Interventions which suppress the reproduction number save a lot of lives. But they do not make the population immune to the disease

こういうとき役に立つのは、いつもlong haulで冷静に問題を見ているEconomist。もちろん集団免疫派だ。 https://t.co/7mrArtu9hf

— 池田信夫 (@ikedanob) March 21, 2020

集団免疫派というのがどうか知らんけど、イギリス政府ははじめから、プラグマティストというのか、極めて現実と人間性を見据えた対応だとは思うね。

もっとも、日本の専門委員会と特に異なるわけでもない、と思う。

A couple of hours before Mr Macron spoke, Boris Johnson, the British prime minister, took a newly tough line, too, though from a more lax starting point. No self-printed papers, or for that matter, enforced closures—but a directive to stay away from both the office and the pub, and a clear steer that further restrictions could be coming in pretty short order


They assigned covid-19 a “basic reproduction number” of 2.4. This means that in a population not taking any precautions, and where no one is immune, each case leads, on average, to 2.4 secondary cases.

Under those conditions the model showed the disease infecting 80% of the British population in three to four months.


Imposing isolation on people with symptoms of the disease, quarantine on their households and protecting the elderly through social distancing—broadly speaking, the sum of British government advice as of last week—cut the height of the epidemic’s peak by two-thirds and pushed it from May to June. But the demand for beds still outstripped the supply by a factor of eight, and the death toll still reached the hundreds of thousands.

Extending social distancing from the over-70s to the whole population, though—a lockdown along the lines of Mr Macron’s, or of what is being seen in Italy—made a big difference. The peak of the disease outstripped the nhs’s surge capacity by just a factor of two. And it was delayed until July or August. That is a time-frame over which increasing that capacity is clearly plausible. I


In early March Chinese scientists circulated a preprint looking at how air-temperature and humidity influenced transmission in 100 Chinese cities, and controlling for population density and wealth, they concluded that transmission became a little less likely when the weather was hotter and more humid. Summer and rainy seasons may not help a lot—but they could help a bit.


The Imperial results shaped the British government’s new plans: everyone to work from home if they can, no travel for fun, no “unnecessary” visits to care homes, avoid pubs, restaurants, theatres and the like, quarantines of 14 days for households where someone develops symptoms.


This summer’s international Euro 2020 tournament has been put off until 2021 (as has the Copa América, which was to take place in Colombia and Argentina). The Glastonbury festival has been cancelled; so will much of the summer be.

Politicians also seem much keener on banning international travel than epidemiologists are. The World Health Organisation does not recommend broad international travel restrictions, or the closure of borders.


Interventions which suppress the reproduction number save a lot of lives. But they do not make the population immune to the disease. So if you relax the interventions, thus letting the disease’s reproduction number rise back up, it will promptly set about infecting the large pool of people it missed the first time round.



The model from Imperial showed this well-known effect in action. After its five months of restrictions were brought to a close, a second wave started to build in late autumn. When the model was run with schools and universities closed down at the beginning of the first outbreak, this second wave was considerably worse, because immunity built up through infection-in-the-course-of-education was absent


Concerns about a second wave are one of the reasons that governments cannot reassure citizens over just how long the disruption to normal life will continue, instead bandying about “the time beings” and “coming weeks and months”.


Even if governments were forthright about how long they expected their impositions to last, it is not clear that the decision will be theirs to make. In free societies the duration will, in practice, depend on how compliant people are, and for how long. The team at Imperial reckons that, if their measures are to show the desired results, compliance with each of those measures must be at least 50-75%


Drugs do not make people immune to disease; but if severe cases could be treated more effectively, there would be much less to fear about letting people circulate quite freely. On March 17th Chinese doctors reported that Avigan (favipiravir), a drug used against influenza in Japan, led to clinical improvements in patients.

どうやっても感染拡大は不可避で、拡大を止めるにはワクチンが必要だが、これには時間がかかる。
対処法としては、手洗い、消毒などの他に、

対人間距離をとる、対人間接触機会を減らす→　店舗閉鎖→　学校閉鎖→　移動禁止→封鎖

とかあるわけだが、自由社会ではじめからきつい禁止令をだしてもだれもがビシッと守れるわけがない。
日本の専門委員会の会見でも、国民が納得して、持続可能なルールじゃないと意味がないみたいなことは言っていたね。

で、一番厳しい処置である封鎖をすれば、当分は感染は防げるわけだが、世界中に感染拡大は不可避だから、いずれにせよ、封鎖された地域にもウイルスの台風はやってくる。また、第２波もくる。徐々にウイルスになれてきた人口にとってはそのとき、衝撃は少ないわけだ。

ワクチンができるまで、発症した場合にアビガン投与して凌ぐとか、発症した感染者とその接触者をはやめに隔離する。

その追跡ができるように、中国なんかは、強力な監視体制があるんだけど、おれなんかは、後で追跡できるように、劇場、映画館など民間人が任意で、観客の連絡先を集めて、記録しておいたほうがいいんじゃないか、と思っているんだよね。

集団免疫派と鎖国・弾圧派という対立があるんじゃなくて、感染拡大は不可避、という大前提のもと、持病持ち高齢者を保護し、感染拡大が台頭してくる度合いにに応じて、現実的に、緩い制限から最大の制限を選択していくしかあるまい。

それがもっともウイルスの現実と人間の現実に即した対処法であろう。