April 19 (UPI) -- This fall's flu vaccine will be 20 percent effective for the dominant circulating strain of influenza A, the same as shots given the past two years, a new study predicts.
Rice University researchers predicted the efficacy of a new H3N2 formulation for the first time since 2015 using a method called pEpitope. Researchers published their findings this week in the journal Clinical Infectious Diseases.
Their study also suggests the pEpitope is a more accurate predictor than long-relied-upon ferret tests. The pEpitope method represents 77 percent of what impacts efficacy of the vaccine in humans.
"The vaccine has been changed for 2018-19, but unfortunately it still contains two critical mutations that arise from the egg-based vaccine production process," Michael Deem, a professor in biochemical and genetic engineering and of physics and astronomy at Rice, said in a press release. "Our study found that these same mutations halved the efficacy of flu vaccines in the past two seasons, and we expect they will lower the efficacy of the next vaccine in a similar manner."
With a 20 percent efficacy, one-fifth percent fewer vaccinated people will get the flu compared with the unvaccinated people.
The pEpitope measures critical differences in the genetic sequences of flu strains. In the new study, the method accurately predicted vaccine efficacy for more than 40 years of flu records.
Flu vaccines are designed to protect against one type of influenza B and two influenza A strains -- H3N2 and H1N1.
The Centers for Disease Control and Prevention is still compiling the 2017-18 flu season data. The pEpitope has predicted it will be around 19 percent against H3N2, the type of influenza A that infected the most people in the United States the past past two years.
In 2016, the vaccine had an efficacy of 20 percent, similar to 19 percent predicted by pEpitope.
In February, the CDC said in its sampling that 36 percent of people who got the vaccine did not get the flu this past season, compared with 40 percent last year and 48 percent two years ago.
Most flu vaccines come from culturing viruses in chicken eggs, which researchers sasy can be difficult to grow.
"Very often there are egg adaptations," Deem said. "There were a couple of these in the vaccine strain the past two seasons that wound up making it a little bit different from the actual circulating virus strain."
The company Medicago has been running clinical trials on a flu vaccine produced from tobacco plants and expects the vaccine to be on the market for the 2020-21 influenza season. The company says its product can be grown quicker and more effectively from a non-live virus.
Researchers also compared the efficacy of the egg-based vaccine with an experimental vaccine produced from insect cells via reverse genetics. The cell-based vaccine had a predicted efficacy of 47 percent, which Deem said is an average value of a perfectly matched H3N2 vaccine.
The researchers said their technique's predictions are three times more accurate than using ferrets, which have been used for decades to test the flu vaccine.
"It's been apparent over the last 10 years that egg adaptations have affected the efficacy of flu vaccines," Deem said. "It's also been apparent that the ferrets have done a really bad job of predicting the reduction of the efficacy due to the egg adaptations. Additionally, it's been difficult to get data from ferrets because the ferrets' immune systems have not recognized the vaccines particularly well over the past 10 years."