South America's 2023 Flu Vaccine Cut Flu Hospitalizations by About Half

The Southern Hemisphere's flu season runs from March to September, ahead of the North's October-to-May season, so its vaccine results can hint at what the North can expect. In 2023, early data from five South American countries showed that the season's vaccine cut the risk of being hospitalized with flu by about half — and by about 70 percent in young children.

How it was measured

Since 2013, countries in the Pan American Health Organization's REVELAC-i network have tracked how well flu vaccines prevent hospitalization for severe acute respiratory infection (SARI). The 2023 analysis pooled 486 sentinel hospitals in Argentina (11), Brazil (455), Chile (8), Paraguay (2) and Uruguay (10), covering March 27 to July 9, 2023.

It compared hospitalized SARI patients who tested positive for flu with those who tested negative for both flu and COVID-19, and asked how many in each group had been vaccinated at least 14 days before falling ill, with vaccination checked in national electronic records. It included three groups prioritized for vaccination — young children, people with preexisting conditions, and older adults (definitions varied slightly by country) — and 2,780 patients after exclusions.

Because flu arrived earlier than usual, before the vaccination campaigns began, each country's evaluation started two weeks after its campaign did. The countries used egg-based vaccines against A/Sydney/5/2021 (H1N1)pdm09-like, A/Darwin/9/2021 (H3N2)-like and B/Austria/1359417/2021 (B/Victoria)-like viruses, with the four-strain version adding B/Phuket/3073/2013 (B/Yamagata)-like virus.

The results

Of the patients with flu, 15.3 percent had been vaccinated, against 28.0 percent of those without it.

A(H1N1)pdm09 dominated: 668 of the 900 flu cases. Against influenza B, the estimate was 46.2 percent but not statistically significant; there were too few H3N2 cases to estimate. Of 1,031 viruses characterized genetically by August 15, 2023, most belonged to the same genetic groups as the vaccine strains.

Share of hospitalized SARI patients testing positive for flu, by week, in the five countries: 2011–2019 against 2023. Credit: CDC, Morbidity and Mortality Weekly Report.

Too few vaccinated

Flu causes an estimated 716,000 to 829,000 respiratory hospitalizations and 41,007 to 71,710 deaths a year in Latin America and the Caribbean. The region once had high vaccination coverage, but it has declined since 2019, and fewer than 30 percent of the patients in this study had been vaccinated before falling ill. In Peru, only one in five people seeking care for a respiratory illness in 2023 had been vaccinated.

What it means for the North

In the weeks before the report, most U.S. flu detections were A(H1N1)pdm09 and B/Victoria — the same pattern as in the South. The 2023–24 Northern Hemisphere vaccine contains similar H1N1 strains (A/Victoria/4897/2022-like for egg-based vaccines, A/Wisconsin/67/2022-like for cell-based vaccines), so it may protect as well if those viruses dominate, though one hemisphere's season does not reliably predict the other's. B/Yamagata has not circulated anywhere since 2020, and whether it belongs in future vaccines needs watching.

CDC and WHO recommend flu vaccination for everyone eligible aged 6 months and older, and especially, by September or October, for people at higher risk of complications: children under 2; people with asthma, obesity, neurologic or neurodevelopmental conditions, blood disorders, chronic lung conditions, or endocrine, kidney, liver, metabolic or heart disease; people who have had a stroke; and people with weakened immune systems.

Limits

These are early estimates from about three months of data, some with wide confidence intervals. Many otherwise eligible patients lacked flu test results. Unmeasured factors may affect who is hospitalized or vaccinated. Results may not apply to countries with different target groups, campaign timing or circulating viruses. And children who needed two doses could not be separated from those who had only one, so effectiveness among fully vaccinated children may be higher.

Sources

Based on "Interim Effectiveness Estimates of 2023 Southern Hemisphere Influenza Vaccines in Preventing Influenza-Associated Hospitalizations — REVELAC–i Network, March–July 2023," by Ashley L. Fowlkes, Francisco Nogareda and colleagues (CDC; Pan American Health Organization; REVELAC-i Network), Morbidity and Mortality Weekly Report, Centers for Disease Control and Prevention; a work of the United States government in the public domain.