Recently CNN.co.jp reported the following:
インドの新型コロナ感染者数が大幅に減少、その理由は？(1
of 2)
2021.02.25 Thu posted at 20:00 JST
（ＣＮＮ） 半年前に新型コロナウイルスの流行で危機に陥っていたインドだが、状況が大幅に改善してきた。１日の新規感染者数は昨年９月のピーク時の９万人あまりから２月には１万人あまりに減少。今月９日には首都ニューデリーでここ９カ月間近くで初めて１日の新型コロナによる死者数がゼロとなった。
インドではニュージーランドやオーストラリアのような大胆なサーキットブレーカー型のロックダウン（都市封鎖）を実施したわけではない。社会的距離の確保や医療体制の支援などはあるものの、経済活動や国内旅行も再開し、人々の日常生活は大部分戻ってきている。
状況の改善はワクチンによるものではなさそうだ。オックスフォード大学が集計したデータによると、１００人当たりの接種回数は１回にも満たない。英国は同２７回、米国は同１９回に達している。
専門家は若年人口の多さや、都市で免疫獲得者が増加している可能性など、いくつかの要因が絡んでいるとにらんでいる。
ただ、感染者数の減少が現実を反映していない可能性もある。インド医学研究評議会（ＩＣＭＲ）によると、検査数は昨年９月の１日１００万回以上から２月には１日６０万～８０万回に落ちた。
検査の陽性率も１月に６％近く、２月後半で５％超と高く、検査数が十分でない可能性がある。世界保健機関（ＷＨＯ）は活動再開の目安を２週間、陽性率５％以下と示している。
さらに一部の専門家は、世界中でより感染力の高い変異株が広まりつつあり、警戒を怠るべきではないと警告している。
大都市での免疫獲得
感染数減少の要因の一つに、抗体を持つ人々の増加がありうる。アショカ大学のウイルス学者、シャヒド・ジャミール氏は「感染にさらされた人が多く、それにより防護されている」と語る。
新型コロナの免疫の働きは研究が進んでいるが、米科学誌サイエンスに今月掲載された研究によると、ウイルスに感染して抗体ができた場合、少なくとも８カ月間は再感染を防ぐ可能性があるという。
米ジョンズ・ホプキンス大学の集計によると、インドではこれまでに１１００万人以上が感染、１５万６０００人以上が死亡している。
ＩＣＭＲが実施した３つの最新研究によると、抗体検査での陽性率は昨年８月下旬～９月には６～７％だったが、１２月～今年１月には２２％近くと３倍以上はね上がった。
人口が密集した都市ではさらに値が高い。ムンバイでは昨年８～９月の調査で住民の半数以上が感染を経験していると示唆する結果が出た。デリーでも今年１月の検査で住民の半分以上が感染しているとの結果が出ている。
地域医療の専門家、ヘマント・シェワデ医師は「これは非常に大きな増加だ。大都市では免疫の高まりで感染が大きく減少するかもしれない」との見方を示す。
仮に報告数が少ないとしても、まだ集団免疫の状態には至っていそうにない。
ジャミール氏はこれまでの抗体検査の結果を踏まえ、「感染にさらされた人数は１億５０００万～９億人と推計可能で、おおよそ３億５０００万～４億人がより合理的な推計値だ」としつつ、それでも人口の４分の１にしかならないと指摘。仮に最も高い９億人と想定すると人口の６５％だが、それでも集団免疫としては十分でない可能性がある。
Translation
(CNN) Six months ago India was in crisis due to the new
coronavirus epidemic, but the situation had improved significantly. The number
of new infections per day decreased from the peak of 90,000 in September last
year to 10,000 in February. On the 9th of this month in the capital city of New
Delhi, the death toll from the new corona in one single day was zero for the
first time in nearly nine months.
India did not implement the bold circuit breaker-type lockdowns
(city lockdown) of New Zealand and Australia. No more than just securing social
distance and medical system support, economic activities and domestic travel resumed,
and most of the people's daily lives had returned.
The improvement in the situation did not seem to be due to
the vaccine. According to data compiled by the University of Oxford, the number
of vaccinations per 100 people was less than one. The United Kingdom had
reached 27 and the United States 19.
Experts suspected that there were several factors involved,
including the possibility of large youth population and the increased in the
number of people with immunity in cities.
However, it was possible that the decrease in the number of infected people did not reflect the reality. According to the Indian Medical Research Council (ICMR), the number of tests performed had dropped from more than 1 million cases per day in September last year to between 600,000 to 800,000 cases a day in February.
The positive rate of tests was close to 6% in January and
higher than 5% in the latter half of February, the number of tests might not
be sufficient. The World Health Organization (WHO) had set a standard for
resuming activities with a positive rate of 5% or less for two weeks.
In addition, some experts warned that as infectious
mutants were becoming more wide spread around the world, caution should be
exercised.
To obtain immunity in big cities
One of the factors behind a decrease in the number of
infections could be an increase in the number of people with antibodies. Shahid
Jameel, a virologist at Ashoka University said "many people have exposed
to the infection and are protected in that way".
Research into the immune function of the new corona was underway, but according to a study published this month in the US scientific journal Science, if the virus had infected and produced antibodies, it could prevent re-infection for at least eight months.
According to data from Johns Hopkins University, in India up to now more than 11 million people had been infected and more than 156,000 had died.
According to three latest studies conducted by ICMR, the positive rate in antibody tests was 6-7% from late August to September last year, but it bounced up more than tripled to nearly 22% from December to January this year.
The value was even higher in densely populated cities. In Mumbai, a survey from August to September last year suggested that more than half of the inhabitants had experienced infection. Also, in Delhi, in a survey in January of this year, more than half of the residents were found to have experienced infection.
Dr. Hemant Shewade, a community medicine expert said, "This
is a huge increase. In big cities, increased immunity may significantly reduce
infections".
However, India as a whole was far from herd immunity. Large
cities could have enough herd immunity to deter infection, but the majority of
the population needed to experience infection or vaccination in order to obtain
immunity across the country. The reported 11 million cases were less than 1% of
the population of 1.4 billion.
Even if the number of reported [new] cases was small, it was unlikely that the state of herd immunity had been reached yet.
Mr. Jameel based on the results of antibody tests so far, said "the number of people exposed to infection can be estimated to be 150 to 900 million, and about 350 to 400 million is a more reasonable estimate”. However, he pointed out that it was still only a quarter of the population. Assuming the highest number of 900 million people, still it was 65% of the population, even that might not be sufficient for herd immunity.
Adar Punawara, CEO of the Serum Institute of India that was India's largest vaccine manufacturer, said " to obtain the magical 90% herd immunity, immunity achieved by vaccination would take 3-4 years, and until then you should be careful. "
(to be continued)
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