In possible sign for US, Chile’s flu hospitalizations were highest in three years, but vaccination helped

In a possible warning sign for the US and other Northern Hemisphere countries, Chile’s 2022 flu season started much earlier than usual and brought more hospitalizations than during the pandemic, but the effectiveness of the vaccine against hospitalization was estimated to be almost 50%, according to a new study published by the US Centers for Disease Control and Prevention.

Researchers look to the Southern Hemisphere when trying to forecast what the North American flu season might look like, and they’ve noted that the southern season has been particularly bad this year.

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n the study, published Thursday in the CDC’s Morbidity and Mortality Weekly Report, researchers found that flu numbers in Chile were at epidemic levels in the first six weeks of the year – much earlier than in 2017, 2018 or 2019, when the flu season began in April or May. This year’s numbers dipped in weeks seven through 17 before rising to epidemic levels again in May and peaking in June.

Chile’s flu hospitalization rates were “substantially higher” this year than in 2020 and 2021, the researchers say. Those pandemic years were marked by especially low numbers of viral illnesses around the world because of Covid-19 mitigation measures, and experts have warned that the lifting of those measures – and reduced exposure to viruses during the pandemic – will bring infection numbers back up.

However, compared with the prepandemic years of 2017-19, this year’s flu hospitalization rates in Chile were “substantially lower.” This is partly credited to the vaccination of more than 92% of residents who were prioritized because of their age or underlying medical conditions, a group that made up 41% of the total population.

The flu vaccine used in Chile, which included a match for the dominant A(H3N2) virus, was found to be 49% effective at preventing hospitalization. The shot used in the Northern Hemisphere includes the same virus clade and antigen as the Southern Hemisphere vaccine, the researchers say, so it may be similarly effective if the same virus dominates.

Flu vaccination can prevent infection, and among those who still become sick with flu, vaccination can reduce the severity of illness and risk of hospitalization.

The researchers say their findings should reinforce the need to prepare for an “atypical season,” and they urge health officials to encourage everyone to get vaccinated and avoid close contact with people who are sick.

According to data from the US Centers for Disease Control and Prevention, flu activity has already been on the rise in the US about a month earlier than usual. Overall respiratory illness activity was “very high” in Washington, DC, and “high” in seven states: Alabama, Georgia, Louisiana, New York, South Carolina, Tennessee and Texas.

More than 4% of laboratory tests were positive for flu in the second week of October, more than doubling over the past two weeks, but not yet at last year’s peak positivity rate. Hospitalizations for flu are also ticking up, but are also not yet at last year’s peak.

So far, US flu vaccination rates are lower than they’ve been at this point in the season for the past few years – about 116 million doses of flu vaccine have been distributed, compared with 129 million at this point last year and 141 million in 2021.

Dr. Nipunie Rajapakse, a pediatric infectious diseases specialist at Mayo Clinic Children’s Center, told CNN on Thursday that people should get vaccinated against Covid-19 and flu and try to prevent any respiratory illness, especially while hospitalizations are rising due to RSV and other viruses.

“Making sure that your kids and anyone over six months of age in your family are getting their flu vaccines this year is even more important because we haven’t seen a lot of influenza the last couple of years, and so everyone’s going into this season with less immunity, less protection from prior infections,” Rajapakse said.

CNN’s Deidre McPhillips contributed to this report.