In November 2002, a farmer in Guangdong province, China, was admitted to the First People’s Hospital of Foshan after he developed breathing problems. Soon after, he died. Within just a few months, the same mysterious illness – which ultimately became known as SARS, or Severe Acute Respiratory Syndrome – had claimed 774 lives in 29 territories.

Reeling from an epidemic of a disease that had killed around 10% of sufferers, some scientists in 2003 feared that there could be a future outbreak of SARS, or of some other disease that causes respiratory failure. So they set in place a way for countries to report clusters of unexplained pneumonia cases to the World Health Organisation. Which is why, when a cluster of people suffering from unexplained pneumonia began arriving at hospitals in Wuhan, China in December 2019, the World Health Organisation already knew what it could be dealing with. The world had been here before, less than twenty years prior. COVID-19 was SARS, take two.

The story of the 2003 SARS outbreak now sounds scarily familiar: a previously unknown virus began in East Asia and caused widespread cases of pneumonia, killing many vulnerable people and spreading to other countries.  Both illnesses have similar symptoms and similar reproduction rates (that’s the number of people a sufferer will infect, on average). So why is COVID-19 spreading in a way SARS never did? And what can the 2003 SARS outbreak tell us about COVID-19?  Watch our video below to find out:

The clue as to why COVID-19 has spread so rapidly when SARS didn’t might be easier to find than you’d think. The answer may lie in one of the World Health Organisation’s papers published in 2003, during the height of the SARS outbreak. The World Health Organisation makes an important observation in one line on page 26:

“There are currently no reports of the transmission of SARS from asymptomatic individuals.”

As the SARS outbreak developed in 2003, it was increasingly clear that the most infectious sufferers were those who were very sick. That’s part of the reason so many of the victims of SARS were healthcare workers and not the general public. Chances are, you had to be so sick you needed to be in hospital before you were most likely to pass the infection on.

SARS did have asymptomatic sufferers, but there are no reports of them ever transmitting the virus to anyone else. Three SARS cases detected in the UK in 2003 were asymptomatic to mild, none of them having fevers above 38°C. It was noted that no secondary transmission was associated with any of these cases.

Fast forward to 2020, and the picture looks quite different. The possibility that COVID-19 could be transmitted from people who don’t have symptoms was an early concern of the medical community. Writing as early in the outbreak as February 20th, Virologists from the National Institute of Allergy and Infectious Diseases, National Institutes of Health, Hamilton, MT, and the Department of Viroscience, Erasmus Medical Center, Rotterdam, the Netherlands, wrote in the New England Journal of Medicine that contact tracing for COVID-19 would be complicated if many infected people remain asymptomatic or only had mild symptoms. They concluded that if people with COVID-19 don’t have symptoms but can still pass on the virus, the virus would be harder to contain.

Similar fears have been expressed by UK officials. On 5th March, the UK’s Chief Medical Officer Professor Chris Whitty explained to a UK Parliament committee the possibility that asymptomatic carriers had played a role in community transmission of COVID-19 in China and so might do elsewhere.  Watch his comments here:

These scientists could be right. There’s mounting evidence that asymptomatic COVID-19 cases may be accelerating the spread of the virus.

One study conducted by the University of Padua in March 2020 involved testing 3,300 inhabitants of Italian village Vò for the virus. They found several asymptomatic but infectious individuals that were able to pass COVID-19 on to other people. Professor of clinical immunology at the University of Florence Sergio Romagnani went on to say that by his estimation the great majority of people infected – between 50 and 75% – were asymptomatic but were able to pass the virus on.

There are similar findings in other countries. The CDC puts their estimate of asymptomatic infections at 12.6%, and on 16th March, an analysis in the American journal Science found that undocumented infections could account for as many as 79% of symptomatic infections that tested positive for COVID-19.

In a recent interview on the current COVID-19 pandemic, Professor Kim Woo-Joo of Korea University Guro Hospital gave South Korea’s estimate of the proportion of asymptomatic carriers as 20%. No matter the numbers, the presence of any asymptomatic carriers has major implications for public health officials. How can any government control a virus that is primarily passed on by people who don’t even know they’re sick?

It’s too early to say whether these reports are accurate. But the only way the international community will know is if countries start widespread testing across the entire population.

As the world recovers from the pandemic, the possibility of future viruses that are spread by asymptomatic carriers will likely be a major consideration by the scientific community. If emergency protocols are put in place after the COVID-19, just as they were with SARS, widespread testing may be a prominent recommendation. After all, testing everyone is the only way to identify asymptomatic carriers, so that they can be isolated and prevented from infecting others who may not go on to be asymptomatic.

In the meantime, the best thing we can all do to fight COVID-19 is to practice social distancing. Because you might feel fine, but these studies show that feeling healthy is no real measure of whether you have the virus or could pass it on. There’s always a risk you could transmit COVID-19 to someone more vulnerable without even knowing you had it. And until testing becomes widespread, we just won’t know who has it and who doesn’t. There’s a lot we don’t understand about this virus and the science is still in its infancy, but it’s better to play it safe. Which is why the best way to protect yourself and those you love is to assume you already have it – and to stay at home.

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