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An atypically early flu season in Chile this year came with an increase in hospitalizations compared with the first two pandemic years, a CDC report detailed, potentially offering a preview of what's in store for the U.S.

In 2022, cumulative influenza-related hospitalizations in the South American nation reached 5.1 per 100,000 person-years (95% CI 4.8-5.4), a sizeable increase compared with the vanishingly small flu hospitalization rates during the pandemic years so far (0.01 and 0.6 per 100,000 person-years in 2020 and 2021), reported Rodrigo Fasce, BSc, of the Public Health Institute of Chile in Santiago, and co-investigators.

And Chile's influenza positivity rates reached epidemic thresholds as early as January, several months sooner than usual.

But the 2022 flu hospitalization rates in Chile were still substantially lower than years prior to the pandemic, which from 2017 to 2019 ranged from 23.0 to 30.4 per 100,000 person-years, according to the findings in .

"Northern Hemisphere countries might face influenza activity with atypical timing and intensity during the 2022-23 season," Fasce and co-investigators wrote. "Health authorities should encourage all eligible persons to seek influenza vaccination and take precautions to reduce transmission of influenza (e.g., avoiding close contact with persons who are ill)."

COVID mitigation measures -- physical distancing, masking -- have been credited for reduced influenza rates globally during the pandemic, but the group also cited Chile's high levels of flu vaccination for vulnerable groups in 2022 as another contributing factor to the lower rates of hospitalizations compared with pre-pandemic years.

Before peak influenza activity in week 24, the government of Chile had already vaccinated 88% of prioritized groups -- health workers, older adults, people with chronic conditions, pregnant women, infants and young children, etc. -- and ultimately 93.5% were vaccinated.

Vaccine effectiveness estimates against hospitalization from the predominant influenza strain -- A(H3N2) clade 3C.2a1b.2a.2 -- was an adjusted 49% (95% CI 23-67).

Fasce's group noted that the Northern Hemisphere 2022-2023 flu vaccine contains the same A(H3N2) clade and antigen used in Southern Hemisphere nations like Chile, indicating that they may be similarly effective at preventing severe cases if that strain proves to dominate up north as well.

Of 59,392 lab tests performed in 2022 for respiratory illness in Chile, influenza was detected more frequently than COVID (6.9% vs 5.3%). About half of the flu specimens were typed, and all but one (99.9%) turned out to be influenza A(H3N2). When 280 of these were sequenced, all came back positive for A(H3N2) clade 3C.2a1b.2a.2. The researchers noted that Chile did identify a limited number of influenza B virus cases, but they were rare and none were subtyped as B/Yamagata.

Overall, hospitalizations for influenza in Chile accounted for 5.6% of the 17,752 total hospitalizations for pneumonia and influenza during January to August 2022. Hospitalizations were highest in patients 65 and over (48.3%), followed by those under 5 years old (27.7%).

Limitations cited by the researchers included that vaccine effectiveness estimates were based on a limited number of hospitalized patients, drawing from nine hospitals. Furthermore, some hospitalizations may have been due to added healthcare seeking behavior because of the COVID-19 pandemic.

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