Trivia

The curious case of the missing Influenza
February 6, 2021 - 4:48 pm

By mid-December, the Northern Hemisphere is usually well into the start of its annual cold and flu season — but so far this year, even as the COVID-19 pandemic surges in dozens of countries, the levels of many common seasonal infections remain extremely low.

The pandemic caused by the SARS-CoV-2 coronavirus has infected at least 67 million people and killed 1.5 million worldwide. The patchwork of responses intended to fight the pandemic — from temporary lockdowns to mask wearing, social distancing, enhanced personal hygiene and reduced travel — has had a huge impact on other common respiratory illnesses, too.

In the Southern Hemisphere — now past its winter — seasonal influenza hardly struck at all. That looks as though it might happen in the north, too. Conversely, some common-cold viruses have thrived, and tantalizing evidence suggests that they might, in some cases, protect against COVID-19.

Despite humanity’s long history with colds and flu, the viruses that cause them still hold many mysteries. Scientists hope this year’s disrupted seasons could reveal new information about the transmission and behaviour of these unwelcome annual guests: how these viruses respond to health measures, how they interact and what that might mean for long-term disease burdens. “This is a natural experiment for so many respiratory viruses,” says Sonja Olsen, an epidemiologist at the National Center for Immunization and Respiratory Diseases, part of the US Centers for Disease Control and Prevention (CDC) in Atlanta, Georgia.

In May, at the tail end of the first wave of COVID-19 deaths in many nations, and when some of the strictest lockdowns were in place, health workers noted an abrupt and early halt to the 2019–20 flu season in the Northern Hemisphere.

That might partly have been an artefact caused by fewer people coming to a clinic for testing, experts say, but it was also attributable to the effectiveness of policies such as social distancing. After the pandemic started, positive tests for the flu virus plummeted by 98% in the United States, for example, whereas the number of samples submitted for testing dropped by only 61%1. In the end, the US 2019–20 flu season was rated as ‘moderate’ by the CDC, which estimates that 38 million people fell ill with influenza, and 22,000 people died. That’s fewer than in recent years, but not unprecedented.

After the flu season in the north ended early, it hardly got going at all in the Southern Hemisphere. There were astonishingly few cases of seasonal flu there from April to July 2020 — even as global COVID-19 cases continued to climb. In Australia, Chile and South Africa, a grand total of just 51 cases of flu were spotted in more than 83,000 tests1. “We know it’s less transmissible than coronavirus, so it makes sense,” says Olsen, but the decline was still “greater than expected”. Influenza’s absence has been attributed to pandemic-response measures, but they don’t tell the whole story.

“Some South American countries haven’t done such a good job controlling COVID, but even there flu is low,” says virologist Richard Webby at St Jude’s hospital in Memphis, Tennessee. “I don’t think we can put it all down to mask wearing and social distancing.” He suspects that the dearth of international travel played a part. Flu typically travels around the world from one winter to another, while maintaining a lower year-round presence in the tropics. Although the mechanisms underlying this behaviour aren’t entirely clear, the movement of people clearly contributes.

By mid-December, the Northern Hemisphere is usually well into the start of its annual cold and flu season — but so far this year, even as the COVID-19 pandemic surges in dozens of countries, the levels of many common seasonal infections remain extremely low.

The pandemic caused by the SARS-CoV-2 coronavirus has infected at least 67 million people and killed 1.5 million worldwide. The patchwork of responses intended to fight the pandemic — from temporary lockdowns to mask wearing, social distancing, enhanced personal hygiene and reduced travel — has had a huge impact on other common respiratory illnesses, too.

In the Southern Hemisphere — now past its winter — seasonal influenza hardly struck at all. That looks as though it might happen in the north, too. Conversely, some common-cold viruses have thrived, and tantalizing evidence suggests that they might, in some cases, protect against COVID-19.

Despite humanity’s long history with colds and flu, the viruses that cause them still hold many mysteries. Scientists hope this year’s disrupted seasons could reveal new information about the transmission and behaviour of these unwelcome annual guests: how these viruses respond to health measures, how they interact and what that might mean for long-term disease burdens. “This is a natural experiment for so many respiratory viruses,” says Sonja Olsen, an epidemiologist at the National Center for Immunization and Respiratory Diseases, part of the US Centers for Disease Control and Prevention (CDC) in Atlanta, Georgia.

In May, at the tail end of the first wave of COVID-19 deaths in many nations, and when some of the strictest lockdowns were in place, health workers noted an abrupt and early halt to the 2019–20 flu season in the Northern Hemisphere.

That might partly have been an artefact caused by fewer people coming to a clinic for testing, experts say, but it was also attributable to the effectiveness of policies such as social distancing. After the pandemic started, positive tests for the flu virus plummeted by 98% in the United States, for example, whereas the number of samples submitted for testing dropped by only 61%1. In the end, the US 2019–20 flu season was rated as ‘moderate’ by the CDC, which estimates that 38 million people fell ill with influenza, and 22,000 people died. That’s fewer than in recent years, but not unprecedented.

After the flu season in the north ended early, it hardly got going at all in the Southern Hemisphere. There were astonishingly few cases of seasonal flu there from April to July 2020 — even as global COVID-19 cases continued to climb. In Australia, Chile and South Africa, a grand total of just 51 cases of flu were spotted in more than 83,000 tests1. “We know it’s less transmissible than coronavirus, so it makes sense,” says Olsen, but the decline was still “greater than expected”. Influenza’s absence has been attributed to pandemic-response measures, but they don’t tell the whole story.

“Some South American countries haven’t done such a good job controlling COVID, but even there flu is low,” says virologist Richard Webby at St Jude’s hospital in Memphis, Tennessee. “I don’t think we can put it all down to mask wearing and social distancing.” He suspects that the dearth of international travel played a part. Flu typically travels around the world from one winter to another, while maintaining a lower year-round presence in the tropics. Although the mechanisms underlying this behaviour aren’t entirely clear, the movement of people clearly contributes.

Increased influenza vaccination might have contributed to the disappearance, too. Australia, for example, saw more than 7.3 million flu jabs administered by 20 May 2020, compared with 4.5 million by that date in 2019, and 3.5 million in 2018. It’s unclear if that trend will hold in the north.

Vaccination rates in the United States for seasonal flu have been trending upwards for years: slightly more than half of the US population over six months of age was vaccinated in 2019–20, up 2.6 percentage points from the previous year. But it is unclear whether Americans will be more or less inclined towards flu vaccinations this year, particularly given the tumultuous backdrop of the pandemic and the change in president.

Most experts are cautiously betting on a very mild flu season for the Northern Hemisphere this year. That would be good news on many fronts — in particular, it would help to alleviate the potential burden on the health system, from hospitals to testing centres, caused by simultaneous waves of flu and COVID-19. But surprises could be in store.

No one really knows, for example, why one nation, such as Australia, can be hit hard by influenza for several years while a neighbouring country, such as New Zealand, sees very low rates, says Webby. Even influenza’s seasonality isn’t entirely understood, nor exactly how it travels around the globe. “We don’t have a real good handle on why it’s a winter disease,” he says. Untangling lessons about flu from this year’s data will be interesting but difficult, Olsen says, because pandemic policies and compliance vary on the national, state and even neighbourhood level.

And the changing trends could have consequences. If this year’s flu season does fizzle out in the Northern Hemisphere, that could make it harder to predict the right strains to put in 2021’s flu vaccine. It could also have intriguing, longer-term consequences. Webby speculates that a low-flu season might kill off less-common variants of influenza. “A lot of different flus have been circulating in recent years. Are they all going to make it out of this or not?” he asks. “It’s possible that what this season will do is actually make the virological picture a lot simpler. That may be permanent, potentially.”

At the same time, Webby adds, the lack of viral competition in human hosts could conceivably open a door for new swine-flu variants in the future. “We get a handful of those every year, in the agricultural-fair season,” Webby says. “One of the things holding those viruses back a lot is natural immunity. If flu is low for a few seasons, that might leave a gap for swine viruses to have more impact.”

“I am sure that flu will come back with a vengeance at some stage in the future,” says Robert Ware, a clinical epidemiologist at Griffith University in Queensland, Australia, “but it might take a few years.”

Source: Nature

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