Flu season in US: What the latest trends show

Flu season in the Southern Hemisphere is winding down and health officials in the Northern Hemisphere are reviewing data on the flu vaccine's effectiveness in anticipation of the fall. According to a CDC report released Sept. 8, the flu vaccine helped reduce influenza-associated hospitalizations by 52 percent in the Southern Hemisphere.

"Circulating influenza viruses were genetically similar to those targeted by the 2023–24 Northern Hemisphere influenza vaccine formulation," the CDC report states. "This vaccine might offer similar protection if these viruses predominate during the coming Northern Hemisphere influenza season."

Because the Southern Hemisphere influenza season happens ahead of the that in the Northern Hemisphere, health experts often pay close attention to emerging trends to inform clinicians. 

Earlier this year, Seema Lakdawala, PhD, associate professor of microbiology and immunology at Emory's School of Medicine, told Becker's that clinicians should closely watch a few key trends from the Southern Hemisphere to make predictions and adjustments:  

  1. Pay attention to vaccine efficacy data because it will "be important to see how that fits with the components for the Northern Hemisphere vaccine," Dr. Lakdawala said in June.

  2. Increase masking, social distancing and vaccination efforts as the fall draws near.

  3. Understand that a 'tripledemic' could occur again, if the flu, COVID-19, and respiratory syncytial virus show signs of beginning to co-circulate and rise at the same time.  

Now, as the U.S. and the Northern Hemisphere enters fall, experts are examining the data, like information released on flu vaccinations from the CDC, and making some tweaks to these final predictions. 

Early on in Australia, influenza A and B were co-circulating, Dr. Lakdawala said, which can typically be a sign of a more severe flu season ahead. That prediction may still hold up, other experts confirmed to Becker's.

"Influenza disease in the Southern Hemisphere over the past few months — during their winter season —  has been severe," Mark Schleiss, MD, professor at the University of Minnesota Medical School told Becker's. "There have been large numbers of cases in Australia, and although there have not been as many cases as in the record-setting year of 2019, disease activity has been intense, with over 200,000 reported cases as of Aug. 25. Influenza during the current Australian winter seems to be disproportionately targeting children, with disease notification rates highest in children 5–9 years of age."

The reason behind the surge, Dr. Schleiss explained, is not new strains, but rather a" two-fold decrease in pediatric vaccination compared to 2020."

Decreases in childhood vaccinations of many kinds has been a trend in the U.S., in recent years, and Dr. Schleiss noted that if this trend continues with the flu vaccine this fall, it will make for a more challenging flu season across the country. 

Despite the possibility of a more severe flu season, many are divided on whether there will be a 'tripledemic' co-circulation of all three: flu, COVID-19, and RSV. 

Dr. Schleiss told Becker's it is "absolutely" a possibility. 

"We are already observing another surge in COVID-19 cases in the U.S., which will surely be exacerbated by kids going back to school. We are also observing an 'early' RSV season. RSV typically circulates in temperate climates between November and March — sometimes with more than one 'outbreak' during the winter season," Dr. Schleiss said. "Just this week, the CDC has issued a warning that we are already seeing a surge in RSV cases, in particular in Florida and Georgia. And, if disease activity in the Southern Hemisphere (particularly Australia) is any indicator, we need to brace ourselves for a difficult influenza season as well."

However, Colleen Kraft, MD, a professor in the department of pathology at Emory University Hospital in Atlanta ,told Becker's she is not certain that will be the case this year. 

"I think what we're seeing is the spike in the fall of COVID. And I think it is going to go down at the time that influenza peaks," she said. "Crystal balls and viruses don't go together, but I really do feel like that we're spiking kind of early with COVID. So, I do think that will make this year less likely to be a 'tripledemic' year, and instead it may be sort of like a wave and then a slow more separately than it all together."

While predictions vary, the one triple-threat experts have consensus on for fall respiratory virus season is betting on the trio of vaccines that will be available to help curb hospital overload.


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