How effective is this year’s vaccine?

New flu data from the southern hemisphere suggests this year’s vaccine isn’t as effective against severe illness as previous iterations, a finding that may not bode well for Americans as the nation kicks off flu season.

Researchers tracked the vaccine’s effectiveness in five South American countries – Argentina, Brazil, Chile, Paraguay and Uruguay – during their flu season from March to July.

They found flu shots reduced the risk of hospitalizations by about 34%, according to the study published Thursday in the Centers for Disease Control and Prevention’s Morbidity and Mortality Weekly Report.

Public health experts typically expect flu vaccines to reduce hospitalization risk by closer to 50%, said Dr. Kawsar Talaat, associate professor of international health at the Johns Hopkins Bloomberg School of Public Health.

“It’s a little bit disappointing,” she said. “This is on the lower end of what would be expected but not outside the range.”

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Vaccines distributed in the U.S. target two influenza A strains – H1N1 and H3N2 – and a B-lineage, the same strains targeted by vaccines used in South America.

A typical flu season occurs between April and 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 to gain insight into how Americans may experience the flu season.

But they may not suffer a fate similar to their South American peers, Talaat said, because multiple variables determine the outcome of a flu season.

One of the most important variables is the circulating strains. It remains to be seen if the strains that appeared in South America will be the same strains that circulate in the U.S.

“It’s very early here… We’re not seeing a lot of flu, yet,” Talaat said. “We might have different virus (strains) and different proportions.”

Vaccination coverage is also an important factor, she said. A high flu vaccination rate will prevent the virus from spreading and infecting people who are likely to become severely ill.

Even if the strains targeted by the current U.S. flu shot don’t match the strains that appear in the northern hemisphere, studies have shown the vaccine will still provide some protection.

The 2018-2019 influenza season is one such case.

The vaccine’s effectiveness during that year was an abysmal 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 U.S.

Sanofi and CSL Seqirus, two of the nation’s largest flu vaccine manufacturers, conceded that the circulating virus strains could differ from those the World Health Organization recommended be included in this year’s flu vaccine. While trends from the southern hemisphere offer valuable insights, they don’t always predict outcomes in the U.S., according to a spokesperson for CSL Seqirus.

“This is a natural phenomenon linked to the unpredictability of the influenza epidemiology: the viruses can mutate as they move across populations and geographies,” said Thomas Grenier, Sanofi’s head of vaccines in North America. “Vaccination remains the best preventative strategy for influenza.”

GSK, another large manufacturer of flu vaccine, did not respond on Friday to requests for comment.

It’s still too early in the flu season to tell if this will be a “mismatch year,” but Talaat still urged eligible Americans to get their vaccinations. Any protection is better than none.

“We’d like to be closer” to that 50% target, she said. “But preventing a third of hospitalizations is still good.”

Adrianna Rodriguez can be reached at adrodriguez@.com.