Infectious Disease

Data from Chile suggest an early, ‘atypical’ flu season for US

October 28, 2022

3 min read

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The authors report no relevant financial disclosures. Belongia reports no relevant financial disclosures.

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Influenza reached epidemic levels in Chile months earlier than a typical influenza season, just like in several other Southern Hemisphere countries, researchers reported.

Although influenza activity in the Southern Hemisphere does not always predict what will happen in the United States, the findings suggest the US and other nations in the Northern Hemisphere should prepare for “influenza activity with atypical timing and intensity” this season, researchers from the CDC , Chile and the Pan American Health Organization wrote in MMWR.

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“As natural immunity to flu is likely lower after 2 years of decreased flu activity, and COVID-19 prevention measures are being eased, it’s possible the season may be more severe,” the CDC said in a summary of the new study sent to reporters .

According to the researchers, although the incidence of influenza-associated hospitalizations in Chile was four to six times lower in 2022 than it was during the 2017-2019 seasons — before the COVID-19 pandemic — it was “much higher” than in 2020- 2021

During Chile’s influenza season, officials there tested 59,392 respiratory specimens, finding that 5.3% were positive for SARS-CoV-2 and 6.9% were positive for influenza, according to the researchers. Among the influenza-positive specimens, 54% were typed, with all but one identified as influenza A(H3N2). The other specimen was influenza A(H1N1).

Overall, 1,002 people in Chile were hospitalized for influenza-associated pneumonia, and more than half those hospitalized were aged 65 years or older.

According to the researchers, Chile’s early influenza start was consistent with trends in other countries, including Australia, Argentina, Peru and South Africa.

Edward A. Belongia, MD, director of the Center for Clinical Epidemiology and Population Health at the Marshfield Clinic Research Institute in Marshfield, Wisconsin, told Healio that early influenza peaks in the Southern Hemisphere were “a bit unusual” but noted that COVID-19 , and mitigation measures to slow its spread, have disrupted the seasonality of influenza.

“There have been all these measures in place since the beginning of the pandemic — those are largely gone now. And so, we’re in a situation where it will be much easier to transmit respiratory viruses,” said Belongia, who was not involved with the new study.

In the week ending Oct. 22, 6.2% of clinical lab tests in the US were positive for influenza, up from 4.4% the week before. Most positive samples reported by public health laboratories were identified as influenza A(H3N2), and 2,332 people were hospitalized with influenza during the week, according to CDC data.

Additionally, 3.3% of US outpatient visits to a health care provider during the week were due to a respiratory illness — up from 3% the week before and the third consecutive week the US has been above the national baseline of 2.5%. It is the first time since the 2009 H1N1 pandemic that the US has been above baseline at this time of year.

According to the authors of the MMWR report, in Chile, influenza vaccines were 49% effective at reducing hospitalization with A(H3N2). Because influenza vaccines being used in the Northern Hemisphere this season are identical to those that were used in the Southern Hemisphere — including the H3N2 component — the CDC said US vaccines could provide similar protection against hospitalization.

In recent years, a mismatched H3N2 vaccine component contributed to a severe influenza season in 2017-2018, during which more than 700,000 people were hospitalized and more than 50,000 died, according to CDC estimates.

“H3N2 is the flu subtype we’re most worried about. H3N2 infections tend to be worse,” Belongia said. “So far, H3N2 viruses going around appear to be genetically matched to the vaccine virus. So, that’s good. I’m hopeful — it would be great if we saw 40% to 50% vaccine effectiveness [like Chile].”

Last season’s US influenza vaccine was less effective, although the season saw low levels of virus activity and peaked early, during the last week of 2021. Many COVID-19 mitigation measures were still in effect across the nation at that time, however. But after two consecutive seasons with low levels of influenza activity, CDC officials are concerned that some people may skip the vaccine — nearly half of US adults in one survey said they do not plan to be vaccinated this year.

“Northern Hemisphere countries could benefit from preparing for an atypical season, which could include early influenza activity with potentially severe disease for the 2022-2023 season, especially in the absence of prevention measures, including vaccination. Health officials should encourage communities to protect themselves by seeking influenza vaccination,” the researchers wrote.

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