Coronavirus: Will summer make a difference?

The idea warmer weather might stop the coronavirus seems to have faded as the virus has spread around the globe. But could new research contain a glimmer of hope?

It's too early to know for sure whether the new coronavirus is seasonal. To really know that, we'd have to watch how cases change in one place across the year.

But we can look at its spread in different climates across the world for clues.

There is some evidence coronavirus cases have particularly clustered around cooler, drier regions.

One study indicated countries particularly affected by the virus - those where it was spreading undetected via community transmission - by 10 March had lower average temperatures than those with fewer cases.

Another paper looked at 100 Chinese cities with more than 40 cases of Covid-19 and suggested the higher the temperature and humidity, the lower the rate of transmission.

And another, not yet peer-reviewed, study suggested that although cases of the new coronavirus could be found all over the world, outbreaks had particularly clustered in "relatively cool and dry areas" - at least until 23 March.

But, as a group of researchers at the London School of Hygiene and Tropical Medicine pointed out, the virus has now spread to every World Health Organization region, "effectively spanning all climatic zones, from cold and dry to hot and humid regions".

With lots of other viruses, including flu, a seasonal pattern is seen in the northern and southern hemispheres. But tropical regions close to the equator don't experience the same pattern.

And some of the hot and humid regions that have seen locally transmitted cases of the Covid-19 virus, such as Malaysia and the Democratic Republic of Congo, are close to the equator and so might not provide the best evidence for what will happen elsewhere.

But looking to the southern hemisphere, Australia and New Zealand - at the tail-end of their summer season when their first cases were seen - have had far fewer cases than many of their northern-hemisphere counterparts.

There are lots of other factors at play, such as global footfall and the density of the population.

And since the virus has gradually spread around the world - initially through global travel - at the same time as seasons have been changing, it's difficult to pinpoint the effect of climate specifically.

There is some evidence other coronaviruses mainly circulate in the winter months, according to a team of researchers from University College London (UCL) and the London School of Hygiene and Tropical Medicine.

The researchers asked almost 2,000 people for a weekly report on whether anyone in their household had symptoms of respiratory illness. And anyone with symptoms was asked to send in a swab for testing for a range of viruses.

From this, the researchers observed large peaks in coronavirus cases in the winter, around the same time as flu season. There were a small number of cases in the summer.

One of the study's authors, Ellen Fragaszy, at UCL said it was therefore "possible we will see a bit of a relief in cases over the summer". But we could not be sure this is how the new coronavirus would behave.

And the large number of cases and their spread across the world suggested we shouldn't be too hopeful of a summer respite.

The new coronavirus, called Sars-Cov-2, which causes the Covid-19 disease, appears to spread in basically the same way as other coronaviruses.

But what makes it distinct is the how ill it makes you and the number of deaths it causes.

Dr Michael Head, at the University of Southampton, said the development and impact of the novel coronavirus was "clearly very different from the existing 'common cold' type coronaviruses".

"It remains to be seen as to whether Covid-19 cases will decline in response to environmental changes such as temperature and humidity," he said.

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