Global COVID-19 cases have halved and experts are looking for explanations

As the number of new coronavirus infections in Canada continues to fall, a similar phenomenon is occurring in many other parts of the world, leading experts to try to better understand why COVID-19 cases are plummeting now.

Stronger public health measures, stricter adherence to rules stemming from fear of more rapidly spreading variants and the natural seasonality of coronaviruses may be playing a role, observers say. In countries with relatively high rates of vaccination and infection, such as the United States and Britain, immunity may also be starting to slow the spread.

In the past six weeks, the number of new coronavirus infections reported globally has dropped by almost half, from about five million in the first week of January to about 2.7 million last week. Worldwide, daily case counts are the lowest since October, according to the World Health Organization.

Canada is part of this trend. The country saw new infections plummet from 57,519 in the week that started on January 4 to 20,776 last week – a 64 percent drop.

“We need to understand what is driving this transmission dynamic,” said Mike Ryan, head of WHO’s health emergency program, at a news conference on Monday. “Is it the natural seasonality and the wave pattern of the disease? Are we building a level of immunity in the population that prevents the disease from finding the next case? And do control measures have an impact on this? I think all of the above, to some extent, are true. “

While scientists seek to decode the downward trend in general cases, they are doing so in the context of an increase in infections caused by more contagious variants of SARS-CoV-2 that threaten to usher in a third wave of the pandemic.

“The problem we face is that when you have a fall like that, you start to see the measures relax,” said Gerald Evans, head of the infectious disease division at Queen’s University School of Medicine. “It puts you at risk.”

An explanation for the drop in cases is easy to detect by comparing the case curves of countries that celebrate Christmas. Put the charts on top of each other and their winter spikes converge around January 10th and 11th, two weeks after families and friends got together for the holiday season, whatever the rules in their respective countries.

The heights of these peaks differ enormously from place to place, but Canada, the United States, Britain, Ireland, South Africa, Brazil, Russia and much of Europe have seen a post-holiday increase, followed by a reduction in cases. (The post-holiday peak is not as marked in large European countries, including France, Italy, Spain and Germany, where cases also reached high levels in late November and early December.)

Coronavirus Tracker: How many COVID-19 cases are there in Canada and worldwide? The latest maps and charts

In countries where increases at Christmas and after the holiday have been particularly sharp, governments have imposed severe public health measures that have led to an equally sharp drop in cases. This was especially true in Britain, Ireland and South Africa, three countries where new variants of faster spread of SARS-CoV-2, the virus that causes COVID-19, have replaced an earlier version of the virus.

“They had huge, rigid blocks and responses. And I think people were terrified of these new variants, “said Ian Michelow, a pediatric infectious doctor and professor at Brown University who is originally from South Africa.” It’s a very worrying spectrum for people, and with good reason. There is no doubt about it. These are the most dangerous viruses [because] they spread more easily. “

Another piece of the puzzle may be the natural seasonality of SARS-CoV-2, said Matthew Binnicker, director of clinical virology at the Mayo Clinic in Minnesota. The four seasonal coronaviruses, which cause mild colds, tend to peak in winter and early spring, according to six years of Mayo Clinic data published last summer.

“We knew that this winter would be extremely difficult, because [SARS-CoV-2] it’s a respiratory virus, like the flu, like other coronaviruses, ”said Dr. Binnicker. In the northern hemisphere, influenza generally increases in December, peaks in early mid-January and decreases in mid-February. “And this is really what we saw with COVID,” said Dr. Binnicker.

Clearing up all the reasons why respiratory viruses tend to develop in winter can be tricky. SARS-CoV-2 has been shown to survive longer in colder temperatures. Dry air keeps viral particles in the air for longer, making them easier to inhale. Some studies have suggested that breathing cold, dry air affects the mucous membranes in the nostrils in a way that lowers your defenses against viruses.

But Peter Juni, scientific director of Ontario’s COVID-19 scientific advisory board and professor of medicine and epidemiology at the University of Toronto, says the most likely explanation is that when the weather gets cold and dark, people gather in barely indoor spaces. ventilated, where viruses breathe spread easily.

Crediting a seasonal effect to the drop in COVID-19 cases, he said, is “a chimera”.

So it is the idea that immunity – whether from vaccination or infection – is making it more difficult for the coronavirus to find Canadian victims, added Dr. Juni.

With only 3.4 doses of vaccine injected for every 100 Canadians and less than 900,000 confirmed infections in Canada since the pandemic began, the vast majority of Canadians remain susceptible to coronavirus.

In the United States, however, “it is possible” that immunity is contributing, at the margin, to the drop in cases, especially in cities that have experienced devastating spikes in previous waves, said Jennie Lavine, a biologist at Emory University in Atlanta. “That’s not what I would bet my money on, but it’s not inconceivable.”

Anyway, something is working for the United States: the country reported just over 55,000 cases of COVID-19 on Monday, compared to a peak of almost 300,000 in a single day on January 8.

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