The Pandemic's Future (Nature 2020.08.06)


What happens in 2021 and beyond?
The pandemic's course next year will depend greatly on the arrival of a vaccine, and on how long the immune system stays protective after vaccination or recovery from infection. Many vaccines provide protection for decades -- such as those against measles or polio -- whereas others, including whooping cough and influenza, wear off over time. Likewise, some viral infections prompt lasting immunity, others a more transient response. "The total incidence of SARS-CoV-2 through 2025 will depend crucially on this duration of immunity," wrote Grad, Harvard epidemiologist Marc Lipsitch and colleagues in a May paper exploring possible scenarios.

To predict how COVID-19 might come and go in temperate regions such as North America and Europe, researchers have modelled the influence of factors including how long immunity to the coronavirus might last, the role of seasons and whether other coronavirus infections might give some immunity to it.

Researchers know little so far about how long SARS-CoV-2 immunity lasts. One study of recovering patients found that neutralizing antibodies persisted for up to 40 days after the start of infection; several other studies suggest that antibody levels dwindle after weeks or months. If COVID-19 follows a similar pattern to SARS, antibodies could persist at a high level for 5 months, with a slow decline over 2-3 years1. Still, antibody production is not the only form of immune protection; memory B and T cells also defend against future encounters with the virus, and little is known so far about their role in SARS-CoV-2 infection. For a clear answer on immunity, researchers will need to follow a large number of people over a long time, says Michael Osterholm, director of the Center for Infectious Disease Research and Policy (CIDRAP) at the University of Minnesota, Minneapolis. "We're just going to have to wait."


If the virus induces short-term immunity -- similar to two other human coronaviruses, OC43 and HKU1, for which immunity lasts about 40 weeks -- then people can become reinfected and there could be annual outbreaks, the Harvard team suggests. A complementary CIDRAP report, based on trends from eight global influenza pandemics, points to significant COVID-19 activity for at least the next 18-24 months, either in a series of gradually diminishing peaks and valleys, or as a "slow burn" of continuing transmission without a clear wave pattern. Yet these scenarios remain only guesses, because this pandemic has so far not followed the pattern of pandemic flu, says Osterholm. "We're in a coronavirus pandemic for which we have no precedents."

Another possibility is that immunity to SARS-CoV-2 is permanent. In that case, even without a vaccine, it is possible that after a world-sweeping outbreak, the virus could burn itself out and disappear by 2021. However, if immunity is moderate, lasting about two years, then it might seem as if the virus has disappeared, but it could surge back as late as 2024, the Harvard team found.


The World Health Organization lists 26 COVID-19 vaccines currently in human trials, with 12 of them in phase II trials and six in phase III. Even a vaccine providing incomplete protection would help by reducing the severity of the disease and preventing hospitalization, says Wu. Still, it will take months to make and distribute a successful vaccine.


There is one thing that every country, city and community touched by the pandemic has in common. "There is so much we still don't know about this virus," says Pulliam. "Until we have better data, we're just going to have a lot of uncertainty.

パンデミックの行方 (Nature 2020.08.06)


来年のパンデミックの行方は、ワクチンの開発と、ワクチン接種や感染からの回復後、どれだけの期間免疫による防御を維持できるかに大きく左右される。麻疹やポリオなどのように、多くのワクチンは何十年もの間免疫力を発揮するが、百日咳やインフルエンザなどのように、時間が経つと効果が薄れてしまうものもある。同様に、ウイルス感染症の中には、免疫が持続するものもあれば、一過性のものもある。「2025年までのSARS-CoV-2の総発生率は、この免疫の持続時間に大きく左右される」とGrad氏、ハーバード大学の疫学者Marc Lipsitch氏らは、可能性のあるシナリオを探った5月の論文で述べている。

北アメリカやヨーロッパなどの温帯地域で COVID-19 がどのように広がるかを予測するために、研究者たちは、コロナウイルスに対する免疫がどのくらいの期間続くか、季節の役割、他のコロナウイルス感染症がSARS-CoV-2にある程度の免疫を与えるかどうかなどの要因の影響をモデル化した。

SARS-CoV-2の免疫がどのくらいの期間持続するかについては、これまでのところほとんどわかっていない。回復した患者を対象としたある研究では、中和抗体は感染開始後40日まで持続したが、他のいくつかの研究では数週間から数ヶ月後には抗体レベルが低下することが示唆されている。COVID-19がSARSと同様のパターンをたどるならば、抗体は5ヶ月間高レベルで持続し、2-3年かけてゆっくりと減少する可能性がある。それでも、免疫防御の形態は抗体産生だけではない;メモリーB細胞とT細胞もまた、将来ウイルスと遭遇した際に防御をするが、SARS-CoV-2感染におけるそれらの役割については、今のところほとんど知られていない。免疫に関する明確な答えを得るためには、研究者は長期間にわたって多数の人々を追跡する必要があると、ミネソタ大学ミネアポリス校の感染症研究・政策センター(CIDRAP)のディレクターであるMichael Osterholm氏は言う。「待つしかない。」









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