Above: daily confirmed cases world-wide, per million population, three-day moving average
About eight months after the Covid-19 pandemic started, in Wuhan, China, the infection count is rising fast again worldwide after two lulls.
It peaked in China in February, and China eased its lockdown measures from 11 March. In the second half of February, worldwide cases declined sharply, and they seemed still to be low in early March.
According to many calculations, that was an illusion. There were tens of thousands of undetected cases in Europe. From about 10 March until early April, detected cases rose exponentially in many European countries, and so did deaths, showing that there must have been many undetected cases three or four weeks earlier.
From a week or so later, there were similar rises in the United States.
Both Europe and the USA reached peaks in early to mid April, and then both case counts and death counts decreased.
Meanwhile, infections started increasing in other parts of the world, especially Latin America. But the world case-count remained fairly steady from early April to mid-May.
Since mid-May the case-count has been rising again, thanks mainly to increases in Latin America and new increases in the USA. The toll in Africa remains relatively low.
Deaths per day fell from 20 April to 31 May, but have risen since, but much less sharply than cases, and much less sharply than in March,
Both case-count and death figures are unreliable. We don't know how much of the recent case-count rise is due to tests finding more cases that otherwise would not be counted. We don't know how much of the slowness in the recent rise of death counts is down to countries like Brazil, Mexico, and India drastically under-reporting deaths. We don't know if the death rate from the virus is actually decreasing, perhaps because it is now predominantly spreading in younger populations.
A number of countries which seemed to have the virus under control have seen localised outbreaks over recent weeks.
The early-July outbreak in Melbourne, Australia, may have originated among contracted-out staff at quarantine hotels and in an abattoir, and then spread in high-density blocks of flats.
South Korea has had outbreaks centred round an e-commerce warehouse and a nightclub; Germany, round a meat-processing plant; China (Beijing), round a big food market.
Spain, Austria, and the Czech Republic have also had spurts of infection. Even New Zealand, which prided itself on taking advantage of its remote-islands status to reduce the infection count to zero, has had a couple of new (imported) cases.
So far all those localised outbreaks have brought relatively few new deaths. Officials say that is because they have been mostly among younger people.
Iran, Israel, and some states of the USA have had something different, that is, full-scale new surges of the virus. These seem to be not so much "second waves" (in the sense that the Spanish Flu of 1928-20, for example, had three distinct "waves"), as new surges of the first wave.
Most lockdown-easings have "worked", in the sense that lockdown measures have been eased without changing declining curves of infection. But really we don't know why - was it just that the measures now eased had little effect anyway? - or how much further easing any time soon can "work". It looks to me as if the lockdown-easings in Iran, Israel, and some states of the USA were simply more bungled than elsewhere. They don't have a second wave. They have the first wave resurgent.
Short of finding a widely available treatment for Covid-19 which cures the disease quickly in the big majority of cases - and there is no sign of that - every strategy has to aim, one way or another, for "herd immunity", that is, a condition where individuals may be susceptible but the population as a collective has "immunity" sufficient to make every spread of the disease peter out.
In the early stages some epidemiologists hoped that large percentages of the population would turn out to have gone through infections without symptoms, and their consequent individual immunity (even if temporary) would help a lot in getting population-immunity. If maybe 60% or 70% of people, predominantly young people who had had few or no symptoms, got infected, then that would block further spread. Even 20 to 40% rates would help considerably.
In fact most estimates now are that even in the hardest-hit countries, relatively small percentages of people - 5 to 10%, maybe - have had the infection. Policies which aimed to "shield" older and frail people seem to have worked nowhere or almost nowhere. In many countries, on the contrary, the chief "clusters" of the disease have been disproportionately among elderly people in care homes and in crowded neighbourhoods.
The other policies which can help towards "herd immunity" include physical barriers (covid-distancing policies, masks, PPE), and a vaccine. Work on vaccines is promising so far, but no-one knows whether any of the many vaccine projects will work, how well they may work, and how soon they may become available.
There is much discussion about how much testing, tracing, and isolation can substitute for blunderbuss across-the-board lockdowns in controlling infection. There is no doubt that they help, and that it is a good thing that testing capacity has vastly increased in many countries. It is much less certain that testing-tracing-isolation can be more than a subordinate element in infection control, even in countries like South Korea where high state surveillance of the population helps such policies.
The Independent SAGE group has, on 7 July, come out for a strategy of reducing infections in one country after another to zero, and regulating that by border controls and by energetic testing, tracing, and isolation policies. It offers no detailed argument on how that can be achieved in countries with porous borders, in contrast to the remote islands of New Zealand.