Many infectious diseases wax and wane with the seasons. Flu typically arrives with the colder winter months, as does the norovirus vomiting bug. Others, such as typhoid, tend to peak during the summer. Measles cases drop during the summer in temperate climates, while in tropical regions they peak in the dry season.
Perhaps unsurprisingly, many people are now asking whether we can expect similar seasonality with Covid-19. Since it first emerged in China around mid-December, the virus has spread quickly, with the number of cases now rising most sharply in Europe and the US.
Many of the largest outbreaks have been in regions where the weather is cooler, leading to speculation that the disease might begin to tail off with the arrival of summer. Many experts, however, have already cautioned against banking too much on the virus dying down over the summer.
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And they are right to be cautious. The virus that causes Covid-19 – which has been officially named SARS-CoV-2 – is too new to have any firm data on how cases will change with the seasons. The closely related Sars virus that spread in 2003 was contained quickly, meaning there is little information about how it was affected by the seasons.
But there are some clues from other coronaviruses that infect humans as to whether Covid-19 might eventually become seasonal.
A study conducted 10 years ago by Kate Templeton, from the Centre for Infectious Diseases at the University of Edinburgh, UK, found that three coronaviruses – all obtained from patients with respiratory tract infections at hospitals and GP surgeries in Edinburgh – showed “marked winter seasonality”. These viruses seemed to cause infections mainly between December and April – a similar pattern to that seen with influenza. A fourth coronavirus, which was mainly found in patients with reduced immune systems, was far more sporadic.
There are some early hints that Covid-19 may also vary with the seasons. The spread of outbreaks of the new disease around the world seems to suggest it has a preference for cool and dry conditions, although it is worth noting that the virus has appeared in countries with a wide range of climates, including hot humid ones.
An unpublished analysis comparing the weather in 500 locations around the world where there have been Covid-19 cases seems to suggest a link between the spread of the virus and temperature, wind speed and relative humidity. Another unpublished study has also shown higher temperatures are linked to lower incidence of Covid-19, but notes that temperature alone cannot account for the global variation in incidence.
Further as-yet-unpublished research predicts that temperate warm and cold climates are the most vulnerable to the current Covid-19 outbreak, followed by arid regions. Tropical parts of the world are likely to be least affected, the researchers say.
Pandemics often don’t follow the same seasonal patterns seen in more normal outbreaks
But without real data over a number of seasons, researchers are relying upon computer modelling to predict what might happen over the course of the year.
Extrapolating data about Covid-19’s seasonality based on endemic coronaviruses – meaning viruses which have been circulating in human populations for some time – is challenging. That's not least because endemic viruses are seasonal for a number of reasons that might not currently apply to the Covid-19 pandemic.
Pandemics often don’t follow the same seasonal patterns seen in more normal outbreaks. Spanish flu, for example, peaked during the summer months, while most flu outbreaks occur during the winter. (Read more about what Spanish flu can teach us about Covid-19.)
“Eventually we would expect to see Covid-19 becoming endemic,” says Jan Albert, a professor of infectious disease control who specialises in viruses at the Karolinska Institute in Stockholm. “And it would be really surprising if it didn't show seasonality then. The big question is whether the sensitivity of this virus to [the seasons] will influence its capacity to spread in a pandemic situation. We don’t know for sure, but it should be in the back of our heads that it is possible.”
We need to be cautious, therefore, when using what we know about the seasonal behaviour of other coronaviruses to make predictions about the current Covid-19 pandemic. But why are related coronaviruses seasonal, and why does that offer hope for this outbreak?
Coronaviruses are a family of so-called “enveloped viruses”. This means they are coated in an oily coat, known as a lipid bilayer, studded with proteins that stick out like spikes of a crown, helping to give them their name – corona is Latin for crown.
Research on other enveloped viruses suggests that this oily coat makes the viruses more susceptible to heat than those that do not have one. In colder conditions, the oily coat hardens into a rubber-like state, much like fat from cooked meat will harden as it cools, to protect the virus for longer when it is outside the body. Most enveloped viruses tend to show strong seasonality as a result of this.
Research has already shown that Sars-Cov-2 can survive for up to 72 hours on hard surfaces like plastic and stainless steel at temperatures of between 21-23C (70-73F) and in relative humidity of 40%. Exactly how the Covid-19 virus behaves at other temperatures and humidity has still to be tested, but research on other coronaviruses suggests they can survive for more than 28 days at 4C. (Read more about how long Covid-19 can survive on surfaces.)
A closely related coronavirus that caused the Sars outbreak in 2003 has also been found to survive best in cooler, drier conditions. For example, dried Sars virus on smooth surfaces remained infectious for over five days at between 22-25C and with a relative humidity of 40–50%. The higher the temperature and humidity, the shorter the virus survived.
“Climate comes into play because it affects the stability of the virus outside the human body when expelled by coughing or sneezing, for example,” says Miguel Araújo, who studies the effects of environmental change on biodiversity at the National Museum of Natural Sciences in Madrid, Spain. “The greater the time the virus remains stable in the environment, the greater its capacity to infect other people and become epidemic. While Sars-Cov-2 has quickly spread all over the world, the major outbreaks have mainly occurred in places exposed to cool and dry weather.”
A recent analysis suggests that this pandemic coronavirus will be less sensitive to the weather than many hope
His computer models certainly seem to match the pattern of outbreaks around the world, with the highest number of cases outside of the tropics.
Araújo believes that if Covid-19 shares a similar sensitivity to temperature and humidity, it could mean cases of coronavirus will flare up at different times around the world.
“It is reasonable to expect the two viruses will share similar behaviour,” he says. “But this is not a one-variable equation. The virus spreads from human to human. The more humans at any given place and the more they get into contact with each other, the more infections there will be. Their behaviour is key to understanding the propagation of the virus.”
A study from the University of Maryland has shown that the virus has spread most in cities and regions of the world where average temperatures have been around 5-11C (41-52F) and relative humidity has been low.
But there have been considerable numbers of cases in tropical regions, too. A recent analysis of the spread of the virus in Asia by researchers at Harvard Medical School suggests that this pandemic coronavirus will be less sensitive to the weather than many hope.
They conclude that the rapid growth of cases in cold and dry provinces of China, such as Jilin and Heilongjiang, alongside the rate of transmission in tropical locations, such as Guangxi and Singapore, suggest increases in temperature and humidity in the spring and summer will not lead to a decline in cases. They say it underlines the need for extensive public health interventions to control the disease. (Read more about why social distancing is so important.)
This is because the spread of a virus depends on far more than simply its ability to survive in the environment. And this is where understanding the seasonality of diseases becomes complicated. For a disease like Covid-19, it is people who are now spreading the virus, and so seasonal changes in human behaviour can also lead to shifts in infection rates.
Measles cases in Europe, for example, tend to coincide with school terms and decrease during the holidays when children are not spreading the virus to one another. The enormous migration of people around the Chinese Lunar New Year on 25 January has also been suggested to have played a key role in the spread of Covid-19 out of Wuhan to other cities in China and around the world.
The weather can also mess with our own immune systems to make us more vulnerable to infections, too. There is some evidence to suggest the vitamin D levels in our bodies can have an affect on how vulnerable we are to infectious diseases. In the winter our bodies make less vitamin D from sunlight exposure, mainly because we spend more time indoors and wrap ourselves in clothing against the cold air. But some studies have found this theory is unlikely to account for seasonal variation seen in diseases like flu.
More controversial is whether cold weather weakens our immune systems – some studies suggest it does, but others find the cold can actually boost the number of cells that defend our bodies from infection.
There is stronger evidence, however, that humidity can have a greater impact on our vulnerability to disease. When the air is particularly dry, it is thought to reduce the amount of mucus coating our lungs and airways. This sticky secretion forms a natural defence against infections and with less of it, we are more vulnerable to viruses.
Stopping contact between people should also bring down the infection rates
One intriguing study by scientists in China suggests there is some sort of relationship between how deadly Covid-19 can be and the weather conditions. They looked at nearly 2,300 deaths in Wuhan, China, and compared them to the humidity, temperature and pollution levels on the day they occurred.
Although it has yet to be published in an academic journal, their research suggests mortality rates were lower on days when the humidity levels and temperatures were higher. Their analysis also suggests that on days where the maximum and minimum temperature ranges were greater, there were higher levels of mortality. But this work is largely also based on computer modelling, so the exact nature of this relationship, and whether it will be seen in other parts of the world, is still to be explored.
As the virus causing the Covid-19 pandemic is new, it is unlikely many people, if anyone, will have immunity against it until they have been infected and have recovered. This means the virus will spread, infect and cause disease in a way quite unlike endemic viruses.
Air travel has been the main route by which the virus has spread around the world so rapidly, says Vittoria Colizza, director of research at the French Institute of Health and Medical Research. But once it starts spreading within a community, it is close contact between people that drives the transmission. Stopping contact between people should also bring down the infection rates. This is exactly what many governments have been attempting to do with the escalating lockdown of public places around the world.
“There’s no evidence yet for a seasonal behaviour of Covid-19,” says Colizza. “The behavioural component may play a role, too.” But she warns it is too early to know if the measures put in place will be enough to stall the spread of the virus. “By itself, it may partially reduce effective contagiousness due to the reduction of contacts along which the disease could be transmitted.”
And if cases of Covid-19 do indeed tail off over the coming months it could be for a number of reasons – prevention measures such as isolation and lockdowns are being successful; there is growing immunity in the population; or it may be an effect of the season, as Albert’s models suggest.
“If there is a seasonal effect, it could mask the true impact of the other two,” warns Albert. “In countries where a strong lockdown has meant not many people have been exposed, then I wouldn’t be surprised that we will see a second wave come the fall and winter.”
Even if Covid-19 does show some seasonal variability, it is unlikely to disappear entirely over the summer months, as some have suggested. But a dip in cases might bring some benefits.
“The steps we are taking to flatten out the curve are expensive in economic terms, but they could help us push this pandemic into the summer,” says Albert. “If there is some seasonality, it might buy health systems the time they need to prepare.”
And in a world scrambling to cope with the rapidly rising number of cases, it might just be time we desperately need.
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