This year’s flu vaccine was less effective than last season in South America, the Centers for Disease Control and Prevention reported Thursday, a clue as to how well the shot will protect people in the United States this winter. It might become.
Interim estimates from a new paper published by the CDC’s Morbidity and Mortality Weekly Report found that the vaccine’s effectiveness against hospitalization was 34.5% among high-risk groups such as young children, people with pre-existing medical conditions, and the elderly. That means vaccinated people in these groups were 34.5% less likely to get sick enough to go to the hospital than unvaccinated people.
A CDC report last year estimated vaccine efficacy in South America at 51.9% against hospitalizations among at-risk groups. A study by the same group, which looked at data from 2013 to 2017, estimated efficacy was about 43% for fully vaccinated young children and about 41% for older adults.
These data come from a research network coordinated by the Pan American Health Organization that includes Argentina, Brazil, Chile, Paraguay and Uruguay.
The biggest drop in efficacy this year may be the result of a decline in the number of people infected with the A(H1N1)pdm09 strain, which has been prevalent since the 2009 H1N1 swine flu pandemic.
Until now, influenza vaccines have generally been more effective against H1N1 than H3N2.
Last year, almost all infections in South America were caused by the H1N1 virus, but this year, the World Health Organization announced that detections of the H3N2 influenza subtype have increased.
If a similar pattern of influenza virus strains is identified in the Northern Hemisphere, “health officials may expect a similar level of protection,” the study authors wrote.
U.S. officials often look to southern hemisphere countries to get a preview of what this year’s flu season will look like, as winter sets in during the northern hemisphere’s summer.
The CDC said in August that this year’s southern hemisphere influenza season was “similar to past influenza seasons,” but it also noted an unusually high number of severe influenza hospitalizations reported from Chile, Ecuador, and Uruguay.
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Alexander Ting
