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In the northern hemisphere, as winter ends, cases of seasonal flu dwindle. Could the same happen with covid-19?

Flu surges in winter for three reasons. First, the virus is more stable in cold, dry conditions with low levels of ultraviolet light. Second, people spend more time together indoors, which facilitates viral spread. Third, our immune systems may be weakened due to the mild vitamin D deficiency a lack of sunlight can cause.

In theory, these factors could also cause the covid-19 virus to dampen down in spring. But we don’t know if this will happen, and the evidence so far is conflicting.

In the first study to look at the effect of weather on covid-19, in February, researchers at Harvard University looked at the effects of temperature and humidity on the virus’s transmission in China, Thailand, Singapore, Japan, South Korea and Taiwan, based on weather reports and data on covid-19 incidence between 23 January and 10 February.

They found no significant difference in transmission rates between cold and dry provinces of China and tropical ones, as well as Singapore, concluding that higher temperature and humidity “will not necessarily lead to declines in case counts”.

However, another , which analysed data from Wuhan, the Chinese city where the coronavirus emerged. Alarmingly, it found that the virus seems to spread better in summery weather, with an optimum temperature of 19˚C, humidity of 75 per cent and less than 30 millimetres of monthly rain. Even more worryingly, the researchers found that cold air destroys the virus. They recommended that, as the weather warms, containment measures should be ramped up.

Since then, at least eleven similar studies have been posted online. Most have found the opposite.

For example, one looked at all 80,981 cases of covid-19 across mainland China between 20 January and 29 February. It found that the optimum temperature for virus transmission is 10˚C, and that lower or higher temperatures suppress it. It found no link at all to humidity.

Another up to 29 February. It found that higher temperatures are associated with lower disease incidence. But the researchers say any conclusions are provisional due to limited data.

“We are currently revising the analysis with more recent data,” says lead researcher Melanie Bannister-Tyrrell of Ausvet, an epidemiology consultancy in Australia. “We prefer to reserve comment until we have the revised analysis available.”

This warning is echoed by biologist Francois Balloux at University College London. “Seasonality is difficult to predict,” he says. Keeping tracking of covid-19 cases over time and as seasons change could create a more accurate picture.

The fact that there is an ongoing epidemic in parts of the southern hemisphere also gives some public health experts cause to worry. “If, for example, we look at the epidemic in – where it is still their summer, moving towards their autumn – there are a lot of cases and they’re having an acceleration of an epidemic there,” says Jimmy Whitworth at the London School of Hygiene and Tropical Medicine.

“So I take from that that warm weather is not going to be highly protective for us.” But, he adds, “this is an unfamiliar virus, so we don’t know”.

It is possible, however, that the virus will eventually become seasonal like flu, says virologist Michael Skinner at Imperial College London. “It will probably become seasonal when it eventually settles down to the normal patterns of transmission we see for the other human respiratory coronaviruses, in a population that consists of immune and immunologically naive individuals.

“That does not mean that it will follow seasonal dynamics during the larger epidemics – there may be just too many people infected so that most transmission is short range and less subject to environmental constraints,” says Skinner.

For now, the World ��ҹ����1000���� Organization says on its website that the virus can be transmitted in all areas, “including areas with hot and humid weather”.

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