As the northern hemisphere gets colder, covid-19 deaths are rising rapidly

ON NOVEMBER 8TH the world recorded its fifty-millionth case of covid-19, according to a tally kept by Johns Hopkins University. The deaths of 1.26m people have been attributed to the disease, although the true total may be two or three times higher. After a more or less normal summer for mortality, deaths in Europe are now rising rapidly: in the past week alone, the continent recorded roughly 4,000 every day (see chart). Meanwhile, in America, the pandemic is once more resurgent. In the past week 750,000 new cases have been reported. Hospitals are filling up and deaths are rising.

Yet as the pandemic has accelerated in the northern hemisphere, it has slowed below the equator. In South America, which emerged as a new centre of the pandemic in the summer, deaths have fallen by 60% since their peak in early September. Australia has not recorded a death from covid-19 since October 27th, allowing Melbourne, the country’s second most populous city, at last to lift its 112-day-long lockdown. Among the 39 countries in the southern hemisphere—where summer is now beginning—reported deaths are now 61% below their July peak.

Covid’s deadly path across the world appears to have followed the changing of the seasons. The Economist has calculated the pandemic’s geographic centre by taking the average latitude and longitude of the countries affected by the virus, weighted by their official death toll. Using this approach, we found that after the disease first broke out in China, its centre of gravity shifted rapidly west to Italy by late February (see map). After the virus spread to America in March, the centre of gravity drifted farther west across the Atlantic, before being pulled south in May as outbreaks emerged in Central and South America. Since the September equinox the pandemic’s centre of gravity has moved north-eastwards to Europe once more.

But the precise relationship between covid-19 and weather is disputed. It is thought that weather can affect the transmission of covid-19 in two ways: it affects the conditions in which the virus survives on surfaces and in the air; and as it gets colder people spend more time indoors, where it is easier for the virus to spread. Other viruses, particularly flu and common colds, peak in the winter months, which also suggests that covid-19 could follow suit.

Some studies have indeed shown that transmission appears to decline as temperature and humidity rise. But a new paper in the International Journal of Environmental Research and Public Health suggests that the effect is mild. After controlling for a battery of factors such as mobility and population density, the researchers found that air temperature alone had only a very limited effect on the spread of covid-19 across the world and within America. And that small effect appears to arise because of the influence of weather on people’s behaviour, rather than on the virus itself.