WITH hindsight, we may come to see late summer in the northern hemisphere as the calm before the storm. While many countries in the north have suppressed the spread of covid-19 for now, there is growing evidence warning us that winter could undo that progress.
Researchers are racing to pinpoint what role temperature and humidity may have in the spread and severity of the illness. They are exploring how SARS-CoV-2, the virus that causes covid-19, will interact with other seasonal respiratory viruses. And people are scouring data from winter in the southern hemisphere to see what the north might face.
“Some leaders assumed that this would go away in hot weather. That could have led to complacency”
Advertisement
These questions are a matter of life and death. In the UK alone, in the event of a resurgence of the coronavirus this winter estimates that there could be as many as 251,000 deaths in hospitals – although the total under the range of possibilities within this scenario is more likely to be less than half that. Whatever the outcome, UK scientific advisers expect cases to increase this winter.
The weather is the obvious change coming. To predict how this will influence the virus’s spread, we need to disentangle the effects of weather from other confounding factors, such as how countries have responded, different demographics and variation in testing rates.
“Some of our motivation for studying this is, early in the pandemic, [US] President Trump and other global leaders were assuming, not backed up by evidence, that this would just go away in hot weather. That could have led to complacency,” says Rachel Lowe at the London School of Hygiene & Tropical Medicine. Trump’s appears to have been proven wrong. found the virus spreading in different countries across the globe regardless of temperature.
Yet despite a flood of research into links between temperature, humidity and various other environmental factors on the transmission of the coronavirus and the severity of covid-19, the results are inconclusive so far. led by Albertus Smit at the University of the Western Cape, South Africa, found only five that could offer “credible insight” and deemed it too early to say if there is a link.
Similarly, Francois Cohen at the University of Oxford and his colleagues concluded in that not enough testing is going on to provide the required data. Analysing weather means looking at day-to-day changes, but we have insufficient data on positive coronavirus cases on individual days. “The inference you can make is restricted,” says Cohen.
If it is too soon to tease out a link, what are the plausible ways winter weather could affect respiratory viruses like influenza and SARS-CoV-2?
“There have been studies showing warmer temperatures and humid climates prohibit these kinds of viruses. That’s why, in temperate regions, we tend to see them peaking in winter. But the mechanisms are not well understood,” says Lowe.
Colder temperatures could directly affect the survival of the coronavirus itself. , and lower temperatures are associated with increased shedding of flu virus by infected people. With winter sun delivering less virus-destroying ultraviolet light, the coronavirus may be able to survive on surfaces for longer. The winter peak in respiratory viruses might also be because colder weather hampers how our immune system functions – through a lack of vitamin D, for example. Or it could simply be about how we behave, crowding indoors in winter.
Indoor humidity, which is usually at its lowest in winter because of domestic heating, could affect things too. Gordan Lauc at the University of Zagreb in Croatia and his colleagues looked at 7000 people with covid-19 in hospitals. They as heating is turned down – after ruling out age and accounting for better care.
Lauc, a biologist and adviser to the Croatian government, says the dry indoor air common in winter dehydrates and weakens a mucosal barrier in our lungs that can stop viral infections. As humidity rose during the springtime, the barrier improved again, he says. “Of course, there are other factors. We cannot claim it is only weather, but I think the weather is a very important factor,” he says. There are caveats. For example, Lauc’s research didn’t account for comorbidities, such as obesity, which play a role in the severity of covid-19.
“251,000
Maximum UK hospital deaths this winter in a worst-case scenario”
If SARS-CoV-2 behaves anything like other seasonal viruses, then knowing exactly how cold it will be this winter will be important for planning, says Lowe. In the UK, for every 1°C drop in outside air temperature below 5°C, respiratory infections and emergency hospital admissions rise about 1 per cent. Whether that holds true for the coronavirus remains to be seen.
Look south for clues
Meanwhile, research efforts are intensifying. The UN World Meteorological Organization held in August looking at the potential impact of the weather. At the event, secretary general Petteri Taalas referred to seasonal environmental factors, saying: “So far, I haven’t clear indications whether there is such a thing behind covid-19.”
On the face of it, though, there seems another way to predict what winter holds: just look at what happened in the southern winter. “Some of the experience in Brazil is not all that encouraging, but on the other hand, New Zealand and Australia are doing pretty well,” says Andrew Hayward at University College London.
Drawing parallels is hard, however, as both Australia and New Zealand had a much more effective initial response to the pandemic than places like the UK and US. Using the south’s winter as an analogue for the north’s is likely to be further complicated by potentially differing background levels of immunity to the coronavirus in populations and socio-economic factors.
“There are scientists in Australia who believe the winter conditions are exacerbating the risk in Melbourne,” says Lowe, referring to the recent spike of cases in the state of Victoria. “I personally think it’s too early to say. I don’t think we have enough data.”
Perhaps the biggest issue this winter in the northern hemisphere will be how a virus new to humanity interacts with the existing ones that peak in the season: influenza, respiratory syncytial virus (RSV) and the four coronaviruses that already circulate widely and can cause the common cold. “Respiratory infections tend to transmit more easily in the winter. That applies to seasonal coronaviruses as well, which are the closest analogue we’ve got to SARS-CoV-2,” says Hayward.
Covid-19 first surged in the northern hemisphere at the end of the peak for respiratory viruses, resulting in countries entering lockdown. This winter promises the opposite. “By the time we hit the winter, maybe we’ve got back more to normal – with more crowded public transport, people using the shops, going into work more – and my concern is, if you add on the extra degree of transmissibility that winter brings, that could kick [the transmission rate] up to higher levels,” says Hayward.
An obvious problem is that other respiratory viruses cause symptoms, such as a dry cough and high temperature, that can also occur with covid-19. At the infection peak of a normal winter in the UK, there are around half a million people with a cough or fever, says Hayward. Overlapping symptoms are likely to increase the workload for the testing-and-contact-tracing schemes governments are relying on to keep outbreaks in check.
Another concern is whether catching flu makes you more susceptible to having severe covid-19, and vice versa. That is why governments, including the UK’s, are .
There is “no strong evidence” from the pandemic’s first wave that co-infection with another virus makes covid-19’s symptoms more severe, concluded , UK, and her colleagues. But the same study notes that those with both influenza and covid-19 in England and Scotland did have higher odds of death than those without flu, after adjusting for age and sex.
Evans and her colleagues also say that other viral infections may possibly spur the transmission of the coronavirus by increasing how much it is shed.
There is one other big question about how different viruses interact this winter. Sema Nickbakhsh at the University of Glasgow, UK, and her colleagues last year at a population level that could explain why respiratory viruses fall in those years. Similarly, infections by rhinoviruses, some of which cause the common cold, of being infected by the 2009 strain of H1N1 swine flu. How this happens isn’t fully understood and we don’t know if this has any relevance for the coronavirus.
That fuzziness makes the coming winter hard to predict, says Nickbakhsh. Based on her previous work, she says: “My first hypothesis would be no interactions with flu, but potential for interactions with RSV.” A harmful effect would be a problem as RSV is one of the world’s biggest killers of children under the age of 5.
A dampener on flu
There may be some good news, though. For one, social distancing and face mask-wearing because of covid-19 could lead to fewer flu infections this winter. That appears to have happened in and .
Restricted foreign travel and quarantines may help too. We don’t have a good handle on where flu goes in the northern summer. It could carry on circulating at low levels or be reseeded from the south each season – or both of these may play a part. If reseeding from the south is a big factor, international air travel restrictions may further dampen the north’s flu season.
“Around half a million people have a cough or fever at the peak of a normal winter in the UK”
Scientists are to the new coronavirus through T-cells, the immune cells that help the body fight invading pathogens. For now, we don’t know.
Testing will be key to working out some of these unknowns. Little simultaneous testing for influenza and the coronavirus was done in the UK at the end of last winter, partly because the flu season was ending and because laboratories were overburdened. “Going forwards, we really need to be able to test for all of the pathogens,” says Nickbakhsh.
The UK government’s goal is to have capacity for by October, up from . But without a significant improvement in testing and tracing as UK schools and society reopen, there could be a second peak this December more than twice the size of the first, according to research by Jasmina Panovska-Griffiths at University College London and her team.
“You don’t have much time to make decisions when infections are increasing. It could be catastrophic”
Winter is coming. How the northern hemisphere fares will hinge on whether nations prepare enough in advance and act fast enough when it arrives.
Monitoring for outbreaks and imposing local lockdowns is a good idea, but speed will be of the essence this winter, says Hayward. “You don’t have much time to make these decisions if things are increasing. In a matter of a few weeks, it could increase to really catastrophic levels. If you wait to see big increases in hospital admissions, that might be too late.”
