを接種すれば、今年半ばにはパンデミックの「急性期」を脱する可能性があると述べている。（資料写真）

■パンデミックからエンデミックへ？
　スペインをはじめとする国々は、新型コロナを「エンデミック」の段階に移行したと見なすアプローチを提唱している。つまり、インフルエンザのように人類が共生可能な季節性の感染症ととらえる考え方だ。
　だが、英オックスフォード大学のウイルス進化学者、アリス・カツラキス氏は「『エンデミック』という言葉は、今回のパンデミックでも最も誤用されている言葉の一つだ」と言う。
　同氏は最近、英科学誌ネイチャーで「エンデミックであると同時に広範で流行する致死率が高い伝染病もある」と述べ、２０２０年にはマラリアで６０万人以上、結核で約１５０万人が全世界で死亡していると指摘した。
　英政府の非常時科学諮問委員会は、想定し得る今後数年間のシナリオをいくつか示している。そのうち「妥当な最良のシナリオ」は、地域や季節で流行する規模が縮小し、また新型コロナの感染者数が多いほどインフルエンザの感染者数が少なくなるというものだ。
　一方、最悪のシナリオは、予測できない変異株の出現と流行の波が繰り返され、厳格な規制を復活せざるを得なくなるというものだ。
　こうしたシナリオを左右するのは、変異株の出現とワクチンの長期的な効果という二つの不確定要素だ。

■変異株の脅威
　ワクチンの観点においては感染力の強い変異株「オミクロン株」は戒めとなると同時に試金石ともなった。
　疫学者の多くは、新型コロナを感染が広がるまま放置しておくと、新しい株に変異する可能性が高まると指摘している。しかも新しい変異株が弱毒化する保証はどこにもない。
「ウイルスは、時間とともにより穏やかなものに進化するという楽観的な誤解が広まっている」とカツラキス氏は警告する。例えばデルタ株は、中国・武漢で発生した最初の株より致死率は高いと指摘した。
　またオミクロン株は、既存のワクチンによる免疫をすり抜けることもある。ただし重症化や死亡を防ぐためにはワクチン、特に現在世界中で実施されている３回目の追加接種（ブースター接種）は、非常に効果的だ。

■ワクチンの今後
　イスラエルやスウェーデンなどでは４回目の接種が始まっているが、専門家はブースター接種を際限なく続けるのは近視眼的な戦略だと懸念している。
　ワクチンがターゲットとする対象を新型コロナ以外に広げる考え方もある。
　米政権のアンソニー・ファウチ首席医療顧問ら３人の研究者は、新型コロナだけではなく、動物由来で新たなパンデミックを引き起こす可能性のある別種のコロナウイルスにも効果がある「万能コロナウイルスワクチン」を開発する必要性を訴えている。
　しかし、克服しなくてはならない問題・障害も多く、人を対象とした最初の試験は始まったばかりだ。
　その一方でＷＨＯは、富裕国がさらに接種を進めるよりもワクチンを他国に提供することが、パンデミックの急性期を終わらせる最善の方法だと訴えている。
　ＷＨＯによると、アフリカ大陸での２回目のワクチン接種完了率は２月末時点で１３％にとどまり、今年半ばまでの目標とされる７０％をはるかに下回っている。【翻訳編集ＡＦＰＢＢＮｅｗｓ】
〔ＡＦＰ＝時事〕（2022/03/14-09:24）

Two years after the official start of the pandemic, some countries are now trying to live with Covid, however scientists warn that potential new variants and unequal vaccination rates threaten any long-awaited return to normality.
When US global health researcher Christopher Murray wrote Covid-19 will continue but the end of the pandemic is near, in The Lancet medical journal in late January, he summed up the hopes of many national health authorities around the world.
In the weeks leading up to the two-year anniversary of the World Health Organization declaring a pandemic in March 2020, countries such as Britain and Denmark lifted all legal Covid restrictions. Many US states also relaxed mask and other rules.
British Prime Minister Boris Johnson said the change marked the beginning of learning to live with Covid, as the global death toll falls after the more transmissible though less severe Omicron variant swept the world.
The WHO has said that the acute phase of the pandemic could end by the middle of this year -- if around 70 percent of the world is vaccinated.
- From pandemic to endemic? -
Spain has been among the nations calling for approaching Covid as having transitioned to an endemic phase, meaning it has milder seasonal outbreaks that humanity can live with, such as the flu.
However some scientists worry governments could use the somewhat vague term to justify lifting life-saving measures.
University of Oxford evolutionary virologist Aris Katzourakis said the word 'endemic' has become one of the most misused of the pandemic.
A disease can be endemic and both widespread and deadly, he wrote in the journal Nature last week, pointing out that malaria killed more than 600,000 people in 2020, while 1.5 million died of tuberculosis.
There are also other options than just pandemic or endemic. The British government's scientific advisory body SAGE has laid out four potential scenarios for the years to come.
Under the reasonable best-case scenario, there will be smaller regional or seasonal outbreaks, as the higher Covid numbers lead to fewer flu cases.
Under the worst-case scenario, new unpredictable variants build into repeated damaging virus waves, requiring the return of harsh restrictions.
The different outcomes hinge on two key uncertainties: the possible emergence of new variants, and the ability of vaccines to protect against the disease in the long term.
- The new variant threat -
When it comes to vaccines, Omicron has served as both a warning and a test.
Many epidemiologists say that simply letting Covid spread unchecked gives it a greater chance of mutating into new strains.
And there is no guarantee that such new variants will be less deadly.
There is a widespread, rosy misconception that viruses evolve over time to become more benign, Katzourakis said.
This is not the case: there is no predestined evolutionary outcome for a virus to become more benign, he said, pointing out that the Delta variant was deadlier than the first strain that emerged in Wuhan, China.
Omicron also partly evades protection from the currently available vaccines.
But they are very effective in preventing severe disease and death -- particularly third booster shots which have been rolled out across the world.
- Vaccines to the rescue? -
Countries such as Israel and Sweden have started administering fourth doses, but experts fear that an endless number of booster shots is a short-sighted strategy. An Israeli trial in January also found that a fourth dose was less effective against Omicron.
Pharma giants have raced to develop a vaccine that specifically targets Omicron, but none seem close to becoming available.
Several recent preliminary results of tests carried out on animals and not peer reviewed have suggested the targeted vaccines are no more effective against Omicron than their predecessors.
But there could be another way: broadening rather than narrowing the scope of the vaccine.
Three researchers including Anthony Fauci -- US President Joe Biden's chief medical adviser -- have called for a universal coronavirus vaccine that would protect not just against Covid but also against future coronaviruses that could spread from animals and trigger another pandemic.
We must now prioritise development of broadly protective vaccines, the researchers wrote in the New England Journal of Medicine over the weekend.
However such a vaccine faces major hurdles and the first efforts have only just begun trials on humans.
In the meantime, the WHO emphasises that the best way to end the acute phase of the pandemic is for rich countries to share their doses rather than boost their populations again.
Only 13 percent of Africans had been fully vaccinated as of late last month, according to the WHO -- far below the 70 percent target needed by mid-year.