Public health experts prepared for a “twindemia” as autumn approached last year: a double threat from coronavirus and seasonal flu.
But even as the cold, dry climate fell in the northern hemisphere and cases of COVID-19 increased, the United States and the United Kingdom experienced historically mild flu seasons.
Between October 1 and January 30, only 155 Americans were hospitalized with the flu, compared with 8,633 in almost the same period a year ago. This is a 98% reduction. US laboratories have collected and tested more than half a million flu samples since the end of September, but only 0.2% of those samples tested positive (1,300 in total), according to the Centers for Disease Control and Prevention.
Each state in the United States is experiencing “minimal” flu activity, the agency said. This is in contrast to last season, when 22,000 Americans died of the flu.
A likely driver of abnormally low infection and hospitalization rates is COVID-19. Measures designed to slow or prevent the spread of coronavirus have also prevented other pathogens such as influenza, according to Sonja Olsen, a CDC epidemiologist at the National Center for Immunization and Respiratory Diseases.
“Measures including extensive reductions in global travel, telecommuting, closing schools, social distance and wearing a face mask may have played a role,” she told Insider. Olsen noted, however, that it is a challenge to find out exactly which of these measures are most important for preventing flu.
‘I don’t think anyone foresaw a season when there was no flu’
Worldwide, flu activity is at lower levels than expected for this time of year, according to the World Health Organization. Since the beginning of the pandemic, the southern hemisphere has had “virtually no influenza circulation. “, reported the CDC.
This is despite the increase in tests for influenza in some countries, according to Olsen.
“There is some flu circulating in tropical countries, but in those countries it seems that the season is weaker compared to other years,” she said.
David Battinelli, medical director at Northwell Health and professor of medicine at Hofstra University, told Insider that experts have always “hoped that social detachment, masking and more hand hygiene would mitigate the flu”.
“But I don’t think anyone foresaw a season when there was no flu,” he said, adding, “no one has seen it on the entire planet.”
The flu season usually peaks in February, so the US is likely to be free, but Battinelli warned that it is too early for a sigh of relief.
“The flu seasons can certainly extend into March,” he said. In the past 40 years, six flu seasons peaked in March, according to the CDC.
Why coronavirus restrictions worked so well against the flu
The flu does not spread as well as the coronavirus. The average number of people infected by a person with the flu – a measure known as the reproductive number – is 1.28. Usually, someone sick with the coronavirus passes it on to 2 to 2.5 people. In part, this difference occurs because the coronavirus may be in the air, remaining suspended in the air for hours. This is not true for the flu, although viruses can jump from person to person through respiratory droplets and contaminated surfaces.
The greater reproductive number of the coronavirus “means that it is more difficult to prevent transmission through non-pharmaceutical interventions than it is for influenza,” said Olsen.
Furthermore, existing immunity in the population – whether from previous infections or vaccinations – can also increase the effects of public health measures such as masking, resulting in a more dramatic reduction in transmission, Olsen said.
The flu vaccine has been around for more than 75 years. Vaccine manufacturers have projected that they will supply the United States with 194 to 198 million doses of flu vaccine for the 2020-2021 season, according to Olsen.
“Compare that to a new coronavirus, to which almost the entire population is susceptible,” she said.
Research also shows that over-spreading events – a circumstance in which one person infects a disproportionately large number of others – are the main form of spread of the coronavirus, unlike the flu.
Decreased travel has played a greater preventive role than flu vaccines
CDC data suggests that US pharmacies and doctors’ offices have administered more flu vaccines to Americans this season than the previous season, when Americans received 174.5 million doses. This increase came after public health experts pressured people to get flu shots; some research has suggested that the injection could reduce the risk of getting COVID-19.
But Olsen said it is unlikely that the extra vaccinations were the reason for the low number of flu cases worldwide.
“Although some places in the world are using more vaccines, this is not universally true,” said Olsen. “We have seen less flu circulation even in places that do not use, or use very little, the vaccine.”
Olsen and Battinelli believe that a drop in travel has played a significant role.
“The traditional flu movement around the globe has not occurred,” said Battinelli.
Usually, the flu is carried from the southern hemisphere, which has its flu season between April and September, to the northern hemisphere in the fall. But travel bans and limited air travel prevented this spread.
‘We hope the flu will return at some point’
Experts do not believe that a non-existent flu season will become a regular occurrence, and a study in Cambodia suggests that flu will begin to circulate if the pandemic-related restrictions are lifted.
“We hope the flu will return at some point,” said Olsen.
Even so, the emergence of more transmissible variants of the coronavirus and the slow pace of global vaccinations could mean that the pandemic will continue until 2022, requiring another year of rigorous masking and social detachment. If that’s the case, Battinelli thinks next year’s flu season will be just as mild.
For him, the conclusion of this unprecedented season is that Americans have the ability to actively limit flu transmission – and prevent tens of thousands of flu deaths – by masking, distancing themselves socially and practicing good hand hygiene.
“This is a warning that we must not tolerate so many deaths,” said Battinelli.
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