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A worker sprays disinfectant at the entrance of Beijing West railway station in an effort to protect against the coronavirus on Monday, Feb. 10, 2020. In public health terms, the outbreak is on the verge of becoming a pandemic.GIULIA MARCHI/The New York Times News Service

In recent days, the coronavirus outbreak has taken a troubling turn, with a sharp jump in cases reported in countries as diverse as South Korea, Italy and Iran.

Equally alarming is the growing number of cases with no link to China, including one important case in Canada, a traveller who appears to have been infected in Iran.

What this suggests is that SARS-CoV-2 is gaining a foothold outside of China, where it originated. (The coronavirus is officially known as SARS-CoV-2; the illness it causes is COVID-19.)

In public-health terms, that means the outbreak is on the verge of becoming a pandemic.

Tedros Adhanom Ghebreyesus, director-general of the World Health Organization, on Friday said containing the outbreak is still possible but “our window of opportunity is narrowing.”

“This outbreak could go in any direction. It could even be messy.”

Dr. Tedros’s analysis is frank, but not comforting.

The WHO has, to date, put all its effort into containment – limiting the outbreak to China.

That was a wise strategy, one likened to firefighters aiming to extinguish a forest fire, to keep it from spreading. But what do you do when the embers result in fires springing up in multiple locations?

As of Sunday evening, there were 78,997 cases and 2,470 deaths worldwide, with all but 2,055 cases and 26 deaths in China. But the number of cases outside China is spreading far and wide, now in 33 countries, including 691 on the cruise ship Diamond Princess, 602 in South Korea, 146 in Japan, 134 in Italy, 89 in Singapore, 74 in Hong Kong, 43 in Iran, 35 in Thailand and 35 in the United States. Canada has 10 cases.

We don’t know yet if any of these hot spots will develop into independent, self-sustaining outbreaks.

Even more concerning is that there are places where the coronavirus is likely spreading that we don’t know about. One mathematical model suggests that two-thirds of cases that have been exported from China have yet to be detected.

So the big question becomes: If we can’t contain the spread, what happens next?

Hard hit countries such as South Korea and Italy are responding by imposing quarantines and cancelling public gatherings. But we know these measures have limited effectiveness and are not sustainable over the long term.

What becomes really important now is surveillance – identifying cases promptly – then aggressive case-based follow-up. That approach allowed the world to halt the spread and eradicate SARS back in 2003.

But SARS and SARS-CoV-2 are significantly different bugs.

SARS patients got sick quickly and seriously; the mortality rate was 10 per cent. In other words, it was easy to spot.

The COVID-19 mortality rate is around 2 per cent, and 20 per cent of the infected have severe symptoms. That means about 80 per cent have minor symptoms; the illness can spread stealthily.

The case last week in British Columbia, a woman in her 30s who had travelled to Iran, was only detected because an ER doctor had a hunch and requested a test.

Routine testing of everyone who travels to a country with a coronavirus case is unrealistic. But we need to do more rigorous screening of travel history, especially if there are signs of illness. We will also have to continue to depend on travellers themselves to be vigilant.

One of the most remarkable things about Canada’s nine cases of COVID-19 is how responsible people have been, practising self-isolation and red-flagging themselves to health authorities.

Globally, there are couple of possible scenarios. Spring is coming and that’s important because respiratory viruses prefer cold, dry weather, not hot, humid weather. But there could still be significant spread in the coming weeks, allowing the virus to become endemic.

Public-health officials are already on the lookout for a second wave of illness – common in viral outbreaks – which could deal another blow to China, and accelerate the spread to other countries.

The big test will likely come next fall.

Will SARS-CoV-2 return, as one of the bugs in the seasonal soup of respiratory viruses? If so, will it return with a deadly vengeance, or more mildly?

There are four coronaviruses that circulate routinely: OC43, 229E, HKU1, NL63 – and they are not especially virulent. SARS-CoV-2 could become the fifth. Or it could become influenza-like, deadly every year. Stay tuned.

In the meantime, it remains in our collective self-interest to slow transmission even if we can’t stop it. If nothing else, it flattens the epidemic and spreads the burden on the health system.