Low southern hemisphere incidence of COVID-19: Is it the sunshine and heat?

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There has been much speculation that the low confirmed incidence of COVID-19 infections in Africa and the rest of the southern hemisphere may be because the virus is vulnerable to heat and sun, writes MedicalBrief. And in the northern hemisphere, hopes are being expressed that the coming of summer will offer respite from infections.

In the northern hemisphere, the flu season generally subsides in April and March, but, asks a National Geographic report, will the coronavirus go with it? Past coronavirus outbreaks can offer clues.

The report says whether the coronavirus that’s quickly spreading around the world will follow the flu season and subside with spring’s arrival is unsatisfyingly uncertain, and many scientists say it’s too soon to know how the dangerous virus will behave in warmer weather.

Dozens of viruses exist in the coronavirus family, but only seven afflict humans. Four are known to cause mild colds in people, while others are more novel, deadly, and thought to be transmitted from animals like bats and camels. Health officials have labelled this new virus SARS-CoV-2 and its disease COVID-19.

The prospect that summer could stave off a pandemic is enticing. Earlier this month, President Donald Trump tweeted about China’s efforts to contain the virus, saying they would be successful, “especially as the weather starts to warm.” Viruses that cause influenza or milder coronavirus colds do tend to subside in warmer months because these types of viruses have what scientists refer to as “seasonality,” so the president’s comments have some scientific backing. But, the report says, it’s highly uncertain that SARS-CoV-2 will behave the same way. Those currently studying the disease say their research is too early to predict how the virus will respond to changing weather.

“I hope it will show seasonality, but it’s hard to know,” says Stuart Weston, a postdoctoral fellow at the University of Maryland School of Medicine, where the virus is being actively studied.

Research about why some viruses are seasonal has been largely centred around those that cause the flu, a disease long associated with winter months. “Flu season” generally lasts from October to April or March. Scientists have a number of theories for why that is. Some suggest it is closer quarters – to escape the cold weather, people cluster indoors, where human-to-human transmission becomes more likely.

To understand why northern latitudes see an uptick in flu cases during winter, researchers have looked at how the virus spreads in different temperature and humidity levels.

And relatively recent research suggests that dry, cold air may also help viruses stay intact in the air or travel farther as they become airborne.

Scientists hypothesise that low humidity, which often occurs in winter, might impair the function of the mucus in your nose, which your body uses to trap and expel foreign bodies like viruses or bacteria. Cold, dry air can make that normally gooey mucus drier and less efficient at trapping a virus.

Ian Lipkin, director of the Columbia University Centre for Infection and Immunity, has been studying the novel coronavirus. He says sunlight, which is less abundant in winter, can also help break down viruses that have been transmitted to surfaces. “UV light breaks down nucleic acid. It almost sterilises (surfaces). If you’re outside, it’s generally cleaner than inside simply because of that UV light,” he says. UV light is so effective at killing bacteria and viruses it’s often used in hospitals to sterilise equipment.

However, the report says, though the coronavirus and the flu are both respiratory infections, not enough is known about SARS-CoV-2 to predict if it will have the same seasonal patterns. To better understand this outbreak, scientists are looking at comparable outbreaks like SARS and MERS. SARS, which started its spread in late 2002, shares almost 90% of its DNA with the current virus. The SARS outbreak started in November and lasted until July, which only hints at seasonality, says Weston, and containment may have resulted from early intervention. In other words, did it disappear with warmer weather, or did treatment and prevention efforts simply work?

MERS began in September 2012 in Saudi Arabia, where temperatures are generally high. Unlike SARS, it was never fully contained, and new cases are occasionally reported. The novel coronavirus has also begun to circulate locally in the Middle East, namely in Iran and the United Arab Emirates. “We don’t see too much evidence of seasonality in MERS,” says Weston.

But whether SARS and MERS were truly seasonal or if this virus will imitate SARS is unclear. Weston says their lab is focused on developing treatments and vaccines for the virus, which he cautions won’t likely be available for a least a year, if not several.

The report says Harvard University epidemiologist Marc Lipsitch doesn’t think any weather changes will put a big dent in how the virus spreads. COVID-19 has now been documented around the world. If the virus is anything like a typical flu virus, it may worsen in Southern Hemisphere regions as the seasons change.

And David Heymann from the London School of Hygiene and Tropical Medicine says not enough is known about this new virus to predict how it will change with different weather conditions. “The risk of making predictions without an evidence base is that they could, if they prove to be wrong, be taken as verity and give a false security,” Heymann says via email. “The emphasis today should continue to be on containment to elimination where possible,” he cautions.

According to the US Centres for Disease Control (CDC), people are most contagious when they’re showing symptoms. However, some experts suspect official counts may underestimate the number of infected people, saying not everyone infected will develop a severe illness. “We’re only seeing the most severe cases,” says Weston. “There may be some infection going on that isn’t being detected.”

To prevent contracting an illness from any virus, the World Health Organisation recommends frequently washing your hands, avoiding close contact with those showing symptoms like coughing or sneezing, and seeking treatment if sick.

Meanwhile, recent leaked comments attributed to Professor John Nicholls from the University of Hong Kong’s department of pathology suggest he believes weather conditions will be a key factor in the demise of the coronavirus.

Speaking during a conference call organised by Hong Kong-based brokerage firm CLSA, Nicholls referenced the 2002–03 SARS outbreak and said environmental conditions – such as temperature, humidity and sunlight – are a “crucial factor” in a virus’ ability to survive and infect people.

“Sunlight will cut the virus” ability to grow in half, so the half-life will be two-and-a-half minutes and in the dark it’s about 13–20 minutes. Sunlight is really good at killing viruses,’ he said.

‘That’s why I believe that Australia and the southern hemisphere will not see any great infection rates because they have lots of sunlight and they are in the middle of summer. And Wuhan and Beijing are still cold, which is why there’s high infection rates.

‘In regards to temperature, the virus can remain intact at 4⁰C or 10⁰C for a longer period of time. But at 30⁰C degrees then you get inactivation. And high humidity, the virus doesn’t like it either.’

But while Australia’s warmer weather has theoretically helped protect the population from a wider outbreak to this point, the coming colder months may have the reverse effect.

Nicholls suggested the common cold is a better comparison to the new coronavirus than SARS or MERS, as he believes there has been a “severe underreporting” of cases in China that has contributed to inflated fatality rate estimates. “Basically, this is a severe form of the cold,” he said.

“The environment is a crucial factor. The evidence is to look at the common cold – it’s always during winter.”

However, Dr Kerry Hancock, chair of the RACGP Specific Interests Respiratory Medicine network, is quoted as saying she thinks the containment of coronavirus disease (COVID-19) in Australia has so far been a result of good quarantine practices and not necessarily related to current weather patterns. “Cooler weather provides more favourable conditions for the spread of most respiratory viruses, of which the most common are the coronaviruses – such as the common cold,” she said. “However … there is currently no available evidence about how temperature and humidity affects transmission of COVID-19.”

Instead, Hancock said good hygiene practices are a more important determining factor.

Hancock’s comments mirror those of Professor Lyn Gilbert, chair of the Infection Control Advisory Group (ICAG), who said hygiene is vital for preventing the spread of coronavirus.

Full National Geographic report

RACGP material

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