Nov. 20, 2023 – For those who have gotten the flu shot this season, there’s good news: This year’s formulation looks like it will be very effective at preventing severe cases and hospitalization. That’s based on how the vaccine performed in the parts of the world where flu season is wrapping up.

As flu season sets in across the United States,  health officials look to see how flu vaccines performed in countries in the Southern Hemisphere, where the flu season tends to run from about April to September. An analysis published by the CDC that looked at mid-season flu data for South America showed that the flu vaccine was more than 50% effective at preventing flu hospitalizations there.

But since that report’s publication earlier this year, the vaccine used in South America was found to be less and less effective as the flu season progressed. So the flu shots in the U.S. were updated to better protect against versions of the virus that were seen toward the end of flu season in parts of South America where the data was collected.

“Final estimates can vary from interim estimates, and it does look like – I did look at some of the final season data the other week – and it does look like the effectiveness probably dropped a bit later in the season, and this is what happens when we see changes in the virus circulating, which was mostly a type called influenza A H1N1,” said epidemiologist Annette Regan, PhD, MPH, a co-author of the CDC report on Southern Hemisphere flu. “It does look like we’ve updated the formulation for the Northern Hemisphere based on that change.”

“It was a good report because it gave us an indication that in the middle of the season, the vaccine was working really well, but when everything’s said and done and we’re at the end of the season, it doesn’t always look exactly the same. So we have a slightly different formulation in the Northern Hemisphere than what went out in South America,” said Regan, who formerly tracked flu data for the CDC and the Australian government. She’s now an associate professor at the University of San Francisco’s Orange County campus.

Tracking flu trends below the equator helps medical teams and public health officials prepare for flu season up north, although the influenza virus is so adept at changing that predictions are helpful but not a guarantee of what’s to come, said Paula Couto, MD, an influenza surveillance epidemiologist with the Pan American Health Organization, which is a regional office of the World Health Organization for the Americas.

She said the interim report, which used data from Argentina, Brazil, Chile, Paraguay, and Uruguay, can help with preparations such as estimating this season’s needs for health services and also demand for antiviral drugs used to treat flu.

“Of course, influenza is a tricky virus because it has pandemic and epidemic potential, so it may not necessarily be the same [between hemispheres], but that’s why we are always monitoring and alert about these viruses,” said Couto, who also co-authored the CDC report.

There is no official start date for flu season each year in the U.S. The start date varies because it’s usually determined after the fact, based on a combination of factors such as increasing rates of positive flu tests and hospitalizations. That tends to be in late November.

“In the U.S., we tend to see the flu season start in the Southeast, like in Florida and Texas, and surrounding states, and then we kind of see it spread from there,” said Regan. “Usually it doesn’t take too long, like we’re talking a couple weeks max … because people move a lot and people are much more mobile nowadays than they were during the height of the pandemic.”

The U.S. appears to be on the verge of flu season.

The CDC indicated that flu cases are on the rise in the Southeast, as well as in South Central and West Coast states, according to the agency’s latest weekly flu report. Currently, about 4% of flu tests are coming back positive nationally. Already, one child has died due to influenza. Children are one of the groups with low flu vaccination rates in the U.S. and around the world, Regan noted.

“I just don’t think any kid should die from flu, not in 2023, if we can help it,” she said.

The CDC recommends that everyone in the U.S. ages 6 months and older get vaccinated for the flu. After getting a shot, it takes the body about 2  weeks to make enough antibodies for maximum protection, Regan said, and the effectiveness typically lasts at least 4 months. That means the time to get one is now, since flu cases are on the rise.

About one-third of U.S. adults and kids have gotten this year’s flu shot, according to the CDC, which is lagging several percentage points compared to last year. The adult vaccination rate varies from one state to another, ranging from 22% to 51%. The severity of flu season can vary, but federal data show there are between 9 million and 41 million cases annually, with up to 710,000 hospitalizations and between 12,000 and 52,000 deaths.

Between Oct. 1 and Nov. 11 this year, the CDC estimates that in the U.S. there have already been:

  • Between 780,000 and 1.6 million cases of flu
  • Between 360,000 and 770,000 flu medical visits
  • Hospitalization of 8,000 to 17,000 people
  • Between 490 and 1,500 flu deaths

The COVID-19 pandemic disrupted seasonal flu trends, resulting in lower-than-normal flu cases when stay-at-home orders and quarantines were common. Last winter, though, flu returned with a vengeance. Both Couto and Regan said all indicators point toward a return to normal this season.

But the risk of serious illness or death from flu remains, particularly among high-risk populations like very young children and older people. Regan urged people, even if they have gotten the flu vaccine, to do other things to prevent the spread of flu in order to protect themselves but also people who are at high risk, like grandparents or newborn babies. 

Vaccines are not 100% effective, Regan said, so it’s vital that people wash their hands and cover their mouths when sneezing and coughing. 

“And I think the big one is staying home when we’re feeling ill, especially because children are very good spreaders of influenza, there’s good data to show that. So keeping them home when they’re under the weather can be really helpful for helping to control the spread of influenza and RSV and helping to stop these epidemics,” she said. 

So just because this flu season is a return to normal or going according to predictions, each person’s actions – from getting vaccinated to washing their hands – is important, Regan and Couto concurred.

“In the particular case of influenza, we know that the next pandemic is going to happen,” Couto said. “You can never really know when, and that’s why being vigilant about influenza, it’s key. Of course, the last pandemic was COVID-19, but we cannot forget that influenza is still a threat.”