Experts are predicting a severe flu season in the U.S. this year after finding that the flu shot was only 10% effective at preventing influenza A (H3N2) in Australia, resulting in record-high numbers of flu illnesses.
Flu shots can still lessen the severity of influenza, so experts are still recommending a flu shot this year — but the shot is only going to be 10% effective at preventing H3N2 because the virus mutated recently.
There may also be other factors contributing to the poor effectiveness of the flu shot, according to a study in the New England Journal of Medicine.
Australia had a record-setting flu season this year, with 215,280 reported illness by mid-October 2017. The number of illnesses was far higher than the 59,022 cases reported in the 2009 H1N1 pandemic.
Flu viruses travel from the Southern Hemisphere to the Northern Hemisphere during the winter, which means the U.S. could potentially have a severe flu season, experts are warning.
Even in good years, the flu shot is only 40-60% effective at preventing infections with influenza viruses. This year, people in the U.S. will have a higher risk of hospitalization and death in outbreaks of the flu.
The low effectiveness is perplexing because the vaccine was well-matched to the influenza A (H3N2) strain that is causing unusually high numbers of hospitalizations and deaths from the flu in Australia.
The researchers had several explanations. Prior flu illnesses and flu shots may be influencing the effectiveness of this year’s vaccine.
Another problem is the manufacturing process. In the U.S., most flu vaccines contain viruses that are grown in eggs. The viruses mutate to better infect birds, which makes the vaccine less effective in humans.
“This possibility underscores the need to strive toward a ‘universal’ influenza vaccine that will protect against seasonal influenza drift variants as well as potential pandemic strains.”
The number of flu illnesses ranges from 140,000 to 710,000 per year in the U.S., with between 12,000 and 56,000 deaths. People who are very young, elderly adults, and people with weak immune systems have the highest risk of hospitalization or death from flu infections.