Global cases of COVID-19 have dropped by half and experts are looking for explanations

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

Stronger public health measures, stricter adherence to rules derived from the fear of more rapidly spreading variants, and the natural seasonality of coronaviruses could play a role, according to observers. In countries with relatively high rates of vaccination and infection, such as the United States and Britain, immunity could also begin to slow the spread.

In the last six weeks, the number of new coronavirus infections reported worldwide has dropped by almost half, from about five million in the first week of January to about 2.7 million last week. According to the World Health Organization, worldwide, general daily accounts are the lowest they have been since October.

Canada is part of this trend. The country has seen new infections go from 57,519 the week it began on Jan. 4 to 20,776 last week, a 64 percent drop.

“We need to understand what is driving these transmission dynamics,” Mike Ryan, head of the WHO health emergency program, said Monday. “Is seasonality natural and the pattern similar to the disease wave? Are we building a level of immunity in the population that prevents the disease from finding the next case? And do control measures influence it? I think that all the above, to a certain extent, is true ”.

While scientists are trying 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 trigger a third wave of pandemic.

“The problem we face is that when you get a fall like this, you start to see relaxation of the measures,” said Gerald Evans, president of the infectious diseases division at Queen’s University School of Medicine. “That puts you at risk.”

An explanation for the fall in cases is easy to spot by comparing the case curves of countries celebrating Christmas. Place the charts on top of each other and their winter peaks converge around January 10 and 11, two weeks after families and friends get together for the holiday season, whatever the year. rules of their respective countries.

The heights of these peaks differ greatly from place to place, but Canada, the United States, Britain, Ireland, South Africa, Brazil, Russia, and most of Europe experienced a post-holiday increase. followed by a reduction in cases. (The post-holiday rise is not so striking 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 Cases of COVID-19 Are There in Canada and Around the World? The latest maps and graphs

In countries where Christmas and post-holiday rises were especially strong, governments imposed strict public health measures that led to an equally sharp drop in cases. This was especially true in Britain, Ireland and South Africa, three countries where new, faster-spreading variants of SARS-CoV-2, the virus that causes COVID-19, have displaced an earlier version of the virus.

“They had huge and strict blockages and responses. And I think people were terrified of these new variants, “said Ian Michelow, a pediatric infectious disease doctor and professor at Brown University in South Africa.” It’s a spectrum that worries people a lot, and with reason.There is no doubt about it.These are more dangerous viruses [because] they spread more easily ”.

Another piece of the puzzle could 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 data from the Mayo Clinic published briefly last summer.

“We knew this winter would be extremely difficult, because [SARS-CoV-2] it’s a respiratory virus, like the flu, like other coronaviruses, “Dr. Binnicker said. In the northern hemisphere, the flu usually rises in December, peaking in early mid-January and falling in mid-February.” And that it’s really what we’ve seen with COVID, ”Dr. Binnicker said.

Untangling all the reasons why respiratory viruses tend to thrive in the winter can be tricky. SARS-CoV-2 has been shown to survive longer in colder temperatures. Dry air keeps viral particles higher, making it easier to inhale. Some studies have suggested that breathing cold, dry air affects the mucous membranes of the nostrils so that it decreases its defense 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 it gathers in poorly ventilated indoor spaces where viruses are found to spread easily.

He said accrediting a seasonal effect by the fall in COVID-19 cases is “a dream.”

So is the idea that immunity, whether through vaccination or infection, makes it harder for the coronavirus to find Canadian victims, Dr. Juni added.

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

In the United States, however, “it’s possible” that immunity will contribute, on the sidelines, to falling cases, especially in cities that suffered devastating waves in previous waves, said Jennie Lavine, a biologist at Emory University in Atlanta. . “It’s not where I would put my money, but it’s not inconceivable.”

Either way, something works for the United States: the country reported more than 55,000 cases of COVID-19 on Monday, up from a high of about 300,000 in a single day on Jan. 8.

Sign up for Coronavirus Update Bulletin to read the essential news of the day, the functions and tellers of the coronavirus written by Globe journalists and editors.