停滞するコロナワクチン接種、投与量見極める研究も

【ワシントンＡＦＰ＝時事】新型コロナウイルス感染症（COVID-19）の2回目のワクチン接種は遅らせるべきなのか。投与量は減らすことが可能なのか。複数の製薬会社のワクチンの組み合わせも、同じように効くのだろうか。（写真は資料写真）
　これらは現在、各国政府が直面している疑問だ。世界各地では新型コロナウイルスのワクチン接種が遅れ、ウイルスは引き続き猛威を振るい、さらに感染力が強いとされる新たな変異株が拍車をかけている。
　英保健当局は、同国で使用が許可されたワクチンの１回目から２回目の接種の間隔を最大３か月延長すると発表した。これは奨励期間の３～４週間を優に超えるものだ。
　これが主眼とするのは、ワクチン接種を直ちに受けられる人数を拡大することだ。たとえ個々人に対する保護レベルが、２回目の接種で達成されるレベルには及ばないとしてもだ。
　世界保健機関（ＷＨＯ）は５日、英国の立場を事実上支持し、米製薬大手ファイザーと独製薬ベンチャーのビオンテックが開発した新型コロナウイルスワクチンの２回目の接種を、「例外的状況」下では数週間遅らせることができるとした。
　英当局はまた、在庫がない場合は、１回目とは異なる製薬会社のワクチンで２回目の接種を行う許可を出している。
　他方、米国はより慎重な方針を取っている。
　米食品医薬品局（ＦＤＡ）のスティーブン・ハーン長官は４日、英国の動きを時期尚早で確実な証拠に基づいていないと懸念を表明。この議論で専門家らの間の見解の違いが露呈した。
　理想的には、方針決定は臨床試験（治験）が実施された枠組みのみに基づいて行われるべきだ。だが、コロナの制御が利かなくなりつつあり、現状は理想的とはほど遠いとする意見もある。
　ワクチンの混合については、エール大学の免疫生物学者、岩崎明子氏は理論上は効くはずだとしているが、専門家らはさらなる研究が必要だという認識で一致している。

■国民を混乱させかねない
　ワクチン研究者でエール・グローバルヘルス研究所の所長サード・オメル氏とフロリダ大学の生物統計学者のナタリー・ディーン氏の両氏ともが指摘するのは、進むべき道は、新型コロナウイルス感染症の発症予防に必要な抗体価などの生物学的マーカーを見つけるためのさらなるデータ分析にあるかもしれないということだ。
　この判断を可能とするには、既存の治験結果を詳細に調べて閾値（いきち）を算出し、次にそこに到達するのに必要なワクチンの投与量を見極める小規模の研究を立ち上げることだ。
　米国立衛生研究所（ＮＩＨ）のジョン・マスコーラ氏は米紙ニューヨーク・タイムズに５日、この方法に基づき、米製薬大手モデルナのワクチン投与量を半分にしても同程度の保護が可能かを見極める研究が進行中であると明らかにしている。
　先のディーン氏は、変更をめぐる議論は国民を混乱させかねないとの懸念を示し、「信頼を損なうおそれのあるあらゆることを案じている」と語った。【翻訳編集ＡＦＰＢＢＮｅｗｓ】
〔ＡＦＰ＝時事〕（2021/01/07-15:14）

Should Covid-19 boosters be delayed? Could the dose levels be reduced by half, and would mixing and matching shots made by different makers work just as well?
These are questions now facing governments around the world as vaccine rollout falters and the coronavirus continues to rage, spurred on by new variants that are believed to be more contagious.
British health officials are setting the pace, announcing they will stretch out the second dose of vaccines authorized for use there up to three months, well beyond the three or four weeks recommended.
The idea is to scale up the number of people who can be reached quickly, even if the level of individual protection falls short of what is reached with the booster.
Authorities in England have also granted permission to give people a second shot from a different vaccine maker if the kind the got the first time has run out.
The United States, meanwhile, has tacked a more cautious line.
On Monday night, Food and Drug Administration chief Stephen Hahn said that while these are reasonable questions to consider and evaluate, the moves are premature and not rooted solidly in available evidence.
Without this, we run a significant risk of placing public health at risk, undermining the historic vaccination efforts to protect the population from COVID-19.
The divide exposes a division among the experts, with respected scientists falling on opposing sides of the argument.
Ideally, policy decisions should rest solely on the parameters in which clinical trials were conducted. But with the virus threatening to spiral out of control, some argue we're in far from an ideal situation.
We didn't pick three weeks for Pfizer or four weeks for Moderna because we knew it to be perfect, Howard Forman, a public health expert at Yale University told AFP.
It was the best guess for an optimal time for the booster dose to be given to augment immunity.
Forman added that much of medicine is based on imperfect data -- such as how long a course of drugs should be taken, and physicians routinely prescribe medicine approved for one purpose for other conditions.
So modest changes to what we've already recommended may make all the difference in getting a lot more bang for the vaccines that we have, he added.
Forman stressed that he's only suggesting delaying the second dose -- seen as vital to ensuring longer term protection -- and only in the case of under-65s and those who are at less medically vulnerable.
- Sluggish rollout -
The debate comes amid unexpected delays in the rollout of Covid-19 vaccines.
The US had set 20 million people as its target for December but as of January 4, it had only reached 4.5 million.
Both the US and UK have covered about 1.4 percent of their populations, Europe is far behind, while Israel is out front having covered some 13 percent.
The Pfizer and Moderna vaccines, based on mRNA technology, reach about 95 percent efficacy on second dose, which is reserved in storage for a person after their first.
The Moderna vaccine in particular has shown high levels of protection after the first shot -- in the region of 90 percent -- but the numbers should be treated cautiously as the sample size is small.
Saad Omer, a vaccine researcher and director of Yale's Institute for Global Health, told AFP that he saw only a need for changing tactics in countries where supply itself is short.
As it stands in the US, the rate at which vaccines are being sent to states is only a little behind schedule, and Omer believes addressing the bottleneck in administering them to people should be given priority.
As for mixing vaccines, noted Yale immunologist Akiko Iwasaki has said it should work in theory, but experts are unanimous that it requires more study and should only be a last resort for now.
- Confusing to public -
Both Omer and Natalie Dean, a biostatistician at the University of Florida, argue that a path forward might lie in further data analysis to find biological markers, such as antibody levels, that correlate to protection against Covid-19.
This could be determined by combing through the results of multiple trials to calculate a threshold value, then setting up small studies to determine what dose of vaccine gets you there.
The advantage of this approach is it would eliminate the need to repeat months-long trials.
This might be a promising avenue to pursue for injecting half-doses of the Moderna vaccine, which data suggests might deliver the same level of protection, said Dean.
She added she was worried that the discussion around changes could be confusing to the public, and I'm concerned about anything that could jeopardize trust.
People should therefore be assured that any changes in the US would go through the same transparent regulatory process that granted the vaccines their emergency authorization, she said.