新型コロナ、症状発現直前に感染力ピークか　研究

【パリＡＦＰ＝時事】新型コロナウイルスに感染した人は、目立った症状が表れる数日前からウイルスを拡散させ始めている可能性があるとする最新のモデル化研究が15日、発表された。（写真は資料写真）
　新型コロナウイルスのパンデミック（世界的な大流行）を食い止めるため、世界では、感染者の接触歴を症状が表れ始めた時点までさかのぼって追跡調査するといった対策が講じられているが、今回の研究では、こうした対策の根拠となっている重要な前提に異を唱える結果が示された。
　自身が感染していることに気付いてすらいない一部の人々がウイルスを他人に感染させていることについては、これまでも専門家らの間でその可能性について指摘されていた。
　だが今回の研究では、感染者からの伝染リスクが、実際には症状が発現する前からすでに非常に高くなっていることが示唆されている。
　医学誌ネイチャー・メディシンに掲載された論文は、「流行を効果的に制御するには、症状発現の２～３日前に発生している可能性がある感染を捕捉する目的で、接触者追跡のより包括的な基準の制定を至急検討するべきだ」としている。

■症状発現前に高い感染性
　今回の研究では、感染可能期間（感染者が他者に感染させることが可能な期間）に関する推論値を導き出すために、中国の病院で確認された患者からのウイルス排出に関する臨床データと、感染者とその人から感染したとみられる他者の組み合わせの「感染ペア」に関する別のデータを比較した。
　香港大学のエリック・ラウ氏らが主導した研究チームは、広州市第８人民医院の入院患者９４人の咽頭検体を採取し、最初に症状が表れた日から３２日間にわたってそれぞれの感染力を評価した。
　その結果、患者はいずれも重症や重篤に分類される症例ではなかったが、保有するウイルス量は症状が表れた直後が最大で、その後に徐々に減少することが明らかになった。
　また、中国国内と世界各地の感染ペア７７組に関する公開データを用いて、ペアのそれぞれの患者に症状が表れるまでの時間の経過を算定した結果、潜伏期間（感染への暴露から症状が表れるまでの時間）は５日強と仮定された。
　分析の結果、感染力が高い状態は症状が表れる２・３日前に始まり、最初の疾病兆候の０．７日前にピークに達すると推察された。ただし、症状が表れた正確なタイミングの特定は患者の記憶を頼りにしたと、論文の執筆者らは注意を促している。
　感染連鎖における二次感染者の４４％は症状が出る前の期間に感染したと、論文の執筆者らは推測している。また感染力は７日以内に速やかに低下すると予測されている。【翻訳編集ＡＦＰＢＢＮｅｗｓ】
〔ＡＦＰ＝時事〕（2020/04/16-15:35）

People infected with the new coronavirus may start spreading the virus several days before they have any noticeable symptoms, according to a new modelling study published Wednesday.
The study comes as nations have broadened restrictions aimed at curbing the epidemic, amid concerns over patients who may be infectious despite not showing signs of ill health.
The findings challenge key assumptions behind measures put in place to stop the spread of the pandemic, such as tracing contacts of an infected person only as far back as the time at which they began to show symptoms.
Experts have long conjectured that some people who do not even know they are infected may transmit the virus.
But the new study suggests that even those who get visibly sick may be highly infectious before the onset of symptoms.
More inclusive criteria for contact tracing to capture potential transmission events two to three days before symptom onset should be urgently considered for effective control of the outbreak, the authors said in the paper published in Nature Medicine.
- Infectious before symptoms show -
Researchers compared clinical data on virus shedding from patients at a hospital in China with separate data on transmission pairs -- where one person is believed to have infected the other -- to draw inferences about periods of infectiousness.
The research team co-led by Eric Lau of the University of Hong Kong took throat swabs from 94 patients admitted to Guangzhou Eighth People's Hospital and measured infectiousness from the first day of symptoms for 32 days.
They found that the patients, none of whom were classed as severe or critical, had the highest viral load soon after the onset of symptoms, which then gradually decreased.
The study used publicly-available data on 77 transmission pairs, within China and internationally, to assess how much time elapsed between the onset of symptoms in each patient.
It assumed an incubation period -- the time between exposure to infection and appearance of symptoms -- of a little over five days.
The authors inferred that infectiousness started 2.3 days before symptoms appeared and was at its peak at 0.7 days before the first signs of illness -- although they cautioned that pinpointing the exact timing of the onset of symptoms relied on patient memory.
They estimated that 44 percent of secondary cases in the transmission chains were infected during the pre-symptomatic stage.
Infectiousness was predicted to decrease quickly within seven days.
- 'Important implications' -
Responding to the study, Babak Javid of Tsinghua University School of Medicine in Beijing said the findings would have important implications for measures to control the pandemic, such as whether masks should be worn by those with no symptoms.
This is important because current public health control measures advised, for example, by the WHO and UK government assume that maximum contagion is after symptom onset. Hence one reason masks are not advocated for wearing by asymptomatic members of the public, he said.
Javid added that several studies had now suggested that a large number of patients shed the virus before they show signs of illness and said the findings are likely to be real and robust.
At the beginning of this month China said it had more than 1,300 asymptomatic coronavirus cases, the first time it has released such data following public concern over people who have tested positive but are not showing symptoms.
As the pandemic has spread, many nations that initially advised only individuals with symptoms to self-isolate and wear masks have expanded their responses to measures that apply across the board.
The US Centers for Disease Control and Prevention (CDC) has said up to a quarter of people who are infected may be asymptomatic.