Public health experts have braced for a “twindemia” as autumn approaches: a double threat from coronavirus and seasonal flu.
But even as cold, dry weather has descended in the northern hemisphere and cases of COVID-19 have increased, the United States and the United Kingdom have experienced historically mild flu seasons.
Between Oct. 1 and Jan. 30, only 155 Americans were hospitalized with the flu, up from 8,633 at about the same time a year ago. That’s a decrease of 98%. U.S. labs have collected and tested over half a million samples for influenza since late September, but only 0.2% of those samples have tested positive (1,300 in total), according to the Centers for Disease Control and Prevention.
Every state in the United States experiences “minimal” influenza activity, the agency reported. This is in contrast to last season when 22,000 Americans died from the flu.
A likely factor behind the abnormally low infection and hospitalization rates is COVID-19. Measures to slow or prevent the spread of the coronavirus have also stopped other pathogens like influenza, according to Sonja Olsen, CDC epidemiologist at the National Center for Immunization and Respiratory Disease.
“Measures such as major cuts in global travel, telecommuting, school closures, social distancing and the use of a face mask may have played a role,” she said to Insider. Olsen noted, however, that it was difficult to determine precisely which of these measures was most important for influenza prevention.
“ I don’t think anyone anticipated a flu-free season ”
Des and Adele Morrow line up to receive a COVID-19 vaccine at the Lakes Regional Library in Fort Myers, Fla., December 30, 2020.
Octavio Jones / Getty Images
Around the world, influenza activity is at lower levels than expected for this time of year, according to the World Health Organization. Since the start of the pandemic, the southern hemisphere has had “virtually no circulation of influenza,” the CDC reported.
This is despite the increase in influenza tests in some countries, according to Olsen.
“There is flu circulating in tropical countries, but in those countries it seems the season is dull compared to other years,” she said.
David Battinelli, chief medical officer at Northwell Health and professor of medicine at Hofstra University, told Insider that experts had always “hoped that social distancing, masking and more hand hygiene would ease the flu.”
“But I don’t think anyone anticipated a flu-free season,” he said, adding that “no one has seen it on the whole planet”.
Flu season usually peaks in February, so the United States is likely to be in the clear, but Battinelli warned it is too early for a sigh of relief.
“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 have worked so well against the flu
Academics study in socially remote reading rooms at Bodleian Libraries in Oxford, England, August 25, 2020.
Christopher Furlong / Getty Images
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. Typically, a person who is sick with coronavirus transmits it to between 2 and 2.5 people. In part, this difference is due to the fact that the coronavirus can be airborne, staying airborne for hours. This is not the case with the flu, although viruses can pass from person to person via respiratory droplets and contaminated surfaces.
The higher reproduction number of the coronavirus “means it is more difficult to prevent transmission through non-pharmaceutical interventions than it is for the flu,” Olsen said.
In addition, the existing immunity in the population – whether from previous infections or from vaccinations – may also enhance the effects of public health measures such as masking, leading to a more dramatic reduction in transmission, a Olsen said.
The flu vaccine has been around for over 75 years. Vaccine makers predicted that they would supply the United States with up to 194 million to 198 million doses of influenza vaccine for the 2020-2021 season, according to Olsen.
“Compare that to a new coronavirus, to which almost everyone is susceptible,” she said.
Research also shows that widespread events – a circumstance in which one person infects a disproportionate number of other people – is the primary means of spread of the coronavirus, unlike the flu.
Reduced travel played a more important preventive role than the flu shot
A nurse administers a flu shot.
zoranm / Getty Images
Data from the CDC suggests that U.S. pharmacies and doctors’ offices have given Americans more flu shots this season than the previous season, when Americans received 174.5 million doses. This increase came after public health experts urged people to get the flu shot; some research has suggested that the shot may reduce the risk of contracting COVID-19.
But Olsen said the additional vaccinations were unlikely to be the reason for the low number of influenza cases worldwide.
“While some parts of the world are using more vaccines, this is not universally true,” Olsen said. “We have seen less circulation of the flu, even in places that do not use or use very little vaccine.”
A flight attendant and traveler wear face masks when flying from Istanbul to Ankara, Turkey.
Arif Hudaverdi Yemen / Anadolu Agency / Getty
Olsen and Battinelli believe, however, that a drop in travel has played a significant role.
“The traditional flu movement around the world has not happened,” Battinelli said.
Typically, influenza is transported from the southern hemisphere, which has its influenza season between April and September, to the northern hemisphere in the fall. But travel bans and limited air travel have mostly stopped this spread.
‘We would expect the flu to come back at some point’
Experts don’t believe that a non-existent flu season will become a regular occurrence, and a study from Cambodia suggests that the flu will begin to circulate if the restrictions related to the pandemic are lifted.
“We would expect the flu to come back at some point,” Olsen said.
Nonetheless, the emergence of more transmissible coronavirus variants and the slowness of global vaccinations could mean that the pandemic will extend until 2022, requiring another year of rigorous masking and social distancing. If so, Battinelli believes next year’s flu season will be just as mild.
A face mask lies on the side of the road in the fall foliage.
Sebastian Gollnow / Getty Images
For him, the takeaway from this unprecedented season is that Americans have the ability to actively limit influenza transmission – and prevent tens of thousands of flu deaths – by masking themselves , social distancing and practicing good hand hygiene.
“This is a wake-up call that we should not tolerate so many deaths,” Battinelli said.
