The second wave of coronavirus: how to recognize it and which countries have already encountered it

In Israel, Germany and other countries, quarantine measures are partially returned. Is this the second wave of the pandemic? Explains «Medusa».

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In the United States on June 27, a record number of confirmed cases of coronavirus infection was recorded - 45 thousand people fell ill during the day. In Germany, in connection with an outbreak at a meat processing plant, restrictions were imposed on 600 thousand people, and 7 thousand were quarantined. In Israel, where things seemed to be going well until June, the rules for wearing medical masks have been tightened since last week. These all seem to be signs of the second wave of the pandemic that has been talked about so much. In reality, the situation is more complicated: in some countries no second wave is yet to be seen, while in others you can see not only the second, but also the third wave. There are no universal reasons why this is happening - it seems that the situation is different everywhere, and much depends not even on officially announced measures to combat the epidemic, but on people's behavior.

In the latest issue of The Shortwave, a popular science podcast on coronavirus infection in the United States, the presenters began a conversation something like this: “Hello! We have an important message for you - the second wave does not exist. Just because the first wave [in the US] did not even end close. Thank you all, Maddy and Emily were with you, bye. ”

This really fully reflects the current situation in America: the number of daily recorded cases in the country in mid-April (at the conditional “first peak”) averaged about 30 thousand, approximately the same number of infections has been recorded in the last two weeks. At the same time, infection rates in the USA have never dropped below April 18 infections every day since April.

If you try to dig deeper and take the situation in different areas in the United States separately (the graphs are convenient to see here), then among different states you will find examples of very simple, monotonous growth of patients (California), and cases where it really seems to be visible the second (Alaska, Montana) or even the third or fourth (West Virginia) wave of patients. But before figuring out whether these are real waves or not, and determining their order, you still need to deal with what can generally be considered a wave.

What is the second wave?

The second wave is a concept that came from epidemiological modeling. When working with models, waves of new infections arise as a result of understandable changes in the model settings - for example, when quarantine measures are removed or resumed, which change the basic reproductive number R₀ and, thus, the rate of spread of the disease. Such changes lead to expected bursts that can be easily measured and numbered.

However, it is important to understand that scientists always have only to guess about the causes of changes in real statistics in a real epidemic. In this case, it is not clear what exactly should be considered the real second wave. Daily and even weekly statistics on registered cases of the disease is a very “noisy” parameter, subject to a lot of accidents, and if you look only at it, then if you want, you can find dozens of different “microwaves”. Therefore, before speaking seriously about the second wave, it is necessary to first agree on the principles of averaging data - that is, which periods are considered sufficient (but not too) large, and how significant a surge in cases of infection can be considered a full-fledged second wave, and which better attributed to chance.

As a simple conservative approach to averaging, you can stop by simply taking data for every two weeks. This is convenient, because during this period, most cases of the disease end in recovery or death.

If we take this approach to averaging, then the first spike in each individual country can be considered the two-week period after which the decline in the number of recorded cases first began. The lowest point, or “bottom,” of the epidemic is the moment when the number of cases in two weeks began to rise again. Fortunately, the recession is still ongoing in most countries, so they have not reached the bottom, but there are a few important exceptions.

By agreeing on such an approach, it is possible to calculate how the number of cases recorded over the past two weeks correlates with the number that was recorded at the bottom of the epidemic. The resulting ratio can be calculated for all countries where the growth in the number of cases has resumed - it will reflect (albeit very roughly) how much the situation has worsened compared to the most calm period from the epidemiological point of view.

Formally, we have long been living on the crest of the second wave. This is due to the fact that the initial outbreak in China was relatively quickly suppressed, even before significant growth began outside of it. The average number of registered cases at the end of February was no more than three thousand people a day, and compared with this level, the current 150 thousand a day can certainly be called a real “second wave." This once again reminds us that the epidemic could be relatively easily prevented if countries took action decisively and ahead of the curve.

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Which countries have already faced the second wave?

If we look at different countries separately, we can distinguish several of those where the secondary increase in the incidence rate has actually already begun after a relative lull.

Israel

This is the most striking example. In mid-May, about 17 cases a day were recorded in Israel on average, and over the past two weeks this average value has grown more than 20 times and has a tendency to increase (on June 25, for example, 532 cases were recorded). The authorities have already begun to react to such an unexpected secondary growth: on Monday, June 22, Prime Minister Benjamin Netanyahu issued a statement on the need to strengthen the distance between people, announced a fine of 500 shekels (about $ 145) for being in public places without a mask and predicted that many areas would again be included in the list of those where movement is limited.

Morocco

Here the situation is developing in a similar way. In April, Morocco also saw a significant surge in incidence, followed by a lull in early June. Over the past two weeks, about 300 cases a day are registered again in the country, despite the fact that the level of testing is constantly growing, and the proportion of positive tests from the total volume of testing only falls.

China

China is in the top of countries with greatly worsened situations, where recently only two or three dozen cases have been registered daily. This may look strange, but actually quite expected - and is explained by the fact that the secondary peak in this country began from a very low base. Before the resumption of air travel and the beginning of the return import of infection from abroad, only (a few cases) of the disease were recorded here (in May). Against this background, the increase to 20-30 cases per day, of course, looks significant.

Australia and South Korea

The data from Australia and South Korea are significantly more modest in scope, but the dynamics are similar - in both cases the current number of cases is not comparable to that observed at the first peak, and getting into the world top in terms of the incidence rate is explained by the almost complete elimination of internal transmission of the infection in the most favorable period after the peak.

Panama, Philippines, Sweden

A completely different situation has developed, for example, in Panama, the Philippines and Sweden. There, the second peak turned out to be significantly (at times) - higher than the strongly “smeared” first, and not only the dynamics, but also the absolute values ​​of the number of cases do not inspire optimism. We can confidently say that here high secondary growth is not exactly associated with a low base. But is there any other general explanation for such growth, it is not clear: these countries are too different. The only thing that unites them is a very high proportion of positive results among all tests performed. This may indicate that testing is not carried out in sufficient volume: the more tests, the more patients are detected, but the proportion of positive tests decreases. Therefore, we don’t really know how fully published statistics reflect the real situation and whether it is fair to talk about the second wave at all.

Why is there a second wave?

It is natural to assume that the emergence of the second wave is due to the fact that in countries where it is recorded, governments removed quarantine measures too early. The example of Israel speaks in favor of this version: in mid-May, kindergartens began to work here, then the beaches opened and the work of public transport resumed. A few weeks after this, the growth of patients began, and the daily number of new cases is already measured in hundreds (on June 28, 621 people fell ill). Experts advising the government believe that in a few weeks it can reach several thousand a day.

But to confirm this hypothesis, based on objective data from different countries, it does not work: people behave in them too differently.

It should be emphasized that we are talking about the formal severity of the measures introduced by governments, and not about the real level of compliance. The latter can be indirectly judged by another parameter - the mobility of residents, which is reflected in the data of mobile operators or navigation services. But here, the differences in the development of the situation in different countries are too significant to make any general conclusions about why the second wave arises in some countries and not in others.

For example, if we compare Israel and Morocco with similar epidemic dynamics, it can be seen that, not only in terms of formal severity, but also in real mobility of the population, quarantine in Morocco was significantly more stringent than in Israel, but this did not help prevent a new outbreak even with a fairly high level of testing.

On the other hand, if we compare the mobility of people in Germany and Russia, we can see that they are very similar - both in terms of depth of decline in activity and in duration of recession (the only difference is that in Germany recession started earlier). The results of such similar “quarantines” turned out to be very different: in Germany now they are talking more and more about the “second peak”, due to which quarantine measures are again tightened in some districts (due to the outbreak at the Tönnies meat processing factory). But this “second peak” is also an order of magnitude (15 times, when compared over two-week periods) less than the “decline” that is observed today in Russia.

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Why the second wave does not occur everywhere?

The fashion for talking about several waves of the epidemic was largely set by the epidemiologists themselves. And now they have to explain why in some countries secondary outbreaks of infection occur, and in others it seems not, although the restrictive measures introduced look very similar.

One of the most famous works that predicted a significant increase in cases is the report of a group of Neil Ferguson from Imperial College in London on the consequences of the uncontrolled spread of infection in the UK. Such bursts are predicted by other, more primitive models, and in all cases they are based on the simple fact that without the formation of collective immunity (a significant number of ill or vaccinated in the population), the removal of quarantine measures inevitably leads to an increase in the incidence of nothing .

But Ferguson's model (one of the most detailed at the time of its publication), and later theoretical models of the epidemic did not take into account everything. Including such important factors as it is now becoming clear, such as the heterogeneity of society and the behavior of people that has changed greatly since the beginning of the epidemic.

Heterogeneity

The spread of COVID-19 seems to be far from the same among people of different occupations and different social groups. This, as it turned out, is very important to take into account when forecasting: some of the carriers, judging by the latest data, may be responsible for a much larger proportion of infections than might be expected. And that means that those average estimates of the coronavirus spreading rate, which are based on the results of their “work,” can be overestimated relative to the real ones - just because at the first stage the spread of coronavirus was provided by just the active “superspreders”.

Behavior of people

It is very difficult to predict the reaction to an unprecedented pandemic, Ferguson himself and his colleagues honestly admitted. And in the same forecast, scientists stipulated that the effectiveness of the introduction of future quarantine measures will greatly depend on how people react to them. But it was impossible to take this reaction into account in advance because there were simply no examples of such large-scale epidemics in the recent past.

Then the researchers were worried, first of all, that no one, as they expected, would observe quarantine for as long as required. However, perhaps in practice it was more important not to refuse to visit public places and communicate with people, but to change habits. Until now, no model has taken into account the effect of universal wearing masks and gloves, maintaining distance in communication and other small, simple but very massive measures. Perhaps they explain the inconsistency of the current situation in prosperous countries with gloomy scenarios that epidemiologists had only recently expected.

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