New flu data from the Southern Hemisphere suggests this year’s vaccines are less effective against severe illness than in previous years, and the findings do not bode well for Americans as flu season begins across the country. There is a possibility.
Researchers tracked vaccine effectiveness in five South American countries during the flu season from March to July: Argentina, Brazil, Chile, Paraguay and Uruguay.
A study published Thursday in the Centers for Disease Control and Prevention’s Morbidity and Mortality Weekly Report found that influenza vaccination reduces the risk of hospitalization by about 34%.
Dr. Kausar Talat, associate professor of global health at the Johns Hopkins Bloomberg School of Public Health, said public health experts typically expect the flu vaccine to reduce the risk of hospitalization by nearly 50%.
“It’s a little disappointing,” she said. “This is on the lower end of expected values, but not out of range.”
Back-to-school vaccinations: Fewer kindergarteners were vaccinated last year, CDC says
The vaccine being distributed in the United States targets two influenza A strains (H1N1 and H3N2) and the B strain, which is the same strain targeted by vaccines used in South America.
The typical influenza season occurs from April to September in the Southern Hemisphere and from October to May in the Northern Hemisphere. Public health experts often look at data from the Southern Hemisphere for insight into how Americans experience the flu season.
But they may not suffer the same fate as their South American compatriots, Talaat said, as multiple variables determine the outcome of a flu season.
One of the most important variables is the circulating strain. It remains to be seen whether the strain emerging in South America is the same strain circulating in the United States.
“It’s very early here… we don’t have a lot of influenza cases yet,” Talaat said. “There may be different virus strains and different ratios.”
Vaccination rates are also an important factor, she said. High influenza vaccination rates can prevent the virus from spreading and infecting people who are more likely to become seriously ill.
Even if the strains targeted by current U.S. flu shots don’t match those occurring in the Northern Hemisphere, studies have shown that the vaccines still offer some protection.
The 2018-2019 influenza season is one of them.
That year’s vaccine efficacy was a dismal 29%, according to the CDC. But a 2020 study found the shot still prevented an estimated 4.4 million infections, 58,000 hospitalizations and 3,500 deaths in the United States.
Sanofi and CSL Sekyrus, the country’s largest flu vaccine makers, acknowledged that the circulating virus strain may be different from the one recommended by the World Health Organization for inclusion in this year’s flu vaccines. A CSL Seqirus spokesperson said Southern Hemisphere trends provide valuable insight but do not necessarily predict U.S. outcomes.
“This is a natural phenomenon related to the unpredictability of influenza epidemiology, as the virus can mutate as it moves through populations and regions,” said Thomas Grenier, Sanofi’s head of vaccines in North America. Ta. “Vaccination remains the best prevention strategy against influenza.”
GSK, another major manufacturer of influenza vaccines, did not respond to a request for comment Friday.
Although it’s still too early in flu season to tell whether this will be the “year of mismatch,” Talaat still urged eligible Americans to get vaccinated. Any protection is better than none.
“We want to get even closer” to that 50% goal, she said. “But preventing a third of hospitalizations is still a good thing.”
Adriana Rodriguez can be reached at adrodriguez@usatoday.com.
