Post by Nadica (She/Her) on Oct 8, 2024 23:21:39 GMT
35% flu vaccine efficacy in South America may predict similar for US season - Published Oct 7, 2024
By Mary Van Beusekom, MS
The effectiveness of the seasonal flu vaccine against hospitalization among high-risk groups during the 2024 seasonal flu season in five Southern Hemisphere countries was low (35%), down from 50% in 2023 but not outside the expected range, which may portend similar efficacy during the upcoming Northern Hemisphere flu season if similar A(H3N2) viruses predominate, Centers for Disease Control and Prevention (CDC) researchers report.
The interim estimates, published late last week in Morbidity and Mortality Weekly Report (MMWR), come from Argentina, Brazil, Chile, Paraguay, and Uruguay, where an average of only 21.3% of patients hospitalized for flu from March to July 2024 (the Southern Hemisphere flu season) had received the flu vaccine.
"While Southern Hemisphere data cannot predict exactly what will happen in the Northern Hemisphere, what we can say is that if we see similar influenza viruses (strains or clades) circulating this season in the Northern Hemisphere, we can expect similar vaccine effectiveness against influenza-related hospitalizations," lead author Erica Zeno, PhD, epidemic intelligence service officer with the CDC’s Influenza Division, told CIDRAP News in an email.
This flu season, the United States is using trivalent (three-strain) vaccines with the same components as those in Southern Hemisphere vaccines, Zeno said.
On August 29, the CDC forecasted that the upcoming flu season "will be similar to or lower than that of the 2023-2024 season rate of 8.9 weekly laboratory-confirmed hospitalizations per 100,000, with moderate confidence," it wrote. "However, past seasons have varied widely in the number of illnesses, hospitalizations, and deaths, depending on the subtypes circulating, population immunity to different subtypes, and vaccine effectiveness against circulating subtypes."
Campaign targeted high-risk groups
The researchers used a test-negative, case-control design to analyze data from a multinational surveillance network to generate estimates of interim vaccine effectiveness (VE) against hospitalization with flu-related severe acute respiratory illness (SARI).
The study population was made up of 11,751 SARI patients from three Pan American Health Organization (PAHO) vaccination target groups: young children (58.3%), older children and people with underlying medical conditions (14.5%), and older adults (27.2%). Case-patients had SARI and tested positive for flu, while control patients had SARI and tested negative for flu and COVID-19.
Data were pooled from 30 hospitals in Argentina, 2,477 in Brazil, 13 in Chile, 5 in Paraguay, and 10 in Uruguay beginning 2 weeks after each country's flu vaccination campaign. Vaccination status was confirmed using national electronic vaccination records.
All countries used World Health Organization (WHO)–recommended egg-based Southern Hemisphere formulations. Argentina, Brazil, Chile, and Uruguay used trivalent (three-strain) vaccines containing antigens from A/Victoria/4897/2022 (H1N1)pdm09–like virus, A/Thailand/8/2022 (H3N2)–like virus, and B/Austria/1359417/2021 (B/Victoria lineage)–like virus, while Paraguay used quadrivalent (four-strain) vaccines that also contained the B/Yamagata lineage–like virus.
Vaccination rate lower than prepandemic norms
The five countries reported 11,751 flu-related SARI cases and an average low seasonal flu vaccination rate. "The documented influenza vaccination coverage levels (21.3%) were below pre–COVID-19 norms," the researchers wrote. "This finding is consistent with postpandemic declines in vaccination coverage across the Americas associated with vaccine misinformation, hesitancy, and disruptions in routine immunization services, prevalent during the COVID-19 pandemic."
The adjusted VE was 34.5% against hospitalization, 36.5% against the predominant A(H3N2) influenza substrain, and 37.1% against the A(H1N1)pdm09 substrain. VE was 58.7% among patients with chronic conditions, 39.0% among young children, and 31.2% among older adults.
"The vaccine effectiveness estimated is within the confidence interval of prior H3N2 (34%–53%) and H1N1 (18%–56%) for this Southern Hemisphere network," Zeno said. "It is lower than what was observed in the United States last season, but that was on the high-end of the historic range."
As of July 19, too few influenza B cases were available to estimate VE, the authors said. Adjusted VE against SARI from any flu virus was 42.2% in Argentina, 30.3% in Brazil, 56.9% in Chile, and 61.0% in Uruguay; VE was not calculated for Paraguay because of insufficient data. A sensitivity analysis excluding Brazil estimated an adjusted VE of 56.5%.
In total, 32.7% of SARI patients tested positive for flu, and 98.6% of identified viruses were influenza A. Only 0.7% of patients were infected with influenza B, all of which were of the B/Victoria lineage. Of 61.9% of subtyped influenza A viruses, 68.3% were A(H3N2), and 31.7% were A(H1N1)pdm09. Most (59.2%) of the case-patients were older adults, followed by people with chronic conditions (50.4%), and the lowest proportion of cases (16.0%) occurred among young children.
Vaccination can reduce illness severity, death
"While flu vaccine effectiveness can vary from season to season, we know that influenza vaccination can offer significant protection for people who get vaccinated and is the best protection against influenza," Zeno said. "Importantly, influenza vaccination also can reduce severity of illness in people who get vaccinated but still get sick."
Vaccination is one of the most effective interventions to prevent flu-related complications, including death, the authors said. "Annual influenza vaccination should be encouraged for young children, persons with comorbidities, and older adults," the authors wrote. "To enhance this year’s modest influenza vaccine protection against hospitalization, providers should treat patients with suspected or confirmed influenza as soon as possible with antivirals."
"Influenza vaccine postintroduction evaluations and knowledge, attitudes, and practices surveys might identify additional reasons for low coverage and strategies for improved coverage for the next Southern Hemisphere season," they added.
By Mary Van Beusekom, MS
The effectiveness of the seasonal flu vaccine against hospitalization among high-risk groups during the 2024 seasonal flu season in five Southern Hemisphere countries was low (35%), down from 50% in 2023 but not outside the expected range, which may portend similar efficacy during the upcoming Northern Hemisphere flu season if similar A(H3N2) viruses predominate, Centers for Disease Control and Prevention (CDC) researchers report.
The interim estimates, published late last week in Morbidity and Mortality Weekly Report (MMWR), come from Argentina, Brazil, Chile, Paraguay, and Uruguay, where an average of only 21.3% of patients hospitalized for flu from March to July 2024 (the Southern Hemisphere flu season) had received the flu vaccine.
"While Southern Hemisphere data cannot predict exactly what will happen in the Northern Hemisphere, what we can say is that if we see similar influenza viruses (strains or clades) circulating this season in the Northern Hemisphere, we can expect similar vaccine effectiveness against influenza-related hospitalizations," lead author Erica Zeno, PhD, epidemic intelligence service officer with the CDC’s Influenza Division, told CIDRAP News in an email.
This flu season, the United States is using trivalent (three-strain) vaccines with the same components as those in Southern Hemisphere vaccines, Zeno said.
On August 29, the CDC forecasted that the upcoming flu season "will be similar to or lower than that of the 2023-2024 season rate of 8.9 weekly laboratory-confirmed hospitalizations per 100,000, with moderate confidence," it wrote. "However, past seasons have varied widely in the number of illnesses, hospitalizations, and deaths, depending on the subtypes circulating, population immunity to different subtypes, and vaccine effectiveness against circulating subtypes."
Campaign targeted high-risk groups
The researchers used a test-negative, case-control design to analyze data from a multinational surveillance network to generate estimates of interim vaccine effectiveness (VE) against hospitalization with flu-related severe acute respiratory illness (SARI).
The study population was made up of 11,751 SARI patients from three Pan American Health Organization (PAHO) vaccination target groups: young children (58.3%), older children and people with underlying medical conditions (14.5%), and older adults (27.2%). Case-patients had SARI and tested positive for flu, while control patients had SARI and tested negative for flu and COVID-19.
Data were pooled from 30 hospitals in Argentina, 2,477 in Brazil, 13 in Chile, 5 in Paraguay, and 10 in Uruguay beginning 2 weeks after each country's flu vaccination campaign. Vaccination status was confirmed using national electronic vaccination records.
All countries used World Health Organization (WHO)–recommended egg-based Southern Hemisphere formulations. Argentina, Brazil, Chile, and Uruguay used trivalent (three-strain) vaccines containing antigens from A/Victoria/4897/2022 (H1N1)pdm09–like virus, A/Thailand/8/2022 (H3N2)–like virus, and B/Austria/1359417/2021 (B/Victoria lineage)–like virus, while Paraguay used quadrivalent (four-strain) vaccines that also contained the B/Yamagata lineage–like virus.
Vaccination rate lower than prepandemic norms
The five countries reported 11,751 flu-related SARI cases and an average low seasonal flu vaccination rate. "The documented influenza vaccination coverage levels (21.3%) were below pre–COVID-19 norms," the researchers wrote. "This finding is consistent with postpandemic declines in vaccination coverage across the Americas associated with vaccine misinformation, hesitancy, and disruptions in routine immunization services, prevalent during the COVID-19 pandemic."
The adjusted VE was 34.5% against hospitalization, 36.5% against the predominant A(H3N2) influenza substrain, and 37.1% against the A(H1N1)pdm09 substrain. VE was 58.7% among patients with chronic conditions, 39.0% among young children, and 31.2% among older adults.
"The vaccine effectiveness estimated is within the confidence interval of prior H3N2 (34%–53%) and H1N1 (18%–56%) for this Southern Hemisphere network," Zeno said. "It is lower than what was observed in the United States last season, but that was on the high-end of the historic range."
As of July 19, too few influenza B cases were available to estimate VE, the authors said. Adjusted VE against SARI from any flu virus was 42.2% in Argentina, 30.3% in Brazil, 56.9% in Chile, and 61.0% in Uruguay; VE was not calculated for Paraguay because of insufficient data. A sensitivity analysis excluding Brazil estimated an adjusted VE of 56.5%.
In total, 32.7% of SARI patients tested positive for flu, and 98.6% of identified viruses were influenza A. Only 0.7% of patients were infected with influenza B, all of which were of the B/Victoria lineage. Of 61.9% of subtyped influenza A viruses, 68.3% were A(H3N2), and 31.7% were A(H1N1)pdm09. Most (59.2%) of the case-patients were older adults, followed by people with chronic conditions (50.4%), and the lowest proportion of cases (16.0%) occurred among young children.
Vaccination can reduce illness severity, death
"While flu vaccine effectiveness can vary from season to season, we know that influenza vaccination can offer significant protection for people who get vaccinated and is the best protection against influenza," Zeno said. "Importantly, influenza vaccination also can reduce severity of illness in people who get vaccinated but still get sick."
Vaccination is one of the most effective interventions to prevent flu-related complications, including death, the authors said. "Annual influenza vaccination should be encouraged for young children, persons with comorbidities, and older adults," the authors wrote. "To enhance this year’s modest influenza vaccine protection against hospitalization, providers should treat patients with suspected or confirmed influenza as soon as possible with antivirals."
"Influenza vaccine postintroduction evaluations and knowledge, attitudes, and practices surveys might identify additional reasons for low coverage and strategies for improved coverage for the next Southern Hemisphere season," they added.