Experts uncertain of flu vaccine’s effectiveness this season
TULSA — As the U.S. flu season approaches its midpoint, health officials say it’s too soon to tell whether the vaccination’s atypical ineffectiveness at repelling the predominant strain in Australia will manifest here.
Australia tallied record numbers of hospitalizations and deaths related to influenza during its flu season this year. The vaccine distributed to the country’s residents was only about 10 percent effective against H3N2, a subtype of the influenza A virus that was most prevalent there, the New England Journal of Medicine reported last week.
With the vaccine for the ongoing flu season in America sharing the same composition as the one used in the Southern Hemisphere, the report warns of the potential for similar results locally.
“There’s no way to know if the flu vaccine is going to behave the same way as it did in Australia,” said Kendra Dougherty, an epidemiologist with the Oklahoma state Health Department. “We’re not going to know vaccine effectiveness estimates until about January or February for the U.S. season.”
The typical success rate for flu vaccines ranges from 40 to 60 percent, according to the Centers for Disease Control and Prevention. But the federal agency noted that vaccines usually work better against influenza B and H1N1 while offering lower protection against H3N2 — the prevalent Australian virus. In the Northern Hemisphere last year, the effectiveness was estimated at 42 percent overall and 34 percent against H3N2.
Although the possibility of a 10 percent success rate exists, Dougherty said its impact would be determined by how predominant H3N2 is in a given community.
In Oklahoma, for instance, H3N2 currently is not the primary strain. Experts are seeing fairly equal cases of H3N2 and the H1N1 subtype of influenza A.
Oklahoma is one of the few states to have declared widespread infections this year. There were 105 influenza-associated hospitalizations and two deaths reported statewide between Sept. 1 and Nov. 28, according to the state Health Department. Updated numbers are expected to be released Thursday.
“While we’ve had a lot of flu hospitalizations so far this season comparatively to other seasons, we haven’t had a lot of deaths,” Dougherty said. “Part of that could be due to that it is not a predominant H3N2 season so far.”
The flu season is far from over, however, and there is still time for H3N2 to take over.
Or it could be phased out by another virus, Dougherty noted. There were a few weeks when she thought that would happen as reports of the milder H1N1 escalated. Now the two seem to switch back and forth each week, making it impossible to predict the remainder of the season, which can last through May.
“They’re just kind of battling it out right now,” she said.
Viral adaptability
Part of why flu strains are able to build an immunity to vaccinations is their ability to adapt. Ellen Niemitalo, who manages the Tulsa Health Department’s Immunization Division, pointed to a theory that H3N2 tends to shift more rapidly than other strains.
Another theory is that small modifications occurring inside the eggs in which the vaccine virus is grown have lowered its effectiveness.
Regardless of the success rate, Niemitalo still stresses the need for vaccinations, as they are the best available defense against the flu.
“The influenza vaccine has benefits beyond just preventing the incidence of the flu,” she said. “It reduces the severity of the disease. It reduces hospitalizations and deaths.”
Niemitalo highlighted a study published last year that directly linked a decrease in pediatric deaths from the flu with vaccinations. She said there also have been several studies showing that vaccinations benefit pregnant women and their unborn babies.