Public health officials and experts watching the dark cloud of the coronavirus pandemic have picked out the tiniest of silver linings: This year’s influenza transmission appears to be one of the lowest in recorded history.
Experts said it is too early to draw conclusions about what the winter months might bring as colder weather descends on the Northern Hemisphere. But data from the Southern Hemisphere suggests the worst-case scenario — a wave of influenza piling onto health care systems already stressed by a surge of coronavirus cases — might not come to pass.
“They had amazingly low transmission rates,” Christine Petersen, an epidemiologist who studies influenza at the University of Iowa, said of the Southern Hemisphere. “There was no decent levels of transmission anywhere, except for Southeast Asia.”
Data collected by the World Health Organization show remarkably low levels of influenza-like illnesses this year, compared to years past. In March — typically the height of influenza season in the Southern Hemisphere — about 36 of every 1,000 outpatients showed an flu-like illness. In the spring of 2017, the most recent year of significant spread, that number was north of 200 per every 1,000 patients.
More recently, transmission rates have dropped even farther.
“Globally, despite continued or even increased testing for influenza in some countries, influenza activity remained at lower levels than expected for this time of year,” the WHO wrote in early November.
In the United States, data collected by the Centers for Disease Control and Prevention (CDC) so far show lower than average flu transmission. Just 1.5 percent of visits to a health care provider were for influenza-like illnesses in the past week, the CDC’s weekly FluView report shows.
Since September, 122,000 people have been tested for the flu, and only 319, less than 0.5 percent, have tested positive.
Experts said the low level of transmission is likely a combination of behavioral changes made during the coronavirus pandemic and a bit of scientific luck — the two dominant flu vaccines developed this year both cover the particular type of influenza that has circulated most widely, though data about how many Americans receive a flu shot will not be available until next year.
But the lion’s share of the credit belongs to the behavioral changes people have made around the world — wearing masks, it turns out, reduces the spread of respiratory infection.
“We all kind of stayed somewhat quiet about that because it requires people to wear masks and then it doesn’t transmit, because this is a respiratory disease,” Petersen said.
Health experts warn that there are still months of cold weather ahead, prime opportunity for the flu to gain footholds.
“People aren’t traveling as widely, so you may see slower spread of flu than normal,” said Kelli Drenner, a public health expert at the University of Houston. “Thanksgiving, all bets are off. That’s going to change.”
The low transmission rates are a small reason for optimism at a time when the coronavirus is spreading more rapidly than ever, just before one or more COVID-19 vaccines hit the market.
“We could still be stupid,” Petersen said. Or, she added: “We could actually see one of the lowest flu years ever.”