Laser Physics Helps to Understand the Corona Virus Crisis

As long as only few are infected in a society, it can be effective to diligently track the few infected person and those who may have infected by them recently. (Here, we have a method which I cannot compare with lasers!) If this is done well enough, one may contain the outbreak, strongly limiting the number of infected peopled. However, it requires rapid well coordinate action, reliable test kits and the like, and it quickly becomes impractical when the number of infected people gets too high. For many countries, including Germany and the United States, that chance has passed. In the United States, it has been minimal from the beginning, since the government essential dismantled the existing structures for pandemic action. A good example for how disastrous (and expensive!) it can be to save some money at the wrong place.

A particular difficulty in that case arises from some characteristics of the SARS-CoV-2 virus: there is a substantial time where someone infected is infectious without feeling any symptoms. Therefore, e.g. it helps little to monitor air passengers for fever: they can be infectious without having any fever yet.

Minimizing the influx of infected people from abroad (e.g. through travel restrictions for foreign visitors) helps only in that early phase, where the internal number of infections is still small. However, that measure becomes insignificant once you get too far. It is simple: it can help only as long as a substantial proportion of the infections is caused by foreigners. Anyway, exponential growth can surely not be stopped with that, once it has really started. Nobody would try to stop a laser pulse by shielding the laser against ambient light!

We can reduce the multiplication factor, mostly with measures of social distancing and with improved hygiene. The goal must obviously be to get that kind of death laser below threshold. Clearly, for that purpose we do not need to be perfect, suppressing any transmissions. However, we need to be sufficiently effective. Reaching 90 % of that will not solve 90 % of the problem, as some may think, because it still means exponential growth. How much would that actually help?

• Still, it would be very valuable because we would win substantial time; flattening the curve would also substantially reduce the required medical capacities, because recovered (or passed) patients free beds for new patients. Over longer times, we can treat more people with the same capacities.
• However, we would still need to accept that a large percentage of the population gets infected sooner or later – which inevitably implies huge numbers of fatalities (unless we soon find a good cure, see below).

I find it appalling that this crucial difference is so often overlooked not only in statements of politicians, but also even of medical experts. That may lead us to doing not quite enough, with the consequence of millions of additional deaths. Aren't they aware of that responsibility?! Or are they more afraid of inducing widespread panic? So far, I think we are far from panic, and much closer to not realizing the real danger.

An interesting aspect is that in contrast to various others approaches, fighting the growth rate with social distancing still works after many have been infected. Also, the above mentioned nasty virus characteristics do not matter much in this context. So it is not too late for this to work.

Unfortunately, while a single country has a good chance of getting the crisis under control, viruses will not be completely wiped out, e.g. coming back from other countries, from which we can never completely isolate ourselves. Therefore, a negative growth rate may have to be maintained over longer times – not just a few weeks, which would be sufficient if we did that effectively worldwide.

A crucial risk in the corona crisis is that we sooner or later exceed the capacities of our medical systems, and that by a very large margin. So a natural reaction seems to be to increase those capacities. But wait a moment – how much can that help?

Imagine what happens if we don't stop the exponential growth and e.g. millions of people in the United State will require intensive care (e.g. 1 % of the population – we may easily top that)? It is easy to see that this would completely exceed the hospital capacities very soon. You may see 100 patients competing for a single hospital bed. But if you would manage to instantly expand those capacities by a factor of ten (which you won't), it would just take roughly 9 days more to reach that higher limit, if we have a growth rate of 30% per day (a bit less than recently calculated for Germany). Obviously, the expansion of capacities can never be enough to more or less solve the problem and avert catastrophic developments. Even if we could still ensure proper care for everyone affected – which we surely cannot –, numerous people would die despite intensive care. The clear conclusion: the expansion of hospital capacities makes sense only in addition to other, really effective measures.

Nevertheless, some public statements create that entirely false impression to those who are not used to think themselves. I find that unbelievably irresponsible.

The British government has recently explained its new strategy: letting the infections spread in the population, except for elderly and sick persons who should be isolated from all others. The idea is that once the younger ones, most of whom would get through the infection without major problems, would afterwards create safety for the whole society in the form of a “herd immunity”.

While the theoretical idea is at least not logically defect, it is obviously not workable, since one cannot isolate all the high-risk persons so well from all the others that their risk to get infected among numerous infected younger persons remains tolerable. For example, where to draw the line – for example on which side of it should all those be placed who care for the ill and elderly? Or should the ill and elderly largely care for themselves for a couple of months, until more and more immune younger ones become available? And how about the large number of younger ones – not in relative, but in absolute terms – who don't cope that well with the virus?

Well, a herd immunity strategy could in principle also be implemented in a softer form: try to control the epidemic in such a way that you always have a substantial number of (mostly younger) sick people, but always within the limits of the health system – hoping that this way you build up herd immunity within a reasonable time. However, this obviously also requires stopping the exponential growth – only slightly later – and therefore about the same strict measures (lock-down). Moreover, this state would need to be maintained for a long time. For example, if Germany would constantly have 400,000 thick people, with roughly 20,000 of those getting intensive care (after a moderate expansion of our capacities), each one for two weeks (probably a bit too optimistic), the system would get through about 800,000 per month, and it would take about 4 years to get half of the population immunized! So we would have to multiply our health care capacities even to get this done within one year. Therefore, it is much wiser to press the infection rate further done – with about the same strictness of measures – and do the immunization later on with a vaccine. By the way, controlling the epidemic so precisely would be another challenge.

For such reasons, this plan is harshly criticized by most experts, including the WHO. I think it was just another insane and dangerous idea of a government formed by irresponsible clowns. It is easy to predict that either they will abandon their plan pretty soon or quickly lead their country into a massive disaster, with the NHS breaking down under the load of seriously ill young and old people. (Addition of 2020-03-17: Indeed, they changed course while I was writing this.)

My estimate are of course based on a rather crude model. However, I have found a much more refined model from Swiss epidemiology experts, which fully confirms the essential results, only in that case for Switzerland. There, you see interesting curves, showing how the epidemic is estimated to progress with different levels of suppressing infections: getting below threshold or not, which of course makes a huge difference. However, they don't tell how much needs to be done to achieve what result. Another interesting study, just published by a group at Imperial College London, goes much further in that direction.

Addition of 2020-03-19: The Dutch government now also goes for herd immunity, but the they seem to be aware that they also will soon have to switch to lock-down as well. Addition on the same evening: Indeed, they also gave up that concept today.

Addition of 2020-03-24, amended 2020-04-30 and 2020-05-17: Donald Trump initially seemed to consider the same herd immunity strategy, which would have turned the United States into the area of a mass experiment with predictably historic impact. However, he was eventually persuaded not to dare that, and is on a meandering course since then – spreading propaganda against the lock-down, while not actively enforcing its end, and promoting silly and dangerous miracle cures. Anyway, the US lock-down became no less severe than elsewhere, although it came too late to prevent enormous fatality numbers. It also demonstrates that it is not just a choice between a health emergency and economic disaster – with suitable political decisions, you can have both combined:

• Tens of thousands of additional fatalities (and enormous suffering by many more who finally survived) have been caused by the government failure to act swiftly and effectively. This is a tremendous responsibility. Most persons would be shattered by having caused the loss of a single human live; other don't seem to care much about having caused tens of thousands of death cases with their neglect.
• Eventually, pressing that R factor below unity must succeed if a catastrophe is to be avoided, and the measures to achieve that at the high infection level are no less severe than if one had done that early on.
• The economic advantage of having started the lockdown a couple of weeks later is probably not sufficient to compensate the consequences. We already see that loss of jobs in the US is terrible – certainly more severe than in Germany, for example.

Addition of 2020-03-27, amended 2020-05-17: The Swedish case appears to be less clear. There, the growth of infections seems to have developed significantly more slowly than expected for exponential growth. Maybe their strategy for reducing infection rates is more effective than one would expect, e.g. based on schools continuing to operate more or less normally at least for the younger pupils. While the idea to solve the problem via herd immunity, created by infections rather than vaccination, is as crazy there as it is anywhere, the reproduction factor is not staying approximately constant as it would for a “let it go” strategy, and that at least gives them more time e.g. to correct their strategy if necessary. Maybe with some further adjustments they can get it below unity, if they turn out to be right with the assumption that young people do not often infect the elder ones. Note also that some conditions in Sweden appear to be favorable, e.g. that the contact between people is traditionally not that intense. However, it does not seem to be quite enough; the government had to admit that the isolation of elderly and sick people against the pandemic hasn't really worked, resulting in much more fatalities than in otherwise comparable countries like Norway. In any case I am sure that a fundamental fact holds for Sweden as for any other country: either you stop exponential growth in time or you ride into a disaster. And doing that exercise somewhat later may in the end well not be cheaper.

Let us turn back to lasers for a moment. Image that you have a Q-switched laser, where you want to lengthen the emitted pulse by a factor of 10 in order to limit its peak power. For that, you need to reduce the round-trip gain (measured in decibels) by a factor of 10, e.g. by inserting an additional loss which compensates for 90% of the gain. If you do a little more – e.g. compensating for 101% of the gain – you can totally suppress the pulse. So in terms of required measures there is no substantial difference – translated to the virus crisis, between controlling the epidemic for creating herd immunity and largely suppressing it.

We heard the argument against a lock-down strategy that the epidemic will come back as soon as you stop it. That's indeed a big concern: if we can't suppress the epidemic worldwide, which we probably won't achieve due to a lack of international cooperation, it will sooner or later come back, and we have to start a new lock-down. However, quite obviously the herd immunity strategy is not immune against that problem, since exponential growth needs to be stopped in any case to avert a catastrophic development.

Experts tell us that it is likely to take at least a year from now, if not substantially more, to obtain a widely usable vaccination which could end this crisis. So it is great that various people continue with that, but don't bet on it for 2020.

There seem to be higher chances to get some medication developed which helps patients substantially – maybe even with some already existing drugs which only need to be tested and proven to be effective and sufficiently safe. I imagine that we might have something like that until summer or so (very rough estimate, very uncertain). However, if this works, it will still not avoid a catastrophe if we get many millions of patients until then. Only perhaps if it were a wonder cure – throw in a few pills, and the patient will stand up and happily walk away! Unfortunately, such miracle cures have so far only been advertised, but never convincingly demonstrated.

It is essential to foster intense cooperation on many levels – at the level of communities, administrations, governments and also on the international level. We need this for many reasons, including the faster data collection, learning on mechanisms, effectivity of measures etc., and for avoiding that viruses will be coming back from regions which are left behind. Nationalistic and populist approaches are bound to fail here just as in many other areas. I suppose that most people working in photonics are aware of that …

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