Influenza epidemics occur in the winter months , and transmission alternates between the northern and southern hemispheres every year as the seasons change .
Thus , each influenza season is ‘ seeded ’ by the genetics of the most recent one in the opposite hemisphere . ”
— Michael A . Ben- Aderet , MD
In most years , the same strains of influenza that cause disease in the Southern Hemisphere predict what strains we will see in the Northern Hemisphere when the seasons flip .”
— Michael Calderwood , MD ,
MPH
public health experts predicted a “ twin-demic ” of sorts , due to influenza season ’ s overlap with COVID ; however , it didn ’ t happen . Instead , while the U . S . flu season was worse than the season prior , it was mild compared to previous pre-pandemic flu seasons .
“ Australia and New Zealand had their most severe flu season in five years ,” says infectious disease specialist Soniya Gandhi , MD , associate chief medical officer at Cedars-Sinai . “ We tend to see similar influenza patterns in the Northern Hemisphere , and while there is no guarantee that this will happen , it really highlights the importance of getting the flu shot this year .”
“ Influenza epidemics occur in the winter months , and transmission alternates between the Northern and Southern Hemispheres every year as the seasons change ,” confirms Michael A . Ben-Aderet , MD , associate director of hospital epidemiology at Cedars-Sinai . “ Thus , each influenza season is ‘ seeded ’ by the genetics of the most recent one in the opposite hemisphere . While there are a number of variables that influence the severity of a flu ‘ season ,’ the circulating influenza strains are one of the most important . Thus , we can look to the prior hemispheric season to get our best prediction of what those may be . While the data from the Southern Hemisphere season is certainly not perfect , it is a fairly good predictor .”
“ The CDC has put a lot of effort into predicting flu seasons over the past decade ,” says Michael Calderwood , MD , MPH , chief quality officer at Dartmouth Hitchcock Medical Center in Lebanon , N . H . “ This began in 2013 with the ‘ Predict the Influenza Season Challenge .’ In most years , the same strains of influenza that cause disease in the Southern Hemisphere predict what strains we will see in the Northern Hemisphere when the seasons flip . There has been much reported on a significant and severe influenza season that just finished up in the Southern Hemisphere . There is concern about what that will predict for our own season this winter , although others debate that some of the increased numbers in the Southern Hemisphere were due to increased testing compared to prior years . Worth mentioning is that there is very little correlation between the peak in influenza-like cases in Australia and the peak in influenza-like cases in the U . S . when the seasons flip . So , I suspect that we will see the same strains circulating and it is possible that the season will have more cases diagnosed , particularly with fewer masks being worn , but time will tell .”
Australia has a robust flu-tracking system , and their flu season , which runs from April to October , offers clues as to what ’ s in store for the U . S . The flu wasn ’ t only severe in Australia this year — it came on fast .
“ Influenza started circulating two months earlier than normal , and the largest number of cases were in children ages 5 to 9 ,” Gandhi says . “ This really emphasizes that even young people should be getting their flu shot .”
The silver lining ? Australia saw lots of influenza A ( H3N2 ), a strain that ’ s included in this year ’ s vaccine , Gandhi says . While it ’ s too early to assess the vaccine ’ s effectiveness in the U . S ., she said it ’ s reassuring to know that this strain of the virus is covered in the current vaccine .
As we know , SARS-CoV-2 dominated in both hemispheres , essentially eliminating flu cases in 2020 and 2021 . Infectivity also evolved .
“ A person infected with influenza will on average infect one other person ,” Calderwood says . “ A person infected with SARS-CoV-2 on average will infect two to three other people with the original Wuhan strain , five other people with the Delta variant , and 10 other people with the Omicron variant .”
Ben-Aderet explains that “ The relative absence of a seasonal influenza epidemic over the past two years is likely due to the unprecedented world-wide infection control work done to combat the spread of COVID-19 . Lack of travel , masking , distancing , and infection precautions all work very well in preventing the spread of influenza as well , which is considered less infectious than SARS-CoV-2 .”
NFID ’ s Schaffner concurs , stating , “ On a positive note , we have more preventive behaviors in our toolbox than we did before the COVID-19 pandemic . We are more accustomed to wearing masks and staying home when sick .”
“ We have the tools to reduce transmission ,” Calderwood emphasizes . “ People are used to wearing masks , and this is still an effective risk reduction strategy when in crowded environments , particularly indoors , when cases of influenza , COVID-19 , or other respiratory viruses ( such as RSV ) are spreading in the community . People need to weigh the risks and the benefits . If you are young , healthy , and up to date on vaccinations , then you may choose not to wear a mask , taking a risk that you will get a viral illness . If you are sick , it is important to stay home . If you must go out with a respiratory illness , it is important to wear a mask and to wash your hands frequently to protect others in the community .”
While people observed COVID-19 pandemic safety measures — like wearing face masks and washing hands frequently — during the past two years , the flu all but disappeared in the U . S . But this year could be different , as mask mandates have lifted , and more people are getting back to socializing .
“ We ’ ve been very protected these past two years , and we haven ’ t had an opportunity for the flu virus to circulate widely ,” says Kimberly Shriner , MD , at Huntington Health , an affiliate of Cedars-Sinai . “ But now , the masks are off .”
“ People are tired of respiratory viruses , and they ’ re trying to resume normal lives ,” Shriner adds . “ I worry that since COVID-19 is beginning to settle down a little , there may be an impression that influenza will as well .”
The NFID survey found that among respondents ,